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- Ryoya Kohata, HyunSeok Lim, Yuki Kanamoto, Akio Murakami, Yuichi Fujita, Ayumi Tanaka, Wesley Swingley, Hisashi Ito, Ryouichi TanakaJournal of Plant Research 136 (1) 107 - 115 0918-9440 2022/11/10 [Refereed][Not invited]
- Hisashi Ito, Hideyuki Saito, Manabu Fukui, Ayumi Tanaka, Keita ArakawaPlant Science 324 111444 - 111444 0168-9452 2022/11 [Refereed]
- Debayan Dey, Masayoshi Nishijima, Ryouichi Tanaka, Genji Kurisu, Hideaki Tanaka, Hisashi ItoProtein science : a publication of the Protein Society 31 (10) e4430 2022/10Chlorophyll degradation plays a myriad of physiological roles in photosynthetic organisms, including acclimation to light environment and nutrient remobilization during senescence. Mg extraction from chlorophyll a is the first and committed step of the chlorophyll degradation pathway. This reaction is catalyzed by the Mg-dechelatase enzyme encoded by Stay-Green (SGR). The reaction mechanism of SGR protein remains elusive since metal ion extraction from organic molecules is not a common enzymatic reaction. Additionally, experimentally derived structural information about SGR or its homologs has not yet been reported. In this study, the crystal structure of the SGR homolog from Anaerolineae bacterium was determined using the molecular replacement method at 1.85 Å resolution. Our previous study showed that three residues-H32, D34, and D62 are essential for the catalytic activity of the enzyme. Biochemical analysis involving mutants of D34 residue further strengthened its importance in the functioning of the dechelatase. Docking simulation also revealed the interaction between the D34 side chain and central Mg ion of chlorophyll a. Structural analysis showed the arrangement of D34/H32/D62 in the form of a catalytic triad that is generally found in hydrolases. The probable reaction mechanism suggests that deprotonated D34 side chain coordinates and destabilizes Mg, resulting in Mg extraction. Besides, H32 possibly acts as a general base catalyst and D62 facilitates H32 to be a better proton acceptor. Taken together, the reaction mechanism of SGR partially mirrors the one observed in hydrolases.
- Hanaki Maeda, Koharu Takahashi, Yoshifumi Ueno, Kei Sakata, Akari Yokoyama, Kozue Yarimizu, Fumiyoshi Myouga, Kazuo Shinozaki, Shin-Ichiro Ozawa, Yuichiro Takahashi, Ayumi Tanaka, Hisashi Ito, Seiji Akimoto, Atsushi Takabayashi, Ryouichi TanakaJournal of plant research 135 (2) 361 - 376 2022/03 [Refereed]
The assembly process of photosystem II (PSII) requires several auxiliary proteins to form assembly intermediates. In plants, early assembly intermediates comprise D1 and D2 subunits of PSII together with a few auxiliary proteins including at least ONE-HELIX PROTEIN1 (OHP1), OHP2, and HIGH-CHLOROPHYLL FLUORESCENCE 244 (HCF244) proteins. Herein, we report the basic characterization of the assembling intermediates, which we purified from Arabidopsis transgenic plants overexpressing a tagged OHP1 protein and named the OHP1 complexes. We analyzed two major forms of OHP1 complexes by mass spectrometry, which revealed that the complexes consist of OHP1, OHP2, and HCF244 in addition to the PSII subunits D1, D2, and cytochrome b559. Analysis of chlorophyll fluorescence showed that a major form of the complex binds chlorophyll a and carotenoids and performs quenching with a time constant of 420 ps. To identify the localization of the auxiliary proteins, we solubilized thylakoid membranes using a digitonin derivative, glycodiosgenin, and separated them into three fractions by ultracentrifugation, and detected these proteins in the loose pellet containing the stroma lamellae and the grana margins together with two chlorophyll biosynthesis enzymes. The results indicated that chlorophyll biosynthesis and assembly may take place in the same compartments of thylakoid membranes. Inducible suppression of the OHP2 mRNA substantially decreased the OHP2 protein in mature Arabidopsis leaves without a significant reduction in the maximum quantum yield of PSII under low-light conditions, but it compromised the yields under high-light conditions. This implies that the auxiliary protein is required for acclimation to high-light conditions. - Koki Fukura, Ayumi Tanaka, Ryouichi Tanaka, Hisashi ItoJournal of plant physiology 266 153535 - 153535 2021/09/25 [Refereed]
During leaf senescence, chlorophyll a and b are degraded through several enzymatic reactions, including chlorophyll b reductase, 7-hydroxymethyl chlorophyll a reductase, and Mg-dechelatase. Considering that the intermediates of the chlorophyll breakdown pathway are highly photoreactive, cooperative and efficient reactions of chlorophyll metabolic enzymes may protect chloroplasts from potential photo-oxidative damage. Here, we investigated the sub-organellar localization and cooperative reactions of the enzymes involved in the chlorophyll breakdown pathway by the fractionation of thylakoid membranes and enzymatic assays using recombinant proteins. We found that these enzymes were enriched in the grana margin fraction. Furthermore, we found that chlorophyll b reductase and Mg-dechelatase efficiently catabolized chlorophylls bound to the chlorophyll-protein complexes when these two enzymes were mixed. These results suggest that the co-localization of chlorophyll catabolic enzymes enables efficient chlorophyll breakdown. The results from this study highlight a key step forward in the investigation of the photosystem breakdown process. - Haruka Suehiro, Ryouichi Tanaka, Hisashi ItoArchives of microbiology 203 (6) 3565 - 3575 2021/08 [Refereed]
In the chlorophyll biosynthesis pathway, the 8-vinyl group of the chlorophyll precursor is reduced to an ethyl group by 8-vinyl reductase. Two isozymes of 8-vinyl reductase have been described in oxygenic photosynthetic organisms: one encoded by BciA and another by BciB. Only BciB contains an [Fe-S] cluster and most cyanobacteria harbor this form; whereas a few contain BciA. Given this disparity in distribution, cyanobacterial BciA has remained largely overlooked, which has limited understanding of chlorophyll biosynthesis in these microorganisms. Here, we reveal that cyanobacterial BciA encodes a functional 8-vinyl reductase, as evidenced by measuring the in vitro activity of recombinant Synechococcus and Acaryochloris BciA. Genomic comparison revealed that BciB had been replaced by BciA during evolution of the marine cyanobacterium Synechococcus, and coincided with replacement of Fe-superoxide dismutase (SOD) with Ni-SOD. These findings imply that the acquisition of BciA confers an adaptive advantage to cyanobacteria living in low-iron oceanic environments. - Ying Chen, Wataru Yamori, Ayumi Tanaka, Ryouichi Tanaka, Hisashi ItoPlant science : an international journal of experimental plant biology 307 110902 - 110902 2021/06 [Refereed]
During leaf senescence, the degradation of photosystems and photosynthetic pigments proceeds in a coordinated manner, which would minimize the potential photodamage to cells. Both photosystem I and II are composed of core complexes and peripheral antenna complexes, with the former binding chlorophyll a and the latter binding chlorophyll a and b. Although the degradation of peripheral antenna complexes is initiated by chlorophyll degradation, it remains unclear whether the degradation of core complexes and chlorophyll is coordinated. In this study, we examined the degradation of peripheral antenna and core complexes in the Arabidopsis sgr1/sgr2/sgrl triple mutant, lacking all the isoforms of chlorophyll a:Mg2+ dechelatase. In this mutant, the degradation of peripheral antenna complexes and photosystem I core complexes was substantially retarded, but the core complexes of photosystem II were rapidly degraded during leaf senescence. On the contrary, the photosynthetic activity declined at a similar rate as in the wild type plants. These results suggest that the degradation of photosystem II core complexes is regulated independently of the major chlorophyll degradation pathway mediated by the dechelatase. The study should contribute to the understanding of the complex molecular mechanisms underlying the degradation of photosystems, which is an essential step during leaf senescence. - Debayan Dey, Dipanjana Dhar, Helena Fortunato, Daichi Obata, Ayumi Tanaka, Ryouichi Tanaka, Soumalee Basu, Hisashi ItoComputational and Structural Biotechnology Journal 19 5333 - 5347 2021/01 [Refereed]
The Mg-dechelatase enzyme encoded by the Stay-Green (SGR) gene catalyzes Mg2+ dechelation from chlorophyll a. This reaction is the first committed step of chlorophyll degradation pathway in plants and is thus indispensable for the process of leaf senescence. There is no structural information available for this or its related enzymes. This study aims to provide insights into the structure and reaction mechanism of the enzyme through biochemical and computational analysis of an SGR homolog from the Chloroflexi Anaerolineae (AbSGR-h). Recombinant AbSGR-h with its intact sequence and those with mutations were overexpressed in Escherichia coli and their Mg-dechelatase activity were compared. Two aspartates – D34 and D62 were found to be essential for catalysis, while R26, Y28, T29 and D114 were responsible for structural maintenance. Gel filtration analysis of the recombinant AbSGR-h indicates that it forms a homo-oligomer. The three-dimensional structure of AbSGR-h was predicted by a deep learning-based method, which was evaluated by protein structure quality evaluation programs while structural stability of wild-type and mutant forms were investigated through molecular dynamics simulations. Furthermore, in concordance with the results of enzyme assay, molecular docking concluded the significance of D34 in ligand interaction. By combining biochemical analysis and computational prediction, this study unveils the detailed structural characteristics of the enzyme, including the probable pocket of interaction and the residues of structural and functional importance. It also serves as a basis for further studies on Mg-dechelatase such as elucidation of its reaction mechanism or inhibitor screening. - Dongjin Shin, Sichul Lee, Tae-Heon Kim, Jong-Hee Lee, Joonheum Park, Jinwon Lee, Ji Yoon Lee, Lae-Hyeon Cho, Jae Young Choi, Wonhee Lee, Ji-Hwan Park, Dae-Woo Lee, Hisashi Ito, Dae Heon Kim, Ayumi Tanaka, Jun-Hyeon Cho, You-Chun Song, Daehee Hwang, Michael D Purugganan, Jong-Seong Jeon, Gynheung An, Hong Gil NamNature communications 11 (1) 2819 - 2819 2020/06/04 [Refereed][Not invited]
Increased grain yield will be critical to meet the growing demand for food, and could be achieved by delaying crop senescence. Here, via quantitative trait locus (QTL) mapping, we uncover the genetic basis underlying distinct life cycles and senescence patterns of two rice subspecies, indica and japonica. Promoter variations in the Stay-Green (OsSGR) gene encoding the chlorophyll-degrading Mg++-dechelatase were found to trigger higher and earlier induction of OsSGR in indica, which accelerated senescence of indica rice cultivars. The indica-type promoter is present in a progenitor subspecies O. nivara and thus was acquired early during the evolution of rapid cycling trait in rice subspecies. Japonica OsSGR alleles introgressed into indica-type cultivars in Korean rice fields lead to delayed senescence, with increased grain yield and enhanced photosynthetic competence. Taken together, these data establish that naturally occurring OsSGR promoter and related lifespan variations can be exploited in breeding programs to augment rice yield. - Lim H, Tanaka A, Tanaka R, Ito HPlant & cell physiology 0032-0781 2019/08 [Refereed][Not invited]
- Obata D, Takabayashi A, Tanaka R, Tanaka A, Ito HMolecular biology and evolution 0737-4038 2019/08 [Refereed][Not invited]
- Ono K, Kimura M, Matsuura H, Tanaka A, Ito HJournal of plant physiology 238 53 - 62 0176-1617 2019/07 [Refereed][Not invited]
Leaf color change through chlorophyll degradation is a characteristic symptom of senescence. Magnesium removal from chlorophyll a is the initial step in chlorophyll a degradation, in a reaction catalyzed by Stay-Green (SGR). Arabidopsis thaliana has three SGR homologs, SGR1, SGR2, and SGR-like. When SGR1 is overexpressed, both chlorophyll a and b are degraded and leaves turn yellow. This process is visually identical to senescence, suggesting that SGR1 overexpression affects various physiological processes in plants. To examine this possibility, gene expression associated with chlorophyll metabolism and senescence was analyzed following dexamethasone-inducible SGR1 introduction into Arabidopsis. When SGR1 was overexpressed following 18 h of dexamethasone treatment, genes involved in chlorophyll degradation were upregulated, as were senescence-associated genes. These observations suggested that chlorophyll a degradation promotes senescence. As jasmonate is the plant hormone responsible for senescence and was expected to be involved in the regulation of gene expression after dexamethasone treatment, the level of jasmonoyl-isoleucine, the active form of jasmonate, was measured. The jasmonoyl-isoleucine level increased slightly after 10 h of SGR1 overexpression, and this increase became significant after 18 h. These observations suggest that jasmonate is produced through chlorophyll a degradation and affects the promotion of senescence. - Chen Y, Shimoda Y, Yokono M, Ito H, Tanaka AThe Plant journal : for cell and molecular biology 97 (6) 1022 - 1031 0960-7412 2019/03 [Refereed][Not invited]
- Tomoaki Sato, Yousuke Shimoda, Kaori Matsuda, Ayumi Tanaka, Hisashi ItoJournal of Plant Physiology 222 94 - 102 0176-1617 2018/03/01 [Refereed][Not invited]
The first step in chlorophyll a degradation is the extraction of the central Mg. This reaction is catalyzed by Mg-dechelatase encoded by Stay-Green (SGR) in land plants. SGR extracts Mg from chlorophyll a but not from chlorophyll b, and chlorophyll b must be converted to chlorophyll a before degradation. The first reaction of the chlorophyll b to chlorophyll a conversion is catalyzed by chlorophyll b reductase. Non-Yellow Coloring 1 (NYC1) and NYC1 like (NOL) are isozymes of chlorophyll b reductase. When SGR was transiently overexpressed in Arabidopsis, both chlorophyll a and b were degraded, suggesting that the chlorophyll b to chlorophyll a conversion is activated by SGR overexpression. To examine the involvement of chlorophyll b reductases in SGR-induced chlorophyll b degradation, SGR was transiently overexpressed in nyc1, nol, and nyc1 nol double mutants by dexamethasone treatment. It was found that in the wild type and nol mutant, chlorophyll a and b were degraded and all the chlorophyll-binding proteins decreased. Meanwhile, in nyc1 and nyc1 nol mutants, chlorophyll b degradation was suppressed and the light-harvesting complex of photosystem II remained. The mRNA and protein levels of NYC1 increased after SGR overexpression in wild type plants. These results suggest that Mg-dechelation of chlorophyll a by SGR activates chlorophyll b degradation by inducing the expression of NYC1. This is an effective regulation of a metabolic pathway. - Kaori Kohzuma, Yutaka Sato, Hisashi Ito, Ayako Okuzaki, Mai Watanabe, Hideki Kobayashi, Michiharu Nakano, Hiroshi Yamatani, Yu Masuda, Yumi Nagashima, Hiroyuki Fukuoka, Tetsuya Yamada, Akira Kanazawa, Keisuke Kitamura, Yutaka Tabei, Masahiko Ikeuchi, Wataru Sakamoto, Ayumi Tanaka, Makoto KusabaPLANT PHYSIOLOGY 173 (4) 2138 - 2147 0032-0889 2017/04 [Refereed][Not invited]
Chlorophyll degradation plays important roles in leaf senescence including regulation of degradation of chlorophyll-binding proteins. Although most genes encoding enzymes of the chlorophyll degradation pathway have been identified, the regulation of their activity has not been fully understood. Green cotyledon mutants in legume are stay-green mutants, in which chlorophyll degradation is impaired during leaf senescence and seed maturation. Among them, the soybean (Glycine max) green cotyledon gene cytG is unique because it is maternally inherited. To isolate cytG, we extensively sequenced the soybean chloroplast genome, and detected a 5-bp insertion causing a frame-shift in psbM, which encodes one of the small subunits of photosystem II. Mutant tobacco plants (Nicotiana tabacum) with a disrupted psbM generated using a chloroplast transformation technique had green senescent leaves, confirming that cytG encodes PsbM. The phenotype of cytG was very similar to that of mutant of chlorophyll b reductase catalyzing the first step of chlorophyll b degradation. In fact, chlorophyll b-degrading activity in dark-grown cytG and psbM-knockout seedlings was significantly lower than that of wild-type plants. Our results suggest that PsbM is a unique protein linking photosynthesis in presenescent leaves with chlorophyll degradation during leaf senescence and seed maturation. Additionally, we discuss the origin of cytG, which may have been selected during domestication of soybean. - Kaori Matsuda, Yousuke Shimoda, Ayumi Tanaka, Hisashi ItoPLANT PHYSIOLOGY AND BIOCHEMISTRY 109 365 - 373 0981-9428 2016/12 [Refereed][Not invited]
Mg removal from chlorophyll by Mg-dechelatase is the first step of chlorophyll degradation. Recent studies showed that in Arabidopsis, Stay Green (SGR) encodes Mg-dechelatase. Though the Escherichia coli expression system is advantageous for investigating the properties of Mg-dechelatase, Arabidopsis Mg-dechelatase is not successfully expressed in E. coli. Chlamydomonas reinhardtii SGR (CrSGR) has a long, hydrophilic tail, suggesting that active CrSGR can be expressed in E. coli. After the incubation of chlorophyll a with CrSGR expressed in E. coil, pheophytin a accumulated, indicating that active CrSGR was expressed in E. coli. Substrate specificity of CrSGR against chlorophyll b and an intermediate molecule of the chlorophyll b degradation pathway was examined. CrSGR exhibited no activity against chlorophyll b and low activity against 7-hydroxymethyl chlorophyll a, consistent with the fact that chlorophyll b is degraded only after conversion to chlorophyll a. CrSGR exhibited low activity against divinyl chlorophyll a and chlorophyll a', and no activity against chlorophyllide a, protochlorophyll a, chlorophyll c(2), and Znchlorophyll a. These observations indicate that chlorophyll a is the most favorable substrate for CrSGR. When CrSGR was expressed in Arabidopsis cells, the chlorophyll content decreased, further confirming that SGR has Mg-dechelating activity in chloroplasts. (C) 2016 Elsevier Masson SAS. All rights reserved. - Ting Jia, Hisashi Ito, Ayumi TanakaPLANTA 244 (5) 1041 - 1053 0032-0935 2016/11 [Refereed][Not invited]
Main conclusionThe photosystem I/II ratio increased when antenna size was enlarged by transient induction of CAO in chlorophyll b -less mutants, thus indicating simultaneous regulation of antenna size and photosystem I/II stoichiometry. Regulation of antenna size and photosystem I/II stoichiometry is an indispensable strategy for plants to acclimate to changes to light environments. When plants grown in high-light conditions are transferred to low-light conditions, the peripheral antennae of photosystems are enlarged. A change in the photosystem I/II ratio is also observed under the same light conditions. However, our knowledge of the correlation between antenna size modulation and variation in photosystem I/II stoichiometry remains limited. In this study, chlorophyll a oxygenase was transiently induced in Arabidopsis thaliana chlorophyll b-less mutants, ch1-1, to alter the antenna size without changing environmental conditions. In addition to the accumulation of chlorophyll b, the levels of the peripheral antenna complexes of both photosystems gradually increased, and these were assembled to the core antenna of both photosystems. However, the antenna size of photosystem II was greater than that of photosystem I. Immunoblot analysis of core antenna proteins showed that the number of photosystem I increased, but not that of photosystem II, resulting in an increase in the photosystem I/II ratio. These results clearly indicate that antenna size adjustment was coupled with changes in photosystem I/II stoichiometry. Based on these results, the physiological importance of simultaneous regulation of antenna size and photosystem I/II stoichiometry is discussed in relation to acclimation to light conditions. - Yousuke Shimoda, Hisashi Ito, Ayumi TanakaPLANT CELL 28 (9) 2147 - 2160 1040-4651 2016/09 [Refereed][Not invited]
Pheophytin a is an essential component of oxygenic photosynthetic organisms because the primary charge separation between chlorophyll a and pheophytin a is the first step in the conversion of light energy. In addition, conversion of chlorophyll a to pheophytin a is the first step of chlorophyll degradation. Pheophytin is synthesized by extracting magnesium (Mg) from chlorophyll; the enzyme Mg-dechelatase catalyzes this reaction. In this study, we report that Mendel's green cotyledon gene, STAY-GREEN (SGR), encodes Mg-dechelatase. The Arabidopsis thaliana genome has three SGR genes, SGR1, SGR2, and STAY-GREEN LIKE (SGRL). Recombinant SGR1/2 extracted Mg from chlorophyll a but had very low or no activity against chlorophyllide a; by contrast, SGRL had higher dechelating activity against chlorophyllide a compared with chlorophyll a. All SGRs could not extract Mg from chlorophyll b. Enzymatic experiments using the photosystem and light-harvesting complexes showed that SGR extracts Mg not only from free chlorophyll but also from chlorophyll in the chlorophyll-protein complexes. Furthermore, most of the chlorophyll and chlorophyll binding proteins disappeared when SGR was transiently expressed by a chemical induction system. Thus, SGR is not only involved in chlorophyll degradation but also contributes to photosystem degradation. - Rei Sato, Hisashi Ito, Ayumi TanakaPHOTOSYNTHESIS RESEARCH 126 (2-3) 249 - 259 0166-8595 2015/12 [Refereed][Not invited]
The light-harvesting chlorophyll a/b binding protein complex of photosystem II (LHCII) is the main antenna complex of photosystem II (PSII). Plants change their LHCII content depending on the light environment. Under high-light conditions, the content of LHCII should decrease because over-excitation damages the photosystem. Chlorophyll b is indispensable for accumulating LHCII, and chlorophyll b degradation induces LHCII degradation. Chlorophyll b degradation is initiated by chlorophyll b reductase (CBR). In land plants, NON-YELLOW COLORING 1 (NYC1) and NYC1-Like (NOL) are isozymes of CBR. We analyzed these mutants to determine their functions under high-light conditions. During high-light treatment, the chlorophyll a/b ratio was stable in the wild-type (WT) and nol plants, and the LHCII content decreased in WT plants. The chlorophyll a/b ratio decreased in the nyc1 and nyc1/nol plants, and a substantial degree of LHCII was retained in nyc1/nol plants after the high-light treatment. These results demonstrate that NYC1 degrades the chlorophyll b on LHCII under high-light conditions, thus decreasing the LHCII content. After the high-light treatment, the maximum quantum efficiency of the PSII photochemistry was lower in nyc1 and nyc1/nol plants than in WT and nol plants. A larger light-harvesting system would damage PSII in nyc1 and nyc1/nol plants. The fluorescence spectroscopy of the leaves indicated that photosystem I was also damaged by the excess LHCII in nyc1/nol plants. These observations suggest that chlorophyll b degradation by NYC1 is the initial reaction for the optimization of the light-harvesting capacity under high-light conditions. - Shin-Ichi Maeda, Akio Murakami, Hisashi Ito, Ayumi Tanaka, Tatsuo OmataLife 5 (1) 432 - 446 2075-1729 2015/02/09 [Refereed][Not invited]
Many of the cyanobacterial species found in marine and saline environments have a gene encoding a putative nitrite transporter of the formate/nitrite transporter (FNT) family. The presumed function of the gene (designated nitM) was confirmed by functional expression of the gene from the coastal marine species Synechococcus sp. strain PCC7002 in the nitrite-transport-less mutant (NA4) of the freshwater cyanobacterium Synechococcus elongatus strain PCC7942. The NitM-mediated nitrite uptake showed an apparent Km (NO2−) of about 8 μM and was not inhibited by nitrate, cyanate or formate. Of the nitM orthologs from the three oceanic cyanobacterial species, which are classified as α-cyanobacteria on the basis of the occurrence of Type 1a RuBisCO, the one from Synechococcus sp. strain CC9605 conferred nitrite uptake activity on NA4, but those from Synechococcus sp. strain CC9311 and Prochlorococcus marinus strain MIT9313 did not. A strongly conserved hydrophilic amino acid sequence was found at the C-termini of the deduced NitM sequences from α-cyanobacteria, with a notable exception of the Synechococcus sp. strain CC9605 NitM protein, which entirely lacked the C-terminal amino acids. The C-terminal sequence was not conserved in the NitM proteins from β-cyanobacteria carrying the Type 1b RuBisCO, including the one from Synechococcus sp. strain PCC7002. Expression of the truncated nitM genes from Synechococcus sp. strain CC9311 and Prochlorococcus marinus strain MIT9313, encoding the proteins lacking the conserved C-terminal region, conferred nitrite uptake activity on the NA4 mutant, indicating that the C-terminal region of a-cyanobacterial NitM proteins inhibits the activity of the transporter. - Ting Jia, Hisashi Ito, Xueyun Hu, Ayumi TanakaPLANT JOURNAL 81 (4) 586 - 596 0960-7412 2015/02 [Refereed][Not invited]
Chlorophyll a and chlorophyll b are interconverted in the chlorophyll cycle. The initial step in the conversion of chlorophyll b to chlorophyll a is catalyzed by the chlorophyll b reductases NON-YELLOW COLORING 1 (NYC1) and NYC1-like (NOL), which convert chlorophyll b to 7-hydroxymethyl chlorophyll a. This step is also the first stage in the degradation of the light-harvesting chlorophyll a/b protein complex (LHC). In this study, we examined the effect of chlorophyll b on the level of NYC1. NYC1 mRNA and NYC1 protein were in low abundance in green leaves, but their levels increased in response to dark-induced senescence. When the level of chlorophyll b was enhanced by the introduction of a truncated chlorophyllide a oxygenase gene and the leaves were incubated in the dark, the amount of NYC1 was greatly increased compared with that of the wild type; however, the amount of NYC1 mRNA was the same as in the wild type. In contrast, NYC1 did not accumulate in the mutant without chlorophyll b, even though the NYC1 mRNA level was high after incubation in the dark. Quantification of the LHC protein showed no strong correlation between the levels of NYC1 and LHC proteins. However, the level of chlorophyll fluorescence of the dark adapted plant (F-o) was closely related to the accumulation of NYC1, suggesting that the NYC1 level is related to the energetically uncoupled LHC. These results and previous reports on the degradation of chlorophyllide a oxygenase suggest that the a feedforward and feedback network is included in chlorophyll cycle. - The Chlorophyll b Reductase NOL Participates in Regulating the Antenna Size of Photosystem II in Arabidopsis Thaliana.Jia T, Ito H, Tanaka AProcedia Chemistry 14 422 - 427 2015 [Refereed][Not invited]
- Hisashi Ito, Ayumi TanakaPLANT AND CELL PHYSIOLOGY 55 (3) 593 - 603 0032-0781 2014/03 [Refereed][Not invited]
Organisms generate an enormous number of metabolites; however, the mechanisms by which a new metabolic pathway is acquired are unknown. To elucidate the importance of promiscuous enzyme activity for pathway evolution, the catalytic and substrate specificities of Chl biosynthetic enzymes were examined. In green plants, Chl a and Chl b are interconverted by the Chl cycle: Chl a is hydroxylated to 7-hydroxymethyl chlorophyll a followed by the conversion to Chl b, and both reactions are catalyzed by chlorophyllide a oxygenase. Chl b is reduced to 7-hydroxymethyl chlorophyll a by Chl b reductase and then converted to Chl a by 7-hydroxymethyl chlorophyll a reductase (HCAR). A phylogenetic analysis indicated that HCAR evolved from cyanobacterial 3,8-divinyl chlorophyllide reductase (DVR), which is responsible for the reduction of an 8-vinyl group in the Chl biosynthetic pathway. In addition to vinyl reductase activity, cyanobacterial DVR also has Chl b reductase and HCAR activities; consequently, three of the four reactions of the Chl cycle already existed in cyanobacteria, the progenitor of the chloroplast. During the evolution of cyanobacterial DVR to HCAR, the HCAR activity, a promiscuous reaction of cyanobacterial DVR, became the primary reaction. Moreover, the primary reaction (vinyl reductase activity) and some disadvantageous reactions were lost, but the neutral promiscuous reaction (NADH dehydrogenase) was retained in both DVR and HCAR. We also show that a portion of the Chl c biosynthetic pathway already existed in cyanobacteria. We discuss the importance of dynamic changes in promiscuous activity and of the latent pathways for metabolic evolution. - Atsushi Takabayashi, Ryosuke Kadoya, Masayoshi Kuwano, Katsunori Kurihara, Hisashi Ito, Ryouichi Tanaka, Ayumi TanakaSPRINGERPLUS 2 (1) 148 2193-1801 2013 [Refereed][Not invited]
Protein-protein interactions are critical for most cellular processes; however, many remain to be identified. Here, to comprehensively identify protein complexes in photosynthetic organisms, we applied the recently developed approach of blue native PAGE (BN-PAGE) coupled with LC-MS/MS to the thylakoid proteins of Arabidopsis thaliana and the whole cell proteins of whole cell proteins of Synechocystis sp. PCC 6803. We identified 245 proteins from the purified Arabidopsis thylakoid membranes and 1,458 proteins from the whole cells of Synechocystis using the method. Next, we generated protein migration profiles that were assessed by plotting the label-free estimations of protein abundances versus migration distance in BN-PAGE. Comparisons between the migration profiles of the major photosynthetic complexes and their band patterns showed that the protein migration profiles were well correlated. Thus, the protein migration profiles allowed us to estimate the molecular size of each protein complex and to identify co-migrated proteins with the proteins of interest by determining the protein pairs that contained peaks in the same gel slice. Finally, we built the protein co-migration database for photosynthetic organisms (PCoM-DB: http://pcomdb.lowtem.hokudai.ac.jp/proteins/top) to make our data publicly accessible online, which stores the analyzed data with a user-friendly interface to compare the migration profiles of proteins of interest. It helps users to find unidentified protein complexes in Arabidopsis thylakoids and Synechocystis cells. The accumulation of the data from the BN-PAGE coupled with LC-MS/MS should reveal unidentified protein complexes and should aid in understanding the adaptation and the evolution of photosynthetic organisms. - Yousuke Shimoda, Hisashi Ito, Ayumi TanakaPLANT JOURNAL 72 (3) 501 - 511 0960-7412 2012/11 [Refereed][Not invited]
Chlorophyll is a deleterious molecule that generates reactive oxygen species and must be converted to non-toxic molecules during plant senescence. The degradation pathway of chlorophyll a has been determined; however, that of chlorophyll b is poorly understood, and multiple pathways of chlorophyll b degradation have been proposed. In this study, we found that chlorophyll b is degraded by a single pathway, and elucidated the importance of this pathway in avoiding cell death. In order to determine the chlorophyll degradation pathway, we first examined the substrate specificity of 7-hydroxymethyl chlorophyll a reductase. 7-hydroxymethyl chlorophyll a reductase reduces 7-hydroxymethyl chlorophyll a but not 7-hydroxymethyl pheophytin a or 7-hydroxymethyl pheophorbide a. These results indicate that the first step of chlorophyll b degradation is its conversion to 7-hydroxymethyl chlorophyll a by chlorophyll b reductase, although chlorophyll b reductase has broad substrate specificity. In vitro experiments showed that chlorophyll b reductase converted all of the chlorophyll b in the light-harvesting chlorophyll a/b protein complex to 7-hydroxymethyl chlorophyll a, but did not completely convert chlorophyll b in the core antenna complexes. When plants whose core antennae contained chlorophyll b were incubated in the dark, chlorophyll b was not properly degraded, and the accumulation of 7-hydroxymethyl pheophorbide a and pheophorbide b resulted in cell death. This result indicates that chlorophyll b is not properly degraded when it exists in core antenna complexes. Based on these results, we discuss the importance of the proper degradation of chlorophyll b. - Saori Nakajima, Hisashi Ito, Ryouichi Tanaka, Ayumi TanakaPLANT PHYSIOLOGY 160 (1) 261 - 273 0032-0889 2012/09 [Refereed][Not invited]
Although seeds are a sink organ, chlorophyll synthesis and degradation occurs during embryogenesis and in a manner similar to that observed in photosynthetic leaves. Some mutants retain chlorophyll after seed maturation, and they are disturbed in seed storability. To elucidate the effects of chlorophyll retention on the seed storability of Arabidopsis (Arabidopsis thaliana), we examined the non-yellow coloring1 (nyc1)/nyc1-like (nol) mutants that do not degrade chlorophyll properly. Approximately 10 times more chlorophyll was retained in the dry seeds of the nyc1/nol mutant than in the wild-type seeds. The germination rates rapidly decreased during storage, with most of the mutant seeds failing to germinate after storage for 23 months, whereas 75% of the wild-type seeds germinated after 42 months. These results indicate that chlorophyll retention in the seeds affects seed longevity. Electron microscopic studies indicated that many small oil bodies appeared in the embryonic cotyledons of the nyc1/nol mutant; this finding indicates that the retention of chlorophyll affects the development of organelles in embryonic cells. A sequence analysis of the NYC1 promoter identified a potential abscisic acid (ABA)-responsive element. An electrophoretic mobility shift assay confirmed the binding of an ABA-responsive transcriptional factor to the NYC1 promoter DNA fragment, thus suggesting that NYC1 expression is regulated by ABA. Furthermore, NYC1 expression was repressed in the ABA-insensitive mutants during embryogenesis. These data indicate that chlorophyll degradation is induced by ABA during seed maturation to produce storable seeds. - Tomo T, Kusakabe H, Nagao R, Ito H, Tanaka A, Akimoto S, Mimuro M, Okazaki SBiochimica et biophysica acta 8 1817 1299 - 1305 0006-3002 2012/08 [Refereed][Not invited]
- Tatsuya Tomo, Hayato Kusakabe, Ryo Nagao, Hisashi Ito, Ayumi Tanaka, Seiji Akimoto, Mamoru Mimuro, Shigetoshi OkazakiBIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1817 (8) 1299 - 1305 0005-2728 2012/08 [Refereed][Not invited]
The luminescence spectrum of singlet oxygen produced upon excitation at 674 nm in the photochemically active photosystem II (PS II) complexes isolated from cyanobacterium Synechocystis sp. PCC 6803 containing different types of chlorophyll, i.e., monovinyl (wild-type) or divinyl (genetically modified) chlorophyll a. The yield of singlet oxygen, estimated using methylene blue as the standard, from the divinyl-chlorophyll PS II complex was more than five times greater than that from the monovinyl-chlorophyll PS II complex. These results are consistent with the observed difference in the sensitivity towards high intensity of light between the two cyanobacterial strains. The yield of singlet oxygen appeared to increase with the level of triplet chlorophyll, in the divinyl-chlorophyll PS II complex. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial. (C) 2012 Elsevier B.V. All rights reserved. - 伊藤 寿, 田中 歩, 田中 亮一光合成研究 日本光合成学会 22 (2) 98 - 105 1884-2852 2012/08 [Not refereed][Not invited]
- Yokono M, Tomo T, Nagao R, Ito H, Tanaka A, Akimoto SBiochimica et biophysica acta 5 1817 754 - 759 0006-3002 2012/05 [Refereed][Not invited]
- Makio Yokono, Tatsuya Tomo, Ryo Nagao, Hisashi Ito, Ayumi Tanaka, Seiji AkimotoBIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1817 (5) 754 - 759 0005-2728 2012/05 [Refereed][Not invited]
The marine cyanobacterium Prochlorococcus marinus accumulates divinyl chlorophylls instead of monovinyl chlorophylls to harvest light energy. As well as this difference in its chromophore composition, some amino acid residues in its photosystem II D1 protein were different from the conserved amino acid residues in other photosynthetic organisms. We examined PSII complexes isolated from mutants of Synechocystis sp. PCC 6803, in which chromophore and D1 protein were altered (Hisashi Ito and Ayumi Tanaka, 2011) to clarify the effects of chromophores/D1 protein composition on the excitation energy distribution. We prepared the mutants accumulating divinyl chlorophyll (DV mutant). The amino acid residues of V205 and G282 in the D1 protein were substituted with M205 and C282 in the DV mutant to mimic Prochlorococcus D1 protein (DV-V205M/G282C mutant). Isolated PSII complexes were analyzed by time-resolved fluorescence spectroscopy. Energy transfer in CP47 was interrupted in PSII containing divinyl chlorophylls. The V205M/G282C mutation did not recover the energy transfer pathway in CP47, instead, the mutation allowed the excitation energy transfer from CP43 to CP47, which neighbors in the PSII dimer. Mutual orientation of the subcomplexes of PSII might be affected by the substitution. The changes of the energy transfer pathways would reduce energy transfer from antennae to the PSII reaction center, and allow Prochlorococcus to acquire light tolerance. (C) 2012 Elsevier B.V. All rights reserved. - Tanaka, R., Takabayashi, A., Ito, H., Tanaka, A.Handbook of Porphyrin Science: With Applications to Chemistry, Physics, Materials Science, Engineering, Biology and Medicine: Volume 16 - 20: Synthetic Developments: Part I 16-20 2012
- Hisashi Ito, Ayumi TanakaPROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 108 (44) 18014 - 18019 0027-8424 2011/11 [Refereed][Not invited]
Acquisition of new photosynthetic pigments has been a crucial process for the evolution of photosynthesis and photosynthetic organisms. In this process, pigment-binding proteins must evolve to fit new pigments. Prochlorococcus is a unique photosynthetic organism that uses divinyl chlorophyll (DVChl) instead of monovinyl chlorophyll. However, cyanobacterial mutants that accumulate DVChl immediately die even under medium-light conditions, suggesting that chlorophyll (Chl)-binding proteins had to evolve to fit to DVChl concurrently with Prochlorococcus evolution. To elucidate the coevolutionary process of Chl and Chl-binding proteins during the establishment of DVChl-based photosystems, we first compared the amino acid sequences of Chl-binding proteins in Prochlorococcus with those in other photosynthetic organisms. Two amino acid residues of the D1 protein, V205 and G282, are conserved in monovinyl chlorophyll-based photosystems; however, in Prochlorococcus, they are substituted with M205 and C282, respectively. According to the solved photosystem II structure, these amino acids are not involved in Chl binding. To mimic Prochlorococcus, V205 was mutated to M205 in the D1 protein from Synechocystis sp. PCC6803 and Synechocystis dvr mutant was transformed with this construct. Although these transgenic cells could not grow under high-light conditions, they acquired light tolerance and grew under medium-light conditions, whereas untransformed dvr mutants could not survive. Substitution of G282 for C282 contributed additional light tolerance, suggesting that the amino acid substitutions in the D1 protein played an essential role in the development of DVChl-based photosystems. Here, we discuss the coevolution of a photosynthetic pigment and its binding protein. - Miki Meguro, Hisashi Ito, Atsushi Takabayashi, Ryouichi Tanaka, Ayumi TanakaPLANT CELL 23 (9) 3442 - 3453 1040-4651 2011/09 [Refereed][Not invited]
The interconversion of chlorophyll a and chlorophyll b, referred to as the chlorophyll cycle, plays a crucial role in the processes of greening, acclimation to light intensity, and senescence. The chlorophyll cycle consists of three reactions: the conversions of chlorophyll a to chlorophyll b by chlorophyllide a oxygenase, chlorophyll b to 7-hydroxymethyl chlorophyll a by chlorophyll b reductase, and 7-hydroxymethyl chlorophyll a to chlorophyll a by 7-hydroxymethyl chlorophyll a reductase. We identified 7-hydroxymethyl chlorophyll a reductase, which is the last remaining unidentified enzyme of the chlorophyll cycle, from Arabidopsis thaliana by genetic and biochemical methods. Recombinant 7-hydroxymethyl chlorophyll a reductase converted 7-hydroxymethyl chlorophyll a to chlorophyll a using ferredoxin. Both sequence and biochemical analyses showed that 7-hydroxymethyl chlorophyll a reductase contains flavin adenine dinucleotide and an iron-sulfur center. In addition, a phylogenetic analysis elucidated the evolution of 7-hydroxymethyl chlorophyll a reductase from divinyl chlorophyllide vinyl reductase. A mutant lacking 7-hydroxymethyl chlorophyll a reductase was found to accumulate 7-hydroxymethyl chlorophyll a and pheophorbide a. Furthermore, this accumulation of pheophorbide a in the mutant was rescued by the inactivation of the chlorophyll b reductase gene. The downregulation of pheophorbide a oxygenase activity is discussed in relation to 7-hydroxymethyl chlorophyll a accumulation. - Yukiko Horie, Hisashi Ito, Makoto Kusaba, Ryouichi Tanaka, Ayumi TanakaJOURNAL OF BIOLOGICAL CHEMISTRY 284 (26) 17449 - 17456 0021-9258 2009/06 [Refereed][Not invited]
The light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) is the most abundant membrane protein in green plants, and its degradation is a crucial process for the acclimation to high light conditions and for the recovery of nitrogen (N) and carbon (C) during senescence. However, the molecular mechanism of LHCII degradation is largely unknown. Here, we report that chlorophyll b reductase, which catalyzes the first step of chlorophyll b degradation, plays a central role in LHCII degradation. When the genes for chlorophyll b reductases NOL and NYC1 were disrupted in Arabidopsis thaliana, chlorophyll b and LHCII were not degraded during senescence, whereas other pigment complexes completely disappeared. When purified trimeric LHCII was incubated with recombinant chlorophyll b reductase (NOL), expressed in Escherichia coli, the chlorophyll b in LHCII was converted to 7-hydroxymethyl chlorophyll a. Accompanying this conversion, chlorophylls were released from LHCII apoproteins until all the chlorophyll molecules in LHCII dissociated from the complexes. Chlorophyll-depleted LHCII apoproteins did not dissociate into monomeric forms but remained in the trimeric form. Based on these results, we propose the novel hypothesis that chlorophyll b reductase catalyzes the initial step of LHCII degradation, and that trimeric LHCII is a substrate of LHCII degradation. - Tatsuya Tomo, Seiji Akimoto, Hisashi Ito, Tohru Tsuchiya, Michitaka Fukuya, Ayumi Tanaka, Mamoru MimuroBIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1787 (3) 191 - 200 0005-2728 2009/03 [Refereed][Not invited]
Chlorophyll (Chl) a in a cyanobacterium Synechocystis sp. PCC 6803 was replaced with di-vinyl (DV)-Chl a by knock-out of the specific gene (slr1923), responsible for the reduction of a 8-vinyl group, and optical and photochemical properties of purified photosystem (PS) II complexes DV-PS II) were investigated. We observed differences in the peak wavelengths of absorption and fluorescence spectra; however, replacement of Chl a with DV-Chl a had limited effects. On the contrary, photochemical reactions were highly sensitive to high-light treatments in the mutant. Specifically, DV-Chl a was rapidly bleached under high-light conditions, and we detected significant dissociation of complexes and degradation of D1 proteins (PsbA). By comparing the SDS-PAGE patterns observed in this study to those observed in spinach chloroplasts, this degradation is assigned to the acceptor-side photoinhibition. The delayed fluorescence in the nanosecond time region at 77 K was suppressed in DV-PS II, possibly increasing triplet formation of Chl molecules. Our findings provide insight into the evolutionary processes of cyanobacteria. The effects of pigment replacement on the optimization of reactions are discussed. (C) 2009 Elsevier B.V. All rights reserved. - Tomo T, Akimoto S, Ito H, Tsuchiya T, Fukuya M, Tanaka A, Mimuro MBiochimica et biophysica acta 1787 (3) 191 - 200 0006-3002 2009/03 [Refereed][Not invited]
- Hisashi Ito, Makio Yokono, Ryouichi Tanaka, Ayumi TanakaJOURNAL OF BIOLOGICAL CHEMISTRY 283 (14) 9002 - 9011 0021-9258 2008/04 [Refereed][Not invited]
The vast majority of oxygenic photosynthetic organisms use monovinyl chlorophyll for their photosynthetic reactions. For the biosynthesis of this type of chlorophyll, the reduction of the 8-vinyl group that is located on the B-ring of the macrocycle is essential. Previously, we identified the gene encoding 8-vinyl reductase responsible for this reaction in higher plants and termed it DVR. Among the sequenced genomes of cyanobacteria, only several Synechococcus species contain DVR homologues. Therefore, it has been hypothesized that many other cyanobacteria producing monovinyl chlorophyll should contain a vinyl reductase that is unrelated to the higher plant DVR. To identify the cyanobacterial gene that is responsible for monovinyl chlorophyll synthesis, we developed a bioinformatics tool, correlation coefficient calculation tool, which calculates the correlation coefficient between the distributions of a certain phenotype and genes among a group of organisms. The program indicated that the distribution of a gene encoding a putative dehydrogenase protein is best correlated with the distribution of the DVR-less cyanobacteria. We subsequently knocked out the corresponding gene ( Slr1923) in Synechocystis sp. PCC6803 and characterized the mutant. The knock-out mutant lost its ability to synthesize monovinyl chlorophyll and accumulated 3,8-divinyl chlorophyll instead. We concluded that Slr1923 encodes the vinyl reductase or a subunit essential for monovinyl chlorophyll synthesis. The function and evolution of 8-vinyl reductase genes are discussed. - Makoto Kusaba, Hisashi Ito, Ryouhei Morita, Shuichi Iida, Yutaka Sato, Masaru Fujimoto, Shinji Kawasaki, Ryouichi Tanaka, Hirohiko Hirochika, Minoru Nishimura, Ayumi TanakaPLANT CELL 19 (4) 1362 - 1375 1040-4651 2007/04 [Refereed][Not invited]
Chlorophyll degradation is an aspect of leaf senescence, which is an active process to salvage nutrients from old tissues. non-yellow coloring1 (nyc1) is a rice (Oryza sativa) stay-green mutant in which chlorophyll degradation during senescence is impaired. Pigment analysis revealed that degradation of not only chlorophylls but also light-harvesting complex II (LHCII) bound carotenoids was repressed in nyc1, in which most LHCII isoforms were selectively retained during senescence. Ultrastructural analysis of nyc1 chloroplasts revealed that large and thick grana were present even in the late stage of senescence, suggesting that degradation of LHCII is required for the proper degeneration of thylakoid membranes. Map-based cloning of NYC1 revealed that it encodes a chloroplast-localized short-chain dehydrogenase/reductase (SDR) with three transmembrane domains. The predicted structure of the NYC1 protein and the phenotype of the nyc1 mutant suggest the possibility that NYC1 is a chlorophyll b reductase. Although we were unable to detect the chlorophyll b reductase activity of NYC1, NOL (for NYC1-like), a protein closely related to NYC1 in rice, showed chlorophyll b reductase activity in vitro. We suggest that NYC1 and NOL encode chlorophyll b reductases with divergent functions. Our data collectively suggest that the identified SDR protein NYC1 plays essential roles in the regulation of LHCII and thylakoid membrane degradation during senescence. - D. Aarti, R. Tanaka, H. Ito, A. TanakaPHOTOCHEMISTRY AND PHOTOBIOLOGY 83 (1) 171 - 176 0031-8655 2007/01 [Refereed][Not invited]
Using the vascular plant Cucumis sativus (cucumber) as a model, we studied the effects of high (intense and excess) light upon chlorophyll biosynthesis during de-etiolation. When illuminated with high light (1500-1600 mu E/m(2)/s), ctiolated cucumber cotyledons failed to synthesize chlorophyll entirely. However, upon transfer to low light conditions (40-45 mu E/m(2)/S), chlorophyll biosynthesis and subsequent accumulation resumed following an initial 2-12 h delay. Duration of high light treatment negatively correlated with chlorophyll biosynthetic activity. Specifically, we found that high light severely inhibited 5-aminolevulinic acid (ALA) synthesis. This effect partly could be because of the decrease in protein level of glutamyl-tRNA reductase (GluTR) observed. Protein level of glutamate-1-semialdehyde (GSA-AT) remained unchanged. It was also found that high light did not suppress HEMA I expression. Therefore, we speculated that this significant inhibition of ALA, synthesis might have occurred mainly because of concomitant inactivation of GluTR and/or inhibition of complex formation between GluTR and GSA-AT. Our further observation that both methyl viologen and rose bengal similarly inhibit ALA synthesis under low light conditions suggested that reactive oxygen species (ROS) could be responsible for the inhibition of ALA synthesis in cotyledons exposed to high light conditions. - 伊藤寿, 永田望, 田中歩月刊海洋 38 (6) 417 - 424 0916-2011 2006/06/01 [Not refereed][Not invited]
- Tanaka, A., Tanaka, R., Ito, H., Yoshida, K.Tanpakushitsu kakusan koso. Protein, nucleic acid, enzyme 41 (2) 0039-9450 1996
MISC
- 伊藤寿, 横野牧生, 田中亮一, 田中歩 日本植物生理学会年会要旨集 49th- 231 2008/03/15 [Not refereed][Not invited]
- 田中 歩, 田中 亮一, 伊藤 寿 蛋白質核酸酵素 41- (2) 136 -145 1996/02
Research Projects
- Japan Society for the Promotion of Science：Grants-in-Aid for Scientific ResearchDate (from‐to) : 2017/04 -2021/03Author : Ito HisashiOne of the central issues in this study is the elucidation of the relationship between chlorophyll degradation and jasmonic acid synthesis. It has been thought that plants senescence is proceeded through the action of plant hormones such as jasmonic acid, which results in the degradation of chlorophyll. However, this study showed that the degradation of chlorophyll leads to the synthesis of jasmonic acid, which induces the expression of senescence-related genes, i.e., accelerates senescence. This result suggests that chlorophyll degradation promotes senescence by plant hormones. This study demonstrates a new role for chlorophyll degradation in plants.
- Japan Society for the Promotion of Science：Grants-in-Aid for Scientific ResearchDate (from‐to) : 2015/04 -2018/03Author : TANAKA Ayumi, TAKABAYASHI ATSUSHI, ITO HISASHIDespite its importance, Mg-dechelatase that catalyzes the first step of chlorophyll degradation pathway was not identified. In this study, we found that Mg-dechelatase is encoded by SGR gene and the dechelating activity was successfully measured with the recombinant protein expressed in E. coli. We also found that substrate specificity differs between SGR1 and SGRL. By these studies, all major enzymes of the chlorophyll metabolic pathway were finally identified. Furthermore, it was revealed that SGR can catalyze not only free chlorophyll but also chlorophyll bound to protein, thus playing a central role in the degradation of chlorophyll-binding proteins and photosystems.