Domain structure, oligomeric state, and mutational analysis of PpsR, the Rhodobacter sphaeroides repressor of photosystem gene expression

The transcription factor PpsR from the facultative photoheterotroph Rhodobacter sphaeroides is involved in repression of photosystem gene expression under aerobic growth conditions. We have isolated a number of spontaneous mutations as well as constructed directed mutations and deletions in ppsR. Repressor activities and the oligomeric state of the wild-type and mutant proteins were assayed. Our results suggest that the wild-type PpsR exists in cell extracts as a tetramer. Analysis of the PpsR mutants confirmed that the carboxy-terminal region of PpsR (residues 400 to 464) is involved in DNA binding. The central region of the protein (residues 150 to 400) was found to contain two PAS domains (residues 161 to 259 and 279 to 367). PAS domains are ubiquitous protein modules involved in sensory transduction as well as in protein-protein interactions. All spontaneously isolated mutations, which significantly impaired repressor activity and which mapped outside the DNA binding region, were positioned in the PAS domains. None of these, however, affected the overall oligomeric state. This implies that the conformation of the PAS domains within the tetramer is critical for repressor activity. Upstream of the first PAS domain resides a putative glutamine-rich hinge (residues 127 to 136) that connects the first PAS domain to the amino-terminal region (residues 1 to 135). The role of the amino terminus of PpsR is not obvious; however, extended deletions within this region abolish repressor activity, thus suggesting that the amino terminus is essential for structural integrity of the protein. We present a model of the domain architecture of the PpsR protein according to which PpsR is comprised of three regions: the carboxy terminus responsible for DNA binding, the central region primarily involved in protein oligomerization and possibly signal sensing, and the amino terminus of unknown function. This model may prove useful for determining the mode of PpsR action.

DNA binding of wild type RegA protein and its differential effect on the expression of pigment binding proteins in Rhodobacter capsulatus

The transcription of genes encoding pigment binding proteins in the facultative photosynthetic bacterium Rhodobacter capsulatus is regulated in response to oxygen partial pressure. Previous results identified RegA and RegB as members of a two component system involved in oxygen dependent synthesis of the photosynthetic apparatus. Here we demonstrate that RegA differentially controls the transcription of the puf and pucoperons which encode proteins of the LHI and LHII antenna complexes, respectively. In a regA mutant strain the level of puf specific mRNA reaches about 30% of the wild type levels and transcription is still responsive to oxygen tension. In contrast, the level of puc specific mRNA is very low and is no longer oxygen regulated. RegA binds to DNA sequences upstream of both the puf and puc operons, although with different affinities. We provide experimental evidence that a putative helix-turn-helix motif in the C-terminal region of RegA is responsible for its specific binding to the puf and puc promoter regions. In contrast to many other response regulators, the affinity of RegA for the target DNA is only slightly modified by phosphorylation.

Abscisic acid biosynthesis in tomato: regulation of zeaxanthin epoxidase and 9-cis-epoxycarotenoid dioxygenase mRNAs by light/dark cycles, water stress and abscisic acid

Two genes encoding enzymes in the abscisic acid (ABA) biosynthesis pathway, zeaxanthin epoxidase (ZEP) and 9-cis-epoxycarotenoid dioxygenase (NCED), have previously been cloned by transposon tagging in Nicotiana plumbaginifolia and maize respectively. We demonstrate that antisense down-regulation of the tomato gene LeZEP1 causes accumulation of zeaxanthin in leaves, suggesting that this gene also encodes ZEP. LeNCED1 is known to encode NCED from characterization of a null mutation (notabilis) in tomato. We have used LeZEP1 and LeNCED1 as probes to study gene expression in leaves and roots of whole plants given drought treatments, during light/dark cycles, and during dehydration of detached leaves. During drought stress, NCED mRNA increased in both leaves and roots, whereas ZEP mRNA increased in roots but not leaves. When detached leaves were dehydrated, NCED mRNA responded rapidly to small reductions in water content. Using a detached leaf system with ABA-deficient mutants and ABA feeding, we investigated the possibility that NCED mRNA is regulated by the end product of the pathway, ABA, but found no evidence that this is the case. We also describe strong diurnal expression patterns for both ZEP and NCED, with the two genes displaying distinctly different patterns. ZEP mRNA oscillated with a phase very similar to light-harvesting complex II (LHCII) mRNA, and oscillations continued in a 48 h dark period. NCED mRNA oscillated with a different phase and remained low during a 48 h dark period. Implications for regulation of water stress-induced ABA biosynthesis are discussed.

Disruption of the Arabidopsis photosystem I gene psaE1 affects photosynthesis and impairs growth

The psae1-1 mutant of Arabidopsis was identified on the basis of a decrease in the effective quantum yield of photosystem II, among a collection of plants subjected to transposon tagging with the Enhancer element. The steady-state redox level and the rate of re-oxidation of P700 are significantly altered in psae1-1 mutants. The responsible mutation was localised to psaE1, one of two Arabidopsis genes that encode subunit E of photosystem I. An additional mutant allele, psae1-2, was identified by reverse genetics. In wild-type plants, the psaE1 transcript is expressed at a higher level than psaE2 mRNA. In the mutants, however, the E1 transcript was barely detectable, and was expressed only in small groups of wild-type cells resulting from somatic reversions. As a consequence, the amount of PsaE protein present in the mutant is significantly reduced. Concomitantly, the levels of other stromal photosystem I subunits (PsaC and PsaD) are also affected. Mutant plants showed a marked increase in light sensitivity and photoinhibition. Additional effects of the psae1 mutation include light green pigmentation, an increase in chlorophyll fluorescence and a decrease of approximately 50% in growth rate under greenhouse conditions.

Disulfide bond formation between RNA binding domains is used to regulate mRNA binding activity of the chloroplast poly(A)-binding protein

Binding of the chloroplast poly(A)-binding protein, RB47, to the psbA mRNA is regulated in response to light and is required for translation of this mRNA in chloroplasts. The RNA binding activity of RB47 can be modulated in vitro by oxidation and reduction. Site-directed mutations to individual cysteine residues in each of the four RNA binding domains of RB47 showed that changing single cysteines to serines in domains 2 or 3 reduced, but did not eliminate, the ability of RB47 to be redox-regulated. Simultaneously changing cysteines to serines in both domains 2 and 3 resulted in the production of RB47 protein that was insensitive to redox regulation but retained the ability to bind the psbA mRNA at high affinity. The poly(A)-binding protein from Saccharomyces cerevisiae lacks cysteine residues in RNA binding domains 2 and 3, and this poly(A)-binding protein lacks the ability to be regulated by oxidation or reduction. These data show that disulfide bond formation between RNA binding domains in a poly(A)-binding protein can be used to regulate the ability of this protein to bind mRNA and suggest that redox regulation of RNA binding activity may be used to regulate translation in organisms whose poly(A)-binding proteins contain these critical cysteine residues.

Role of the H protein in assembly of the photochemical reaction center and intracytoplasmic membrane in Rhodospirillum rubrum

Rhodospirillum rubrum is a model for the study of membrane formation. Under conditions of oxygen limitation, this facultatively phototrophic bacterium forms an intracytoplasmic membrane that houses the photochemical apparatus. This apparatus consists of two pigment-protein complexes, the light-harvesting antenna (LH) and photochemical reaction center (RC). The proteins of the photochemical components are encoded by the puf operon (LHalpha, LHbeta, RC-L, and RC-M) and by puhA (RC-H). R. rubrum puf interposon mutants do not form intracytoplasmic membranes and are phototrophically incompetent. The puh region was cloned, and DNA sequence determination identified open reading frames bchL and bchM and part of bchH; bchHLM encode enzymes of bacteriochlorophyll biosynthesis. A puhA/G115 interposon mutant was constructed and found to be incapable of phototrophic growth and impaired in intracytoplasmic membrane formation. Comparison of properties of the wild-type and the mutated and complemented strains suggests a model for membrane protein assembly. This model proposes that RC-H is required as a foundation protein for assembly of the RC and highly developed intracytoplasmic membrane. In complemented strains, expression of puh occurred under semiaerobic conditions, thus providing the basis for the development of an expression vector. The puhA gene alone was sufficient to restore phototrophic growth provided that recombination occurred.

Degradation of the Photosystem II D1 and D2 proteins in different strains of the cyanobacterium Synechocytis PCC 6803 varying with respect to the type and level of psbA transcript

The turnover of the D1 and D2 proteins of Photosystem II (PSII) has been investigated by pulse-chase radiolabeling in several strains of the cyanobacterium Synechocystis PCC 6803 containing different types and levels of the psbA transcript. Strains lacking psbA1 and psbA3 gene and containing high levels of the psbA2 transcript showed the selective synthesis of D1 whose degradation could be slowed down by the protein synthesis inhibitor lincomycin. In contrast, in strains containing just the psbA3 gene, the intensity of the D1 protein labeling was lower and labeling of the D2 and CP43 proteins was stimulated in comparison to the psbA2-containing strains. In addition, the rate and selectivity of the D1 degradation and its dependence on the presence of lincomycin was proportional to the level of the psbA3 transcript in the particular strain. Consequently, there was parallel, lincomycin-independent and slowed-down breakdown of the D1 and D2 proteins in strains with the lowest level of psbA3 transcript. These results are discussed in terms of a model in which the rate of D1 and D2 degradation in cyanobacteria is affected not only by the rate of PSII photodamage, but also by the availability of newly synthesized D1 protein. Moreover, the comparison of the non-oxygen-evolving D1 mutants D170A** and Y161F*** differing by the presence of tyrosine Z has indicated a minor role of the oxidized form of this secondary PSII electron donor in the donor side mechanism of D1 and D2 protein breakdown.

Degradation and de novo synthesis of D1 protein and psbA transcript levels in Chlamydomonas reinhardtii during UV-B inactivation of photosynthesis and its reactivation

UV-B induces intensity and time dependent inhibition of photosynthetic O2 evolution and PS II electron transport activity in Chlamydomonas reinhardtii. The D1 and D2 proteins of chloroplast membranes are rapidly and specifically degraded in the course of irradiation of cells to UV-B. Continuous synthesis of the two proteins was essential for the repair of damaged PS II as chloramphenicol accelerated UV-B inactivation of photosynthesis and prevented photoreactivation. Northern analysis revealed that UV-B also affected the expression of psbA gene coding for the D1 protein. Cells showing 72% inhibition of PS II activity, revealed a modest net loss of 25% in the level of D1 protein. This shows that synthesis of D1 protein is not the only component involved in the recovery process. Our results indicate that besides affecting the synthesis of the D1 protein UV-B may impair certain post-translational events, which in turn may limit the repair of damaged PS II.

Differential expression of photosynthesis and nitrogen fixation genes in the cyanobacterium Plectonema boryanum

The filamentous non-heterocystous cyanobacterium Plectonema boryanum fixes dinitrogen at a high rate during microaerobic growth in continuous illumination by temporal separation of oxygen-evolving photosynthesis and oxygen-sensitive dinitrogen fixation. The onset of nitrogen fixation is preceded by a depression in photosynthesis that establishes a sufficiently low level of dissolved oxygen in the growth medium. A several-fold reduction in the level of transcripts coding for phycocyanin (cpcBA) and the chlorophyll a binding protein of photosystem II (psbC) and psbA accompanied the depression in photosynthetic oxygen evolution. Unlike most of the other organisms examined to date, in P. boryanum, psbC and psbD do not appear to be co-transcribed. The psbC transcripts were down-regulated several fold, while the psbD transcript declined marginally during the nitrogen fixation phase. A decrease in dissolved oxygen and a dramatic increase in the level of nifH transcripts and the enzyme activity of nitrogenase were characteristic of the nitrogen fixation phase. The level of transcript for glnA, which encodes glutamine synthetase, was not altered. Reciprocal regulation of gene expression was well orchestrated with the alternating cycles of photosynthesis and nitrogen fixation in P. boryanum.

Stability determinants in the chloroplast psbB/T/H mRNAs of Chlamydomonas reinhardtii

The chloroplast gene psbB encodes the chlorophyll-a binding protein P5 (CP47), one of the core subunits of photosystem II (PSII). The psbB mRNA and the downstream psbT and psbH transcripts fail to accumulate in the Chlamydomonas reinhardtii nuclear mutant 222E affected in the Mbb1 gene (Monod et al. 1992, Mol. Gen. Genet. 231, 449-459). By introducing chimeric genes consisting of sequences from psbB and the reporter gene aadA into the chloroplast, the target site of Mbb1 was mapped in the psbB 5' untranslated region (UTR). Primer extension analysis indicates that the psbB RNA exists in a less abundant long form and a more abundant short form, with 5' ends at positions -147 and -35 relative to the AUG initiation codon, respectively. The longer transcript is present both in the wild type (WT) and 222E mutant, but the shorter one accumulates only in the WT. Two putative stem-loop structures in the longer 5' UTR can be deleted individually without affecting psbB mRNA accumulation. Insertion of a poly G cassette in the long leader stabilizes a chimeric psbB transcript in the 222E mutant, suggesting the involvement of a 5'-3' exonuclease. We also show that psbH and psbT are transcribed from the upstream psbB gene promoter, and that the psbH mRNA has its own target sequence for Mbb1 function. We discuss the role of this nucleus-encoded factor, required for specific chloroplast gene expression, in the assembly of the multi-protein PSII complex.

Proteomics of the chloroplast: systematic identification and targeting analysis of lumenal and peripheral thylakoid proteins

The soluble and peripheral proteins in the thylakoids of pea were systematically analyzed by using two-dimensional electrophoresis, mass spectrometry, and N-terminal Edman sequencing, followed by database searching. After correcting to eliminate possible isoforms and post-translational modifications, we estimated that there are at least 200 to 230 different lumenal and peripheral proteins. Sixty-one proteins were identified; for 33 of these proteins, a clear function or functional domain could be identified, whereas for 10 proteins, no function could be assigned. For 18 proteins, no expressed sequence tag or full-length gene could be identified in the databases, despite experimental determination of a significant amount of amino acid sequence. Nine previously unidentified proteins with lumenal transit peptides are presented along with their full-length genes; seven of these proteins possess the twin arginine motif that is characteristic for substrates of the TAT pathway. Logoplots were used to provide a detailed analysis of the lumenal targeting signals, and all nuclear-encoded proteins identified on the two-dimensional gels were used to test predictions for chloroplast localization and transit peptides made by the software programs ChloroP, PSORT, and SignalP. A combination of these three programs was found to provide a useful tool for evaluating chloroplast localization and transit peptides and also could reveal possible alternative processing sites and dual targeting. The potential of proteomics for plant biology and homology-based searching with mass spectrometry data is discussed.

Limitation in electron transfer in photosystem I donor side mutants of Chlamydomonas reinhardtii. Lethal photo-oxidative damage in high light is overcome in a suppressor strain deficient in the assembly of the light harvesting complex

Strains of Chlamydomonas reinhardtii lacking the PsaF gene or containing the mutation K23Q within the N-terminal part of PsaF are sensitive to high light (>400 microE m(-2) s(-1)) under aerobic conditions. In vitro experiments indicate that the sensitivity to high light of the isolated photosystem I (PSI) complex from wild type and from PsaF mutants is similar. In vivo measurements of photochemical quenching and oxygen evolution show that impairment of the donor side of PSI in the PsaF mutants leads to a diminished linear electron transfer and/or a decrease of photosystem II (PSII) activity in high light. Thermoluminescence measurements indicate that the PSII reaction center is directly affected under photo-oxidative stress when the rate of electron transfer becomes limiting in the PsaF-deficient strain and in the PsaF mutant K23Q. We have isolated a high light-resistant PsaF-deficient suppressor strain that has a high chlorophyll a/b ratio and is affected in the assembly of light-harvesting complex. These results indicate that under high light a functionally intact donor side of PSI is essential for protection of C. reinhardtii against photo-oxidative damage when the photosystems are properly connected to their light-harvesting antennae.

A novel precursor recognition element facilitates posttranslational binding to the signal recognition particle in chloroplasts

Signal recognition particles (SRPs) in the cytosols of prokaryotes and eukaryotes are used to target proteins to cytoplasmic membranes and the endoplasmic reticulum, respectively. The mechanism of targeting relies on cotranslational SRP binding to hydrophobic signal sequences. An organellar SRP identified in chloroplasts (cpSRP) is unusual in that it functions posttranslationally to localize a subset of nuclear-encoded thylakoid proteins. In assays that reconstitute thylakoid integration of the light harvesting chlorophyll-binding protein (LHCP), stromal cpSRP binds LHCP posttranslationally to form a cpSRP/LHCP transit complex, which is believed to represent the LHCP form targeted to thylakoids. In this investigation, we have identified an 18-aa sequence motif in LHCP (L18) that, along with a hydrophobic domain, is required for transit complex formation. Fusion of L18 to the amino terminus of an endoplasmic reticulum-targeted protein, preprolactin, led to transit complex formation whereas wild-type preprolactin exhibited no ability to form a transit complex. In addition, a synthetic L18 peptide, which competed with LHCP for transit complex formation, caused a parallel inhibition of LHCP integration. Translocation of proteins by the thylakoid Sec and Delta pH transport systems was unaffected by the highest concentration of L18 peptide examined. Our data indicate that a motif contained in L18 functions in precursor recruitment to the posttranslational SRP pathway, one of at least four different thylakoid sorting pathways used by chloroplasts.

Identification of the proton pathway in bacterial reaction centers: replacement of Asp-M17 and Asp-L210 with asn reduces the proton transfer rate in the presence of Cd2+

The reaction center (RC) from Rhodobacter sphaeroides converts light into chemical energy through the reduction and protonation of a bound quinone molecule Q(B) (the secondary quinone electron acceptor). We investigated the proton transfer pathway by measuring the proton-coupled electron transfer, k(AB)((2)) [Q(A)Q(B) + H(+) --> Q(A)(Q(B)H)(-)] in native and mutant RCs in the absence and presence of Cd(2+). Previous work has shown that the binding of Cd(2+) decreases k(AB)((2)) in native RCs approximately 100-fold. The preceding paper shows that bound Cd(2+) binds to Asp-H124, His-H126, and His-H128. This region represents the entry point for protons. In this work we investigated the proton transfer pathway connecting the entry point with Q(B) by searching for mutations that greatly affect k(AB)((2)) ( greater, similar10-fold) in the presence of Cd(2+), where k(AB)((2)) is limited by the proton transfer rate (k(H)). Upon mutation of Asp-L210 or Asp-M17 to Asn, k(H) decreased from approximately 60 s(-1) to approximately 7 s(-1), which shows the important role that Asp-L210 and Asp-M17 play in the proton transfer chain. By comparing the rate of proton transfer in the mutants (k(H) approximately 7 s(-1)) with that in native RCs in the absence of Cd(2+) (k(H) >/= 10(4) s(-1)), we conclude that alternate proton transfer pathways, which have been postulated, are at least 10(3)-fold less effective.

DNA sequence analysis of the photosynthesis region of Rhodobacter sphaeroides 2.4.1

This paper describes the DNA sequence of the photosynthesis region of Rhodobacter sphaeroides 2.4.1 (T). The photosynthesis gene cluster is located within a approximately 73 kb Ase I genomic DNA fragment containing the puf, puhA, cycA and puc operons. A total of 65 open reading frames (ORFs) have been identified, of which 61 showed significant similarity to genes/proteins of other organisms while only four did not reveal any significant sequence similarity to any gene/protein sequences in the database. The data were compared with the corresponding genes/ORFs from a different strain of R.sphaeroides and Rhodobacter capsulatus, a close relative of R. sphaeroides. A detailed analysis of the gene organization in the photosynthesis region revealed a similar gene order in both species with some notable differences located to the pucBAC = cycA region. In addition, photosynthesis gene regulatory protein (PpsR, FNR, IHF) binding motifs in upstream sequences of a number of photosynthesis genes have been identified and shown to differ between these two species. The difference in gene organization relative to pucBAC and cycA suggests that this region originated independently of the photosynthesis gene cluster of R.sphaeroides.

X-ray crystal structure of the YM210W mutant reaction centre from Rhodobacter sphaeroides

The X-ray crystal structure of a reaction centre from Rhodobacter sphaeroides with a mutation of tyrosine M210 to tryptophan (YM210W) has been determined to a resolution of 2.5 A. Structural conservation is very good throughout the body of the protein, with the tryptophan side chain adopting a position in the mutant complex closely resembling that of the tyrosine in the wild-type complex. The spectroscopic properties of the YM210W reaction centre are discussed with reference to the structural data, with particular focus on evidence that the introduction of the bulkier tryptophan in place of the native tyrosine may cause a small tilt of the macrocycle of the B(L) monomeric bacteriochlorophyll.

Characterization of a zebra mutant of rice with increased susceptibility to light stress

The rice zebra mutant TCM248 is a single recessive mutant. This mutant develops transverse-striped leaves with green and white sectors under alternate light/dark growth conditions. Mutants that were grown under a higher light intensity during the light period showed a more intense striped phenotype. The white tissues contained abnormal chloroplasts with few internal membrane structures, while the green tissues in the mutants contained normal chloroplasts. The white tissue contained only trace amounts of Chls and carotenoids, and mRNA accumulation of nuclear genes encoding chloroplast proteins (rbcS, cab) was strongly suppressed compared to that in the wild type plants. A series of growth condition shift experiments demonstrated that the mutant displayed the striped phenotype only if it was exposed to the alternate light/dark growth conditions during a limited stage of early leaf development. These data suggest that the zebra gene is involved in the acquisition of photoprotective capacity of the plants and that this gene functions at an early stage of chloroplast differentiation.

Insertion of light-harvesting chlorophyll a/b protein into the thylakoid topographical studies

The major light-harvesting chlorophyll a/b-binding protein (Lhcb1,2) of photosystem II is inserted into the thylakoid via the signal recognition particle dependent pathway. However, the mechanism by which the protein enters the membrane is at this time unknown. In order to define some topographical restrictions for this process, we constructed several recombinant derivatives of Lhcb1 carrying hexahistidine tags at either protein terminus or in the stromal loop domain. Additionally, green fluorescent protein (GFP) was fused to either terminus. None of the modifications significantly impair the pigment-binding properties of the protein in the in vitro reconstitution of LHCII. With the exception of the C-terminal GFP fusion, all mutants stably insert into isolated thylakoids in the absence of Ni2+ ions. The addition of low concentrations of Ni2+ ions abolishes the thylakoid insertion of C-terminally His-tagged mutants whereas the other His-tagged proteins fail to insert only at higher Ni2+ concentrations. The C-terminus of Lhcb1 must cross the membrane during protein insertion whereas the other sites of Lhcb1 modification are positioned on the stromal side of LHCII. We conclude that a Ni2+-complexed His tag and fusion to GFP inhibit translocation of the protein C-terminus across the thylakoid. Our observations indicate that the N-terminal and stromal domain of Lhcb1 need not traverse the thylakoid during protein insertion and are consistent with a loop mechanism in which only the C-terminus and the lumenal loop of Lhcb1 are translocated across the thylakoid.

Minicircular plastid DNA in the dinoflagellate Amphidinium operculatum

Plastid DNA was purified from the dinoflagellate Amphidinium operculatum. The genes atpB, petD, psaA, psbA and psbB have been shown to reside on single-gene minicircles of a uniform size of 2.3-2.4 kb. The psaA and psbB genes lack conventional initiation codons in the expected positions, and may use GTA for translation initiation. There are marked biases in codon preference. The predicted PsbA protein lacks the C-terminal extension which is present in all other photosynthetic organisms except Euglena gracilis, and there are other anomalies elsewhere in the predicted amino acid sequences. The non-coding regions of the minicircles contain a "core" region which includes a number of stretches that are highly conserved across all minicircles and modular regions that are conserved within subsets of the minicircles.

Correction of the DNA sequence of the regB gene of Rhodobacter capsulatus with implications for the membrane topology of the sensor kinase regB

We corrected the previously published sequence for the regB gene, which encodes a histidine sensor kinase in Rhodobacter capsulatus. The deduced RegB amino acid sequence has an additional putative transmembrane domain at the N terminus. Analysis of RegB-PhoA and RegB-LacZ fusion proteins supports a topology model for RegB with six membrane-spanning domains.

The ycf 9 (orf 62) gene in the plant chloroplast genome encodes a hydrophobic protein of stromal thylakoid membranes

There are still some open reading frames, orfs, with unknown function in the higher plant chloroplast genome. Of these conserved orfs, designated as ycfs (hypothetical chloroplast open reading frames), one is ycf 9 (orf 62) in the transcription unit with the psbC and psbD genes. The aim of this work was to investigate the function of ycf 9 by insertional inactivation of the gene with a selectable marker cassette, consisting of the aadA coding region connected to the trc promoter and rrnB terminator. This cassette was inserted 19 bp downstream from the start of the coding region of the tobacco ycf 9 gene. Two DNA constructs with the aadA cassette in opposite orientations were precipitated on 1 micron gold particles and delivered into leaves of Nicotiana tabacum, cultivar Samsun, by the biolistic method. Spectinomycin-resistant plants regenerated following bombardment with only the construct containing the aadA gene in the opposite orientation as ycf 9. In spite of several subsequent regeneration cycles on spectinomycin, the transplastomic plants did not reach homoplasmicity. This suggests that the ycf 9 gene product is essential for chloroplast function. Using a polyclonal antibody raised against the inner part of the gene product, the polypeptide was localized in the stromal thylakoid membranes of chloroplasts.

Mutations at three different nuclear loci of Chlamydomonas suppress a defect in chloroplast psbD mRNA accumulation

In the photosynthetic chloroplast mutant PRB2A of Chlamydomonas reinhardtii the psbD mRNA is unstable. Three strains were isolated, in which the underlying site-directed mutation within the psbD 5' UTR (untranslated region) is suppressed. In all three suppressors, psbD RNA levels and RNA 5' maturation are restored to a varying extent, suggesting a tight coupling of RNA stabilization and 5' processing. Expression of the psbA gene is not compromised in these strains. Genetic crosses revealed that the suppressor mutations affect three unlinked nuclear loci, which may encode new factors involved in psbD gene expression.

Redox control of psbA gene expression in the cyanobacterium Synechocystis PCC 6803. Involvement of the cytochrome b(6)/f complex

We investigated the role of the redox state of the photosynthetic and respiratory electron transport chains on the regulation of psbA expression in Synechocystis PCC 6803. Different means to modify the redox state of the electron carriers were used: (a) dark to oxidize the whole electron transport chain; (b) a shift from dark to light to induce its reduction; (c) the chemical interruption of the electron flow at different points to change the redox state of specific electron carriers; and (d) the presence of glucose to maintain a high reducing power in darkness. We show that changes in the redox state of the intersystem electron transport chain induce modifications of psbA transcript production and psbA mRNA stability. Reduction of the intersystem electron carriers activates psbA transcription and destabilizes the mRNA, while their oxidation induces a decrease in transcription and a stabilization of the transcript. Furthermore, our data suggest that the redox state of one of the electron carriers between the plastoquinone pool and photosystem I influences not only the expression of the psbA gene, but also that of other two photosynthetic genes, psaE and cpcBA. As a working hypothesis, we propose that the occupancy of the Q(0) site in the cytochrome b(6)/f complex may be involved in this regulation.

Translation of chloroplast psbA mRNA is modulated in the light by counteracting oxidizing and reducing activities

Light has been proposed to stimulate the translation of Chlamydomonas reinhardtii chloroplast psbA mRNA by activating a protein complex associated with the 5' untranslated region of this mRNA. The protein complex contains a redox-active regulatory site responsive to thioredoxin. We identified RB60, a protein disulfide isomerase-like member of the protein complex, as carrying the redox-active regulatory site composed of vicinal dithiol. We assayed in parallel the redox state of RB60 and translation of psbA mRNA in intact chloroplasts. Light activated the specific oxidation of RB60, on the one hand, and reduced RB60, probably via the ferredoxin-thioredoxin system, on the other. Higher light intensities increased the pool of reduced RB60 and the rate of psbA mRNA translation, suggesting that a counterbalanced action of reducing and oxidizing activities modulates the translation of psbA mRNA in parallel with fluctuating light intensities. In the dark, chemical reduction of the vicinal dithiol site did not activate translation. These results suggest a mechanism by which light primes redox-regulated translation by an unknown mechanism and then the rate of translation is determined by the reduction-oxidation of a sensor protein located in a complex bound to the 5' untranslated region of the chloroplast mRNA.