Chapter 7 Physiological Impact of Thioredoxin- and Glutaredoxin-Mediated Redox Regulation in Cyanobacteria

Physiological Impact of Thioredoxin- and Glutaredoxin-Mediated Redox Regulation in Cyanobacteria

YOSHITAKA NISHIYAMA* AND TORU HISABORI{,1

*Department of Biochemistry and Molecular Biology, Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan { Chemical Resource Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 226-8503, Japan

I. II. III. IV. V. VI.

Introduction: The Redox-Balancing System in Cyanobacteria . . . . . . . . . . . . . Synchronization Between Redox Equilibrium and Photosynthesis. . . . . . . . . Physiological Phenomena Controlled by Redox: Gene Expression. . . . . . . . . Physiological Phenomena Controlled by Redox: Protein Synthesis . . . . . . . . The Proteomic Approach Reveals a Variety of Trx Target Proteins . . . . . . . Perspectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

188 189 191 192 194 200 201 201

ABSTRACT Cyanobacteria are photosynthetic bacteria, which are thought to be derived from ancestral oxygen-evolving photosynthetic organisms. Recent progress in proteomics using redox-protein affinity chromatography, two-dimensional electrophoresis and mass spectrometry has improved our understanding of the complicated redox-regulation networks that exist in photosynthetic organisms, and studies with cyanobacteria have

1

Corresponding author: Email: [email protected]

Advances in Botanical Research, Vol. 52 Copyright 2009, Elsevier Ltd. All rights reserved.

0065-2296/09 $35.00 DOI: 10.1016/S0065-2296(09)52007-1

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made a great contribution to this area. Moreover, a number of remarkable differences relating to redox-regulated proteins between higher plants and cyanobacteria have also been uncovered as a result of these studies. In addition, novel redox-regulation systems that govern gene expression and protein synthesis have also been recently described for cyanobacteria. The redox-regulation system is an important multiphasic control system that ensures cell viability for this photosynthetic organism.

I. INTRODUCTION: THE REDOX-BALANCING SYSTEM IN CYANOBACTERIA The architecture of the photosynthetic apparatus suggests that the ancestral oxygen-evolving photosynthetic organisms were closely related to extant cyanobacteria. Photosynthetic organisms have evolved the capability to use water as an electron donor for photosynthesis, leading to the synthesis of molecular oxygen. Over one billion years, oxygen concentrations in the earth’s atmosphere increased, enabling the evolution of a new metabolic pathway, respiration. The organisms that were subjected to these novel atmospheric conditions rapidly evolved a set of new defence systems which allowed protection against oxidative damage caused by reactive oxygen species (ROS), a by-product of the presence of molecular oxygen within the cell. Two major defence systems that these ancestral organisms evolved are the redox-balancing system, which involves proteins such as thioredoxin (Trx) and glutaredoxin (Grx), and the anti-oxidative stress system, which involves proteins such as catalases, superoxide dismutases, peroxiredoxins and peroxidases. Moreover, the cellular redox-balancing system has become an efficient system which controls the metabolic activity of photosynthetic organisms allowing them to adapt to the alternating light–dark conditions. Trx, the key player of the redox-balancing system, is a ubiquitous protein which is found across an extremely wide range of living organisms. Genomic analysis of various organisms indicates the presence of a strikingly wide variety of isoforms of Trx and Grx proteins. Even in the cyanobacterium Synechocystis sp. PCC 6803, whose genome size is only 3.57 Mb, four Trx isoform genes are present (Kaneko et al., 1996a,b), and these genes are all expressed in physiological growth conditions (Florencio et al., 2006; Hishiya et al., 2008). These findings suggest that Trx is a critical protein, especially for photosynthetic organisms. In this review, we describe the significance of the redox-balancing system in cyanobacterial cells and discuss the newly identified functions of the redox-balancing system in sustaining cyanobacterial cell viability.

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II. SYNCHRONIZATION BETWEEN REDOX EQUILIBRIUM AND PHOTOSYNTHESIS The significance of the redox-balancing system in photosynthetic organisms is very often emphasized by the relevance between the reducing equivalents that are generated by the electron transfer reactions and the reduction levels of redox-sensitive chloroplast enzymes that are important for the carbon assimilation pathway. This relation is already defined for the thiol enzymes in chloroplasts, which work in ATP synthesis (Mills and Mitchell, 1982) and the Calvin–Benson cycle (Buchanan, 1991). The thiol enzymes involved in these systems are directly reduced and activated by Trx in chloroplasts in order to promote efficient photosynthesis. The redox-regulation system therefore seems to be a reasonable switching system for photosynthetic organisms to maintain the efficiency of metabolic pathways under light conditions and to avoid futile reverse reactions under dark conditions (Schurmann, 2003). When the Synechocystis sp. PCC 6803 genome was fully sequenced (Kaneko et al., 1996a,b), the existence of four genes for Trx (sll1057, slr0233, slr0623 and slr1139) and three genes for Trx-like proteins (sll0685, sll1980 and slr1796) was revealed. In addition, the genes for two Trx reductases, ferredoxin-Trx reductase (FTR, the heterodimer consisting of the gene products of sll0554 and ssr0330) and NADPH-Trx reductase (NTR, the gene product of slr0600), were also identified. These two reduction pathways for Trx clearly perform different functions: disruption of the ntr gene results in cells which are very sensitive to oxidative stress, whereas disruption of the ftr-v gene caused slow cell growth but did not affect cell viability under oxidative stress conditions (Hishiya et al., 2008). The reduction levels of Trx isoforms were also shown to vary within the gene disruptants described earlier. In addition, these gene disruptions strongly affected the expression levels of other redox-balancing system proteins, such as Trx and other Trx reductases, suggesting that the redox level in the cyanobacterial cells is an important determinant in the regulation of the expression of certain specific proteins, as mentioned elsewhere in this review. The identification of novel proteins capable of interacting with Trx isoforms in photosynthetic organisms has been significantly enhanced by way of proteomic approaches, such as two-dimensional gel electrophoresis analysis (Yano et al., 2001) and Trx-affinity chromatography (Balmer et al., 2003; Motohashi et al., 2001; Yamazaki et al., 2004). More than 100 proteins in higher plant chloroplasts are currently reported to be potential Trx-interacting partners. Although many of the proteins in the list have not actually been confirmed biochemically as Trx target proteins (Hisabori et al., 2007),

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the data accumulated certainly enhance our understanding of the redoxbalancing system in chloroplasts and in cells of photosynthetic organisms (Montrichard et al., 2009). Another well-studied key protein in the redox-balancing system is Grx. Grx is related to Trx in terms of its three-dimensional structure, also possessing the so-called Trx fold, and the cysteine residues that form a similar active site. In Synechocystis sp. PCC 6803, two Grx genes, slr1562 and ssr2061, have been identified (Kaneko et al., 1996a,b): both proteins contain the conserved CPFC motif, a nearly classical active site sequence for Grx proteins. However, based on the growth phenotype of their disruptants, both proteins appear to be dispensable for cell growth under normal growth conditions (Marteyn et al., 2009). In particular, the grx1 gene disruptant (slr1562) showed no difference in growth compared to the wild type. In contrast, the grx2 (ssr2061) disruptant did show some sensitivity to oxidative stress. Protein– protein interaction experiments have shown that the redox exchange reaction between Grx1 and Grx2 is observed in vivo as well as in vitro (Marteyn et al., 2009). Furthermore, Grx1 seems to be reduced by the NTR system, suggesting the existence of crosstalk between the Trx system and the Grx system in cyanobacterial cells. Currently, the function of Grx is believed to be very diverse in various organisms, ranging from activation of ribonucleotide reductase (Holmgren, 1976), reduction of dehydroascorbate (Wells et al., 1990), regulation of transcription factors, to protection of cells against apoptosis and cellular defence system for ROS (Holmgren, 2000). Grx is also involved in reversible protein inactivation by glutathionylation of the cysteine residues of the target proteins (Rouhier et al., 2008). Grx target proteins in Synechocystis sp. PCC 6803 were recently extensively surveyed by a proteomic approach, using monothiol Grx2 mutant (Li et al., 2007), and these results can be compared to a similar study carried out with a Grx from poplar, a land plant (Rouhier et al., 2005). In Synechocystis, 42 proteins were captured as potential target proteins, including anti-oxidative stress proteins, such as catalase and peroxiredoxin; several Calvin cycle enzymes, which are well known as thiol enzymes like phosphoribulokinase, glyceraldehyde 3-phosphate dehydrogenase and fructose 1,6-bisphosphatase; molecular chaperone; and elongation factor Tu. Of these suggested Grx target proteins, 13 proteins were included in the list of proteins captured by affinity chromatography using monothiol Trx in Synechocystis cells. Critical information relating to the existence of conserved cysteines in the molecule must be evaluated in order to discuss the overlap of the target proteins for Grx and Trx. As discussed in this review, there are certainly remarkable differences in the conserved cysteines among cyanobacteria, green algae and higher plants. For the comprehensive evaluation of these newly proposed redox pathways,

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individual biochemical experiments concerning the proteins listed as Trx or Grx targets are essential in order to further define the redox network system in the cells of photosynthetic organisms.

III. PHYSIOLOGICAL PHENOMENA CONTROLLED BY REDOX: GENE EXPRESSION As mentioned earlier, light is an important signal that controls the synthesis of proteins in photosynthetic organisms. Indeed, synthesis of various proteins involved in photosynthesis is induced in light conditions. The lightdependent synthesis of proteins is regulated at several steps that include transcription, post-transcriptional modification of mRNA (e.g., RNA editing, splicing, processing), translation and post-translational modification (e.g., phosphorylation, formation of disulfide bonds). Primarily, light absorbed by chlorophylls is used to drive electron transport in the photosynthetic machinery located in and around the thylakoid membrane. Thus, the majority of light signals are converted to redox signals in photosynthetic organisms. However, limited information is available regarding the mechanisms of redox regulation of gene expression and protein synthesis. Initially the redox state of plastoquinone, an electron carrier that connects photosystem II (PSII) and the cytochrome b6/f complex, was proposed to be important for the transcriptional regulation of photosynthesis-related genes in plants and algae (reviewed in Allen and Pfannschmidt, 2000; Li et al., 2008; Oelze et al., 2008; Pfannschmidt, 2003). In fact, the redox state of the plastoquinone pool regulates the transcription of the genes of the lightharvesting complex proteins in unicellular green algae Dunaliella tertiolecta and Chlamydomonas reinhardtii (Chen et al., 2004; Durnford and Falkowski, 1997; Durnford et al., 2003; Escoubas et al., 1995) and the psbA gene for the D1 protein, a reaction centre protein of PSII, and psaAB genes for reaction centre proteins of photosystem I (PSI) in mustard (Pfannschmidt et al., 1999). In contrast, studies with barley and transgenic tobacco plants have demonstrated that the redox state of the plastoquinone pool is not involved in the expression of genes for the light-harvesting proteins or for photoacclimative responses (Anderson et al., 1997; Montane et al., 1998). Cyanobacteria constitute a suitable photosynthetic organism in which to thoroughly clarify the role of the redox signal. Recent DNA microarray analysis using inhibitors of the photosynthetic electron transport has revealed that, in Synechocystis sp. PCC 6803, the redox state of components located downstream of plastoquinone is more critical for transcriptional regulation than that of the plastoquinone pool (Hihara et al., 2003).

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DNA microarray analysis in Arabidopsis also suggests that the redox state of the components on the acceptor side of PSI is important for the lightdependent regulation of the expression of nuclear-encoded genes (Piippo et al., 2006). In Synechocystis sp. PCC 6803, a small LuxR-type regulator, PedR, has been identified as a component that is regulated by the redox state of photosynthetic electron transport (Nakamura and Hihara, 2006). This transcription factor activates the expression of chlL, chlN, chlB and slr1957 and represses the expression of ndhD2, rpe and the pedR-sll0296 operon, when the activity of the photosynthetic electron transport is low. Under high light conditions, the supply of reducing equivalents increases as a result of the stimulation of the photosynthetic electron transport, and PedR is transiently inactivated with a concomitant conformational change. Thus, redox regulation of PedR enables transient activation or repression of the target genes in response to rapid changes in light conditions. Recent in vitro and in vivo studies of PedR have revealed that its activity is regulated by Trx (Y. Hihara, personal communication). Thus, Trx-mediated redox signals must be critical for the regulation of the light-dependent gene expression in cyanobacteria.

IV. PHYSIOLOGICAL PHENOMENA CONTROLLED BY REDOX: PROTEIN SYNTHESIS The most striking example of proteins whose synthesis in photosynthetic organisms is controlled by redox conditions is the D1 protein of PSII. The synthesis of the D1 protein is markedly induced during the shift from dark to light conditions in chloroplasts and cyanobacteria. The light-dependent synthesis of the D1 protein occurs at the translational level in chloroplasts of plants (Klein and Mullet, 1987; Zhang et al., 2002) and algae (Trebitsh and Danon, 2001), while it occurs at both the transcriptional and the translational levels in cyanobacteria (Tyystjarvi et al., 2001, 2004). In C. reinhardtii, the translational initiation of psbA mRNA for the D1 protein is regulated in a redox-dependent manner (reviewed in MarinNavarro et al., 2007). Under light conditions, a complex of four proteins, namely, RB47, RB38, RB60 and RB55, is bound to the 50 untranslated region (50 UTR) of the psbA mRNA where they promote translational initiation (Kim and Mayfield, 1997). Within the protein complex, RB47 and RB60 are redox-active proteins: RB47 interacts with the 50 UTR of psbA mRNA under reducing conditions, while RB60, a protein disulfide isomerase, regulates the redox state of RB47 (Alergand et al., 2006; Kim and Mayfield, 1997). Although it has been suggested that RB60 receives reducing equivalents from

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PSI in a redox pathway via ferredoxin and Trx (Trebitsh et al., 2000), whether RB60 is directly reduced by Trx in vitro and in vivo remains to be clarified. In contrast, light-dependent synthesis of the D1 protein is mainly regulated at the elongation step of translation in higher plants (Edhofer et al., 1998; Kim et al., 1991). The translational elongation of the product of psbA mRNA with its concomitant insertion into the PSII complex is activated by reducing equivalents from PSI (Kuroda et al., 1996; Zhang et al., 2000) or by a proton gradient across the thylakoid membrane, which is generated by photosynthetic electron transport (Muhlbauer and Eichacker, 1998). Binding of some trans-acting factors to the 50 UTR of psbA mRNA in a redox-dependent manner in vitro in Arabidopsis was reported (Shen et al., 2001), suggesting that translation might also be regulated at the initiation step as well as at the elongation step. However, detailed mechanisms of the redox-dependent regulation of translation in chloroplasts remain to be elucidated. Recently, remarkable progress has been made towards understanding the mechanisms of redox regulation of protein synthesis in cyanobacteria. Oxidative stress enhances PSII photoinhibition by inhibiting the repair of PSII (Nishiyama et al., 2001, 2004; reviewed in Nishiyama et al., 2006). This inhibition is initially induced due to ROS-induced suppression of the synthesis of proteins required for repair, such as the D1 protein, at elongation step of translation (Nishiyama et al., 2001, 2004). A study with an in vitro translation system, derived from Synechocystis sp. PCC 6803, has revealed that elongation factor G (EF-G), a key protein for translational elongation, is a primary target of ROS-induced inactivation within the translational machinery (Kojima et al., 2007). Inactivation of EF-G has been shown to be attributable to the oxidation of two specific cysteine residues and formation of a disulfide bond (Kojima et al., 2009). The disulfide bond in the oxidized EF-G is reduced by Trx and the resulting reduced form of EF-G regains its activity to mediate translation in vitro (Kojima et al., 2009). Furthermore, the reduction and the subsequent activation of EF-G by Trx have been observed in vivo (Kojima et al., 2009). This phenomenon might also explain aspects of the light-dependent control of translation. Activation of the synthesis of the D1 protein requires reducing equivalents derived from PSI in Synechocystis sp. PCC 6803 (Allakhverdiev et al., 2005) as well as in plants (Kuroda et al., 1996; Zhang et al., 2000). Thus, it is likely that the translational machinery is regulated by the redox state of EF-G, which is oxidized by ROS and reduced by reducing equivalents that are generated by the photosynthetic electron transport and mediated by Trx. Figure 1 depicts the control of D1 translation by EF-G itself reduced by the Trx system. Overexpression of EF-G in cyanobacteria resulted in the enhancement of not only the synthesis of the D1 protein, but also that of almost all proteins on

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EF-G

SH

e– Trx

SH

NTR

Activation Ribosome EF-G

FTR

SH

NADPH

e–

SH e–

mRNA Fd

Thylakoid membrane

PSII

Nascent D1

PSI

Fig. 1. A model for the mechanism of the redox-regulation of the translational machinery in cyanobacteria. Reducing equivalents that are generated at PSI as a result of the photosynthetic electron transport are transmitted to EF-G in a Trx-mediated redox pathway. The resultant reduction of EF-G activates the lightdependent synthesis of proteins, such as the D1 protein. Thus, the photosynthetic machinery and the translational machinery are interconnected via redox regulation. Black arrows indicate the pathways of reducing equivalents. Fd, ferredoxin.

thylakoid membranes under photo-oxidative conditions (Kojima et al., 2007). This observation suggests that many of the light-induced proteins might be regulated by redox signals that are mediated by Trx and EF-G in cyanobacteria. EF-G proteins of Synechocystis sp. PCC 6803 (Lindahl and Florencio, 2003) and spinach chloroplasts (Balmer et al., 2003) have been captured by Trxaffinity column as potential targets of Trx. Since specific cysteine residues that are targets of Trx are conserved in EF-G proteins of cyanobacteria and plant chloroplasts (Kojima et al., 2009), it is possible that the translational machinery in chloroplasts might also be regulated by Trx-mediated redox signals.

V. THE PROTEOMIC APPROACH REVEALS A VARIETY OF Trx TARGET PROTEINS As mentioned earlier, the target proteins of Trx and its related proteins have been largely revealed by the proteomic approach carried out within this decade. Presently, approximately 500 proteins are described as potential

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targets for the cytosolic, mitochondrial and chloroplast Trx proteins in higher plants (Balmer et al., 2003, 2004a,b, 2006; Marchand et al., 2004; Motohashi et al., 2001; Yamazaki et al., 2004). In cyanobacteria, the Trx target proteins have been mainly studied in Synechocystis sp. PCC 6803, and 82 proteins in total have already been assigned as target proteins, including membrane-integrated and soluble proteins (Florencio et al., 2006; HosoyaMatsuda et al., 2005; Lindahl and Florencio, 2003; Perez-Perez et al., 2006). Trx targets have also been studied in the green algae C. reinhardtii (Lemaire et al., 2004), and the anaerobic photosynthetic bacteria Chlorobaculum tepidum (Hosoya-Matsuda et al., 2009). Recently, Montrichard et al. described all the Trx target proteins that have been reported to date in photosynthetic organisms (Montrichard et al., 2009). The comparison of the target proteins from higher plants, green algae and cyanobacteria in the article indicates that 60% of the cyanobacterial target proteins of the list are unique to this organism. In order to obtain the global picture of conserved cysteines present in these unique target protein candidates in cyanobacteria, the cysteine residues in each of the listed proteins have been checked. To this end, we have categorized the conserved cysteines as ‘cyano-Cys’, which are only observed in the target proteins and their homologs in four cyanobacteria, Synechocystis sp. PCC 6803, Anabaena sp. PCC 7120, Thermosynechococcus elongatus BP-1 and Gloeobacter violaceus PCC 7421, and also as ‘global-Cys’, which is observed in the target proteins and their homologs in these four cyanobacteria, plus the higher plant Arabidopsis thaliana. When given Cys residues are only observed in these cyanobacteria and in the target proteins in C. reinhardtii, these were categorized as ‘unicell-Cys’. Although the number of the analyzed proteins is limited in our analysis, we found that 18 unique target proteins observed only in cyanobacteria possess one or multiple ‘cyano-Cys’ residues, and 14 proteins did not have ‘global-Cys’ (Table I). In total, the ratios of the ‘cyano-Cys’ in the conserved cysteines are 71% for the target proteins unique in cyanobacteria, and 30% for the target proteins observed among cyanobacteria, green algae and higher plants. These results suggest that the potential target disulfide for Trx on the target proteins unique in cyanobacteria may be mainly formed by the ‘cyano-Cys’ residues. In contrast, phosphoglucomutase (only one Cys in the molecule) and sugarnucleotide epimerase (seven Cys in the molecule) do not have ‘cyano-Cys’, although they were observed as Trx targets in Synechocystis, suggesting that the reported interaction between Trx and these proteins might be unique in Synechocystis, or simply due to non-specific interaction. In addition to these specific Cys residues in cyanobacterial enzymes linked to redox regulation, there has been high interest concerning the insertion of peptide sequences containing redox-sensitive Cys in eukaryotic

TABLE I Thioredoxin target proteins revealed by proteomics studies Categorya (1)

Trx target proteins

Synechocystis sp. PCC6803

Cyano-Cys

Acetolactate synthase, small subunit Aspartyl-tRNA synthetase Carboxysomal protein

sll0065

sll1031

217, 279

ClpB1 ClpC

slr1641 sll0535

Ferredoxin sulfite reductase

slr0963

312 10, 13, 32, 35, 39, 417 569

GDP-mannose dehydratase Glucan branching enzyme Glycogen phosphorylase

sll1212

184

sll0158

75, 353, 657, 682, 740 85, 309, 832 (slr1367) 127, 169, 400 28 365 25, 293

Glycogen synthase 2 Heme oxygenase 1 Lysyl-tRNA synthetase Oxyanion-translocating ATPase, ArsA PAPS sulfotransferase

Global-Cys

81

sll0945 sll1184 slr1550 sll0086 slr1791

References Mata-Cabana et al. (2007)

slr1720

sll1356

Unicell-Cys

6, 222, 568

Mata-Cabana et al. (2007) Mata-Cabana et al. (2007), Lindahl and Florencio (2003) Mata-Cabana et al. (2007) Mata-Cabana et al. (2007)

139, 451, 497, 501

Mata-Cabana et al. (2007), Lindahl and Florencio (2003) Lindahl and Florencio (2003) Lindahl and Florencio (2003)

144, 782

Perez-Perez et al. (2006)

360

Lindahl and Florencio (2003) Mata-Cabana et al. (2007) Perez-Perez et al. (2006) Mata-Cabana et al. (2007)

230

Schmidt and Christen (1978)

Phosphoglucomutase Photosystem I protein PsaD Polyribonucleotide nucleotidyltransferase Porphobilinogen synthase RNA polymerase, subunit  RNA polymerase subunit RNA polymerase subunit 0 Serine-O-acetyl transferase Sugar-nucleotide epimerase Sulfate adenylyltransferase Universal stress proteinfamily, Usp1 Valyl-tRNA synthetase

sll0726 slr0737

None

sll1043 sll1994

65

Lindahl and Florencio (2003) Mata-Cabana et al. (2007)

455

Perez-Perez et al. (2006)

119, 121, 129

Lindahl and Florencio (2003)

sll1818

8, 261

Mata-Cabana et al. (2007)

sll1787

413, 620

Lindahl and Florencio (2003)

sll1789 slr1348

214, 286, 293, 296, 652, 999 28, 234

Lindahl and Florencio (2003) Mata-Cabana et al. (2007)

sll0576

None

Lindahl and Florencio (2003)

slr1165

213, 243, 285, 332

Lindahl and Florencio (2003)

slr0244

215, 227

87, 274

Mata-Cabana et al. (2007)

slr0557

40, 179, 272, 404

671

Lindahl and Florencio (2003)

(2)

Argininosuccinate synthetase

slr0585

20

120

Lindahl and Florencio (2003)

(3)

ADP-glucose pyrophosphorylase Argininosuccinate lyase Carbonic anhydrase, type FtsH

slr1176

55, 325, 330

Lindahl and Florencio (2003)

slr1133 slr1347 sll1463

274 76, 138

347

Perez-Perez et al. (2006) Mata-Cabana et al. (2007)

268

Mata-Cabana et al. (2007) (continues)

TABLE I Categorya

Trx target proteins Glyceraldehyde 3-phosphate dehydrogenase 2 Isocitrate dehydrogenase (NADP) 1-Cys peroxiredoxin

(4)

Synechocystis sp. PCC6803 sll1342

(continued )

Cyano-Cys 75

slr1289 slr1198

167

Peroxiredoxin II (YLR109-homolog)

sll1621

Phosphoglycerate kinase Pyruvate dehydrogenase component E1, subunit  Pyruvate dehydrogenase component E1, subunit Transketolase

slr0394 slr1934

ATP synthase, subunit  ATP synthase, subunit Chaperonin 1 60 kDa GroEL

sll1326 slr1329 slr2076

344, 519

Elongation factor G

slr1463

105, 388, 547Vb

Global-Cys

References

19, 154, 158

Mata-Cabana et al. (2007), Perez-Perez et al. (2006)

131

Papen et al. (1983)

45

Mata-Cabana et al. (2007), Lindahl and Florencio (2003), , Hosoya-Matsuda et al. (2005) Lindahl and Florencio (2003), Hosoya-Matsuda et al. (2005) Perez-Perez et al. (2006) Mata-Cabana et al. (2007)

56, 80 97, 216 39, 163, 178

sll1721 sll1070

Unicell-Cys

313

Mata-Cabana et al. (2007) 155, 368, 423

570

Perez-Perez et al. (2006)

194 53

Mata-Cabana et al. (2007) Mata-Cabana et al. (2007) Mata-Cabana et al. (2007), Lindahl and Florencio (2003) Lindahl and Florencio (2003)

257Gb

Elongation factor Tu

sll1099

Fructose 1,6bisphosphate aldolase, class II Glutamate synthase GOGAT (Fdx)

sll0018

91, 145, 316

sll1499

1247

GST Type 2 NADH dehydrogenase, NdbC Phosphoribulokinase

sll1545 sll1484

None 146

RubisCO large subunit

slr0009

sll1525

82

Mata-Cabana et al. (2007), Lindahl and Florencio (2003) Perez-Perez et al. (2006)

27, 53, 60, 192, 500, 673, 681, 691, 897, 1163, 1169, 1174, 1390, 1427, 1428

Lindahl and Florencio (2003)

Mata-Cabana et al. (2007) Mata-Cabana et al. (2007) 19, 41, 229, 235

312

79, 167, 187, 242, 279, 422, 454

Mata-Cabana et al. (2007), Perez-Perez et al. (2006) Mata-Cabana et al. (2007), Lindahl and Florencio (2003)

a Category: (1) Trx target proteins observed only in cyanobacterium Synechocystis sp. PCC 6803; (2) Trx target proteins observed in both cyanobacterium Synechocystis sp. PCC 6803 and Chlamydomonas reinhardtii; (3) Trx target proteins observed in both cyanobacterium Synechocystis sp. PCC 6803 and Arabidopsis thaliana; (4) Trx target proteins observed in these three organisms. b Conserved cysteine in three cyanobacteria but not in Synechocystis PCC6803 was shown as the original amino acid.

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Y. NISHIYAMA AND T. HISABORI

photosynthetic enzymes. There is a large variety of Cys insertion motives in redox-regulated enzymes, and they were summarized by Jacquot et al. (1997). So far, there is no general rule as to how the Cys insertion links the change in properties of redox-sensitive enzymes compared to redox-insensitive enzymes. Thus, biochemical analysis of the proteins listed from the proteomic studies will be absolutely required in order to draw definitive conclusions on the interaction with Trx, the critical disulfide bond as a target of Trx, the physiological significance of the suggested interaction and the molecular evolution of the redox-regulated enzymes.

VI. PERSPECTIVES Our knowledge concerning the Trx and Grx target proteins, and the glutathionylated proteins in the cells has dramatically increased during the past decade, and we now have to figure out the very complex picture of the redox-related protein networks, particularly in photosynthetic organisms. However, biochemical and physiological studies on the listed proteins are still required in order to determine whether the suggested interaction between the target proteins and Trx/Grx is of physiological significance. Particularly enigmatic are the mechanisms by which redox-balancing system proteins can detect the change in redox balance within cells. From the study of the cyanobacterial-disruptant strains of NTR and FTR proteins, the average reduction levels of Trx isoforms of these disruptants were found to undergo significant changes as compared to those of the wild-type cells (Hishiya et al., 2008). For instance, the reduction level of Trx-m in the ntr disruptant was decreased to about 25% of that of the wild-type cells, whereas that in the ftr mutant was about 65%. These results indicate that levels of the reduced form Trx-m in the ntr mutant cells decrease dramatically, although a portion of this protein is still reduced. Since certain amounts of the reducing equivalents seem to be required to maintain the function of the anti-oxidative stress system proteins, it is possible that the observed decrease of the reduced form Trx-m directly affects the cell viability under oxidative stress conditions. In contrast, the decrease of the reduced form of Trx-m was not so significant in the case of the ftr mutant, although the mutant showed an obvious delay in cell growth. This phenomenon could not be explained just by the redox regulation of protein synthesis, which is deeply linked to cell growth rate. A threshold of the redox level of the susceptible proteins, the ratio of reduced form within the whole Trx proteins and/or the amounts of reduced forms, may be present, constituting an important component in

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allowing them to exert their function. To evaluate the significance of this hypothetical threshold, the change of the redox level of the desired protein under the various conditions, for example, under high light, or in the dark, and also in the presence of various chemicals, must be examined. In addition, studies on the redox changes of the thiol enzymes in the ntr- and ftr-disruptant cells under various conditions that directly affect the redox balance in the cells will be useful to help understand the whole redox control network which operates within cyanobacterial cells.

ACKNOWLEDGMENTS This study is supported by the grants-in-aid for science research to T. H. (No. 17GS0316) from the Japan Society for the Promotion of Science.

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