cDNA sequence and predicted primary structure of the gamma subunit from the ATP synthase from Chlamydomonas reinhardtii

The Deep Thioredoxome in Chlamydomonas reinhardtii: New Insights into Redox Regulation

Thiol-based redox post-translational modifications have emerged as important mechanisms of signaling and regulation in all organisms, and thioredoxin plays a key role by controlling the thiol-disulfide status of target proteins. Recent redox proteomic studies revealed hundreds of proteins regulated by glutathionylation and nitrosylation in the unicellular green alga Chlamydomonas reinhardtii, while much less is known about the thioredoxin interactome in this organism. By combining qualitative and quantitative proteomic analyses, we have comprehensively investigated the Chlamydomonas thioredoxome and 1188 targets have been identified. They participate in a wide range of metabolic pathways and cellular processes. This study broadens not only the redox regulation to new enzymes involved in well-known thioredoxin-regulated metabolic pathways but also sheds light on cellular processes for which data supporting redox regulation are scarce (aromatic amino acid biosynthesis, nuclear transport, etc). Moreover, we characterized 1052 thioredoxin-dependent regulatory sites and showed that these data constitute a valuable resource for future functional studies in Chlamydomonas. By comparing this thioredoxome with proteomic data for glutathionylation and nitrosylation at the protein and cysteine levels, this work confirms the existence of a complex redox regulation network in Chlamydomonas and provides evidence of a tremendous selectivity of redox post-translational modifications for specific cysteine residues.

Assembly of F1F0-ATP synthases

F1F0-ATP synthases are multimeric protein complexes and common prerequisites for their correct assembly are (i) provision of subunits in appropriate relative amounts, (ii) coordination of membrane insertion and (iii) avoidance of assembly intermediates that uncouple the proton gradient or wastefully hydrolyse ATP. Accessory factors facilitate these goals and assembly occurs in a modular fashion. Subcomplexes common to bacteria and mitochondria, but in part still elusive in chloroplasts, include a soluble F1 intermediate, a membrane-intrinsic, oligomeric c-ring, and a membrane-embedded subcomplex composed of stator subunits and subunit a. The final assembly step is thought to involve association of the preformed F1-c10-14 with the ab2 module (or the ab8-stator module in mitochondria)--mediated by binding of subunit δ in bacteria or OSCP in mitochondria, respectively. Despite the common evolutionary origin of F1F0-ATP synthases, the set of auxiliary factors required for their assembly in bacteria, mitochondria and chloroplasts shows clear signs of evolutionary divergence. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Redox regulation of rotation of the cyanobacterial F1-ATPase containing thiol regulation switch

F(1)-ATP synthase (F(1)-ATPase) is equipped with a special mechanism that prevents the wasteful reverse reaction, ATP hydrolysis, when there is insufficient proton motive force to drive ATP synthesis. Chloroplast F(1)-ATPase is subject to redox regulation, whereby ATP hydrolysis activity is regulated by formation and reduction of the disulfide bond located on the γ subunit. To understand the molecular mechanism of this redox regulation, we constructed a chimeric F(1) complex (α(3)β(3)γ(redox)) using cyanobacterial F(1), which mimics the regulatory properties of the chloroplast F(1)-ATPase, allowing the study of its regulation at the single molecule level. The redox state of the γ subunit did not affect the ATP binding rate to the catalytic site(s) and the torque for rotation. However, the long pauses caused by ADP inhibition were frequently observed in the oxidized state. In addition, the duration of continuous rotation was relatively shorter in the oxidized α(3)β(3)γ(redox) complex. These findings lead us to conclude that redox regulation of CF(1)-ATPase is achieved by controlling the probability of ADP inhibition via the γ subunit inserted region, a sequence feature observed in both cyanobacterial and chloroplast ATPase γ subunits, which is important for ADP inhibition (Sunamura, E., Konno, H., Imashimizu-Kobayashi, M., Sugano, Y., and Hisabori, T. (2010) Plant Cell Physiol. 51, 855-865).

The ferredoxin/thioredoxin system of oxygenic photosynthesis

Forty years ago, ferredoxin (Fdx) was shown to activate fructose 1,6-bisphosphatase in illuminated chloroplast preparations, thereby laying the foundation for the field now known as "redox biology." Enzyme activation was later shown to require the ubiquitous protein thioredoxin (Trx), reduced photosynthetically by Fdx via an enzyme then unknown-ferredoxin:thioredoxin reductase (FTR). These proteins, Fdx, FTR, and Trx, constitute a regulatory ensemble, the "Fdx/Trx system." The redox biology field has since grown beyond all expectations and now embraces a spectrum of processes throughout biology. Progress has been notable with plants that possess not only the plastid Fdx/Trx system, but also the earlier known NADP/Trx system in the cytosol, endoplasmic reticulum, and mitochondria. Plants contain at least 19 types of Trx (nine in chloroplasts). In this review, we focus on the structure and mechanism of action of members of the photosynthetic Fdx/Trx system and on biochemical processes linked to Trx. We also summarize recent evidence that extends the Fdx/Trx system to amyloplasts-heterotrophic plastids functional in the biosynthesis of starch and other cell components. The review highlights the plant as a model system to uncover principles of redox biology that apply to other organisms.

Human mitochondrial thioredoxin. Involvement in mitochondrial membrane potential and cell death

Thioredoxins (Trx) are a class of small multifunctional redox-active proteins found in all organisms. Recently, we reported the cloning of a mitochondrial thioredoxin, Trx2, from rat heart. To investigate the biological role of Trx2 we have isolated the human homologue, hTrx2, and generated HEK-293 cells overexpressing Trx2 (HEK-Trx2). Here, we show that HEK-Trx2 cells are more resistant toward etoposide. In addition, HEK-Trx2 are more sensitive toward rotenone, an inhibitor of complex I of the respiratory chain. Finally, overexpression of Trx2 confers an increase in mitochondrial membrane potential, DeltaPsi(m). Treatment with oligomycin could both reverse the effect of rotenone and decrease the membrane potential suggesting that Trx2 interferes with the activity of ATP synthase. Taken together, these results suggest that Trx2 interacts with specific components of the mitochondrial respiratory chain and plays an important role in the regulation of the mitochondrial membrane potential.

A chloroplast-resident DNA methyltransferase is responsible for hypermethylation of chloroplast genes in Chlamydomonas maternal gametes

Chloroplast DNA of the green alga Chlamydomonas reinhardtii is maternally inherited. Methylation mapping directly revealed that, before mating, chloroplast DNA of maternal (mating type plus; mt(+)) gametes is heavily methylated whereas that of paternal (mating type minus; mt(-)) gametes is not. Indirect immunofluorescence analyses with anti-5-methylcytosine mAbs visually showed methylation to occur exclusively in chloroplast DNA of mt(+) gametes, and not in mt(-) gametes or nuclear DNA of either mt. To clarify the relationship between methylation and maternal inheritance of chloroplast DNA, we have isolated and characterized a cDNA encoding a DNA methyltransferase. The deduced protein, CrMET1, consists of 1,344 aa and contains a conserved catalytic domain at the C terminal and a nonconserved N-terminal region. The predicted N-terminal region has an arginine-rich domain, suggesting CrMET1 is transferred to chloroplasts. This finding could be directly shown by green fluorescent protein epifluorescence microscopy analyses. CrMET1 transcripts were found to be absent in both mt(+) and mt(-) vegetative cells. Upon gametogenesis, however, transcript levels clearly increased in mt(+) but not mt(-) cells. These experiments suggest that the CrMET1 protein is located in chloroplasts and that it specifically methylates cytosine residues of chloroplast DNA in mt(+) gametes. This conclusion was further strengthened by the observation that, during gametogenesis, CrMET1 is expressed in a mt(-) mutant, mat-1, whose chloroplast DNA is heavily methylated in gametes and paternally inherited. The results provide evidence that cytosine methylation plays a critical role in maternal inheritance of chloroplast genes in C. reinhardtii.

A coil-helix instead of a helix-coil motif can be induced in a chloroplast transit peptide from Chlamydomonas reinhardtii

A synthetic peptide MQVTMKSSAVSGQRVGGARVATRSVRRAQLQV corresponding to the 32 amino acid chloroplast transit sequence of the ribulose bisphosphatase carboxylase/oxygenase activase preprotein from Chlamydomonas reinhardtii, required for translocation through the envelope of the chloroplast, has been characterized structurally using CD and NMR under the same experimental conditions as used previously for the 32 amino acid presequence of preferredoxin from the same organism [Lancelin, J.-M., Bally, I., Arlaud, G. J., Blackledge, M., Gans, P., Stein, M. & Jacquot, J.-P. (1994) FEBS Lett. 343, 261-266]. The peptide is found to undergo a conformational transition in aqueous 2,2,2-trifluoroethanol, characterized by three turns of amphiphilic alpha-helix in the C-terminal region preceded by a disordered coil in the N-terminal region. Compared with the preferredoxin transit peptide, the helical and coiled domains are arranged in the reverse order along the peptide sequence, but the positively charged groups are distributed analogously as well as the hydrophobic residues within the amphiphilic alpha-helix. It is proposed that such coil-helix or helix-coil motifs, occasionally repeated, could be an intrinsic structural feature of chloroplastic transit peptides, adapted to the proper translocase and possibly to each nuclear-encoded chloroplast preproteins. This feature may distinguish chloroplastic transit sequences from the other organelle-targeting peptides in the eukaryotic green alga C. reinhardtii, particularly the mitochondrial transit sequences.

A gamete-specific, sex-limited homeodomain protein in Chlamydomonas

During fertilization in Chlamydomonas, gametes of opposite mating types interact with each other through sex-specific adhesion molecules on their flagellar surfaces. Flagellar adhesion brings the cell bodies of the gametes into close contact and initiates a signal transduction pathway in preparation for cell-cell fusion. We have identified a cDNA, gsp1, whose transcript levels are upregulated during flagellar adhesion. The GSP1 polypeptide is a novel, gamete-specific homeodomain protein, the first to be identified in an alga. Its homeodomain shows significant identity with several higher plant homeodomain proteins. Although encoded by a single copy gene present in cells of both mating types, immunoblot analysis showed that GSP1 was expressed in mating type (mt)+ gametes, but was not detectable in mt- gametes or in vegetative cells of either mating type. Moreover, GSP1 appeared late during gametogenesis, suggesting that it may function during adhesion with mt- gametes or after zygote formation. GSP1 is expressed in imp11, mt- mutant gametes, which have a lesion in the mid gene involved in sex determination and exhibit many phenotypic characteristics of mt+ gametes. Thus, gsp1 is negatively regulated by mid and is the first molecule to be identified in Chlamydomonas that shows sex-limited expression.

Transit peptide mutations that impair in vitro and in vivo chloroplast protein import do not affect accumulation of the gamma-subunit of chloroplast ATPase

We have begun to take a genetic approach to study chloroplast protein import in Chlamydomonas reinhardtii by creating deletions in the transit peptide of the gamma-subunit of chloroplast ATPase-coupling factor 1 (CF1-gamma, encoded by AtpC) and testing their effects in vivo by transforming the altered genes into an atpC mutant, and in vitro by importing mutant precursors into isolated C. reinhardtii chloroplasts. Deletions that removed 20 or 23 amino acid residues from the center of the transit peptide reduced in vitro import to an undetectable level but did not affect CF1-gamma accumulation in vivo. The CF1-gamma transit peptide does have an in vivo stroma-targeting function, since chimeric genes in which the stroma-targeting domain of the plastocyanin transit peptide was replaced by the AtpC transit peptide-coding region allowed plastocyanin to accumulate in vivo. To determine whether the transit peptide deletions were impaired in in vivo stroma targeting, mutant and wild-type AtpC transit peptide-coding regions were fused to the bacterial ble gene, which confers bleomycin resistance. Although 25% of the wild-type fusion protein was associated with chloroplasts, proteins with transit peptide deletions remained almost entirely cytosolic. These results suggest that even severely impaired in vivo chloroplast protein import probably does not limit the accumulation of CF1-gamma.

Kinetics and thioredoxin specificity of thiol modulation of the chloroplast H+-ATPase

The kinetics of thiol modulation of the chloroplast H+-ATPase (CF0CF1) in membrana were analyzed by employing thioredoxins that were kept reduced by 0.1 mM dithiothreitol. The kinetics of thiol modulation depend on the extent of the proton gradient. The process is an exponential function of the thioredoxin concentration and reaction time and can be described by an irreversible second order reaction. The results indicate that the formation of the complex between thioredoxin and CF0CF1 is slow compared with the subsequent reduction step. Furthermore we have compared the efficiencies of the Escherichia coli thioredoxin Trx and the two chloroplast thioredoxins Tr-m and Tr-f. The second order rate constants are 0.057 (Tr-f), 0.024 (Trx), and 0.010 s-1 microM-1 (Tr-m) suggesting that Tr-f rather than Tr-m is the physiological reductant for the chloroplast ATPase. The often employed artificial reductant dithiothreitol exhibits a second order rate constant in thiol modulation of 1.02.10(-6) s-1 microM-1.

Identification, cDNA sequence and deduced amino acid sequence of the mitochondrial Rieske iron-sulfur protein from the green alga Chlamydomonas reinhardtii. Implications for protein targeting and subunit interaction

Specific oligonucleotide probes were used to isolate a cDNA clone for the mitochondrial Rieske iron-sulfur protein of the green alga Chlamydomonas reinhardtii. The protein is synthesized as a longer precursor with a cleavable N-terminal presequence of 54 amino acids but without a C-terminal extension. Comparison of the predicted secondary structure of this N-terminal sequence with that of the targeting signal of the chloroplast Rieske protein from C. reinhardtii [de Vitry (1994) J. Biol. Chem. 269, 7603-7609] indicates that, although they both have the potential to form amphiphilic alpha helices, the mito-chondrial presequence may form a more hydrophobic helix that could penetrate deeper into the membrane. The N-terminal part of the mature mitochondrial Rieske protein is characterized by a long, strongly hydrophilic N-terminal domain and by a positive charge in the middle of the hydrophobic stretch that is presumed to interact with the bc1 complex. Thus, the protein from C. reinhardtii differs from the Rieske proteins from mammals or fungi.

Energizing effects of illumination on the reactivities of lysine residues of the gamma subunit of chloroplast ATP synthase

Incubation of chloroplast thylakoids with pyridoxal 5'-phosphate for a short time (5 s) modified the lysine residues of the gamma subunit of ATP synthase. Energization of thylakoids by illumination increased the reactivity of Lys24 by a factor of three and decreased the reactivity of Lys30 to 60%. The reactivities of these residues reached their maximum and minimum values, respectively, within 1 s after the onset of illumination. Illumination of thylakoids increased the reactivities of Lys222 and Lys231 in two steps by a factor of three. The first step was completed within 1 s and the second step was completed 20-30 s after the onset of illumination. In the presence of 10 mM NH4Cl, illumination of thylakoids did not change the reactivities of these lysine residues. These results suggest that the Lys24- and Lys30-containing region of the gamma subunit changes its conformation rapidly in response to delta mu H+ and that the Lys222- and Lys231-containing region of the gamma subunit changes its conformation in two steps in response to delta mu H+ formation.

Isolation of CF0CF1 from Chlamydomonas reinhardtii cw15 and the N-terminal amino acid sequences of the CF0CF1 subunits

CF0CF1 was isolated from chloroplasts of the cell wall-deficient Chlamydomonas reinhardtii strain cw15. The subunit pattern was analyzed by SDS-gel electrophoresis and the N-terminal amino acid sequences of all nine subunits were determined by microsequencing. The amino acid sequences of subunits alpha, beta, gamma and epsilon match with those derived from the corresponding Chlamydomonas DNA sequences. In variance with the previously assumed N-terminus of beta; however, it was found that the first 11 amino acids are lacking. The subunits delta, I, II, III and IV were identified by comparison with known sequences of homologous polypeptides of higher plant chloroplasts and cyanobacteria, respectively.

Increased transcript levels of a methionine synthase during adhesion-induced activation of Chlamydomonas reinhardtii gametes

Chlamydomonas gametes of opposite mating types interact through flagellar adhesion molecules called agglutinins leading to a signal transduction cascade that induces cell wall loss and activation of mating structures along with other cellular responses that ultimately result in zygote formation. To identify molecules involved in these complex cellular events, we have employed subtractive and differential hybridization with cDNA from mt+ gametes activated for fertilization and non-signaling, vegetative (non-gametic) cells. We identified 55 cDNA clones whose transcripts were regulated in activated gametes. Here we report the molecular cloning and characterization of the complementary DNA (cDNA) for one clone whose transcripts in activated gametes were several-fold higher than in normal gametes. Regulation of the transcript was not related simply to protein synthesis because it was not increased in cells synthesizing new cell wall proteins. The cDNA contained a single open reading frame (ORF) of 815 amino acids encoding a polypeptide of calculated relative mass of 87 kDa. Database search analysis and sequence alignment indicated that the deduced amino acid sequence exhibited 42% identity and 62% similarity to a class of prokaryotic methyl transferases (5-methyltetrahydrofolate-homocysteine methyl transferase; EC 2.1.1.14) known to be involved in the terminal step of de novo biosynthesis of methionine. This enzyme catalyzes transfer of a methyl group from 5-methyltetrahydrofolate to homocysteine resulting in methionine formation. Affinity-purified polyclonal antibodies raised against a bacterially produced GST-fusion protein identified a 85 kDa soluble protein in Chlamydomonas gametes. Southern blot hybridization indicated that the enzyme is encoded by a single-copy gene. The evidence presented in this paper raises the possibility that, in addition to its participation in de novo biosynthesis and regeneration of methionine, Chlamydomonas methionine synthase may play a role in adhesion-induced events during fertilization.

Role of the Chlamydomonas reinhardtii coupling factor 1 gamma-subunit cysteine bridge in the regulation of ATP synthase

The gamma-subunit of coupling factor 1 (CF1) contains a cysteine bridge that is thought to be involved in the redox control of enzymatic activity. In order to test the regulatory significance of this disulfide bond, genetic transformation experiments with Chlamydomonas reinhardtii were performed. C. reinhardtii strain atpC1 (nit1-305, cw 15, mt-), which is null for the gamma-subunit, was transformed and complemented with gamma-subunit constructs containing amino acid substitutions localized to the cysteine bridge between Cys198 and Cys204. Successful complementation was confirmed by phenotypic selection, Northern blot analysis, reverse transcription polymerase chain reaction, and cDNA sequencing. CF1 ATPase activities of the soluble enzymes were measured in the presence and absence of dithiothreitol (DTT). Mutant CF1 enzymes showed no effect of DTT although increased activity was observed for the wild-type enzyme. In vitro, phenazine methosulfate-dependent photophosphorylation assays revealed that wild-type CF1 exhibits a 2-fold stimulation in the presence of 25 mM DTT, whereas each of the mutant enzymes has activities that are DTT-independent. Growth measurements indicated that despite the absence of a regulatory disulfide/dithiol, the mutant strains grew with the same kinetics as wild type. This study provides evidence to illustrate the involvement of the gamma-subunit in the redox regulation of ATP synthesis in vivo. This work is also the first demonstration in C. reinhardtii of stable nuclear transformation using mutated genes to complement a known defect.

Complementation of a Chlamydomonas reinhardtii mutant defective in the nuclear gene encoding the chloroplast coupling factor 1 (CF1) gamma-subunit (atpC)

Chlamydomonas reinhardtii strain atpC1 is a mutant defective in the nuclear gene that encodes the CF1 ATP synthase gamma-subunit polypeptide. Photoautotrophic growth was restored to atpC1 after it was transformed with wild-type DNA. Transformed strains were acetate-independent and arsenate-sensitive, similar in phenotype to the progenitor wild-type strain from which atpC1 was generated. Three transformed strains were examined in detail. Southern blot analyses demonstrated that the transformants were complements and not revertants. The transforming DNA integrated into the nuclear genome in a nonhomologous manner and at a low copy number. Northern blot analyses showed that the gamma-subunit mRNA in the complemented strains was expressed at the same relative level as that of wild-type. Western blots of total protein showed that whereas atpC1 was unable to synthesize any CF1 gamma-subunit, all three complemented strains could. Furthermore, the Western blot analyses demonstrated that the mutation in atpC1 had a pleiotropic effect on the accumulation of the CF1 beta-subunit which was relieved upon complementation. Cell extracts from atpC1 did not have any CF1-dependent catalytic activity, whereas extracts from all of the complemented strains and the wild-type strain had identical activities.

Structure of the nuclear encoded gamma subunit of CF0CF1 of the diatom Odontella sinensis including its presequence

Using a PCR-product as homologous probe for screening of a cDNA library of the diatom Odontella sinensis overlapping cDNA clones were obtained which showed homologies to atpC-genes of F0F1-ATPases from different sources. Comparison of the deduced amino acid sequence with the N-terminal sequence of the Odontella gamma subunit obtained by protein sequencing, indicated that the complete 370 amino acid protein is processed to a mature protein of 315 amino acids. The 55 amino acids comprising the presequence consists of two segments, one resembling a signal sequence for cotranslational transport through ER membranes and one showing characteristics of a transit sequence for transport of proteins into chloroplasts of higher plants. This result is discussed with respect to the particular envelope structure of chromophytic plastids consisting of four membranes. The outer membrane contains ribosomes on its cytosolic surface. As in cyanobacterial gamma subunits the regulatory sequence region, which is involved in thiol modulation of chloroplast ATPase of green algae and higher plants, is absent in the Odontella gamma subunit.

Structure, evolution and expression of the mitochondrial ADP/ATP translocator gene from Chlamydomonas reinhardtii

The first AUG in the Chlamydomonas reinhardtii ADP/ATP translocator (CRANT) mRNA initiates an open reading frame (ORF) which is very similar (51-79% amino acid identity) to other ANT proteins. In contrast to higher plants, no evidence for a long amino-terminal extension was obtained. The 5' non-transcribed region of the single-copy CRANT gene contains sequence motifs present in other C. reinhardtii nuclear genes. Four introns, whose positions are not conserved in other ANT genes, interrupt the protein coding region. A short heat shock specifically reduces CRANT mRNA levels. CRANT mRNA levels were unaffected by a mutation in photosynthesis. In a dark/light regime CRANT mRNA levels are high in the dark phase and low in the early light phase. Data on translation initiation sites, splice junctions and the codon preferences of C. reinhardtii nuclear genes were compiled. With the exception of two rare codons, ACA and GGA, the CRANT gene exhibits the biased codon usage of C. reinhardtii nuclear genes that are highly expressed during normal vegetative growth.

Import and processing of the precursor form of the gamma subunit of the chloroplast ATP synthase from tobacco

A cDNA clone encoding the complete precursor of the gamma subunit of chloroplast ATP synthase has been isolated from a tobacco (Nicotiana tabacum) leaf cDNA library in lambda gt11. The 1.4 kb insert encodes a polypeptide of 377 amino acid residues, of which 55 residues constitute an N-terminal presequence and 322 residues make up the mature gamma subunit. Hybridisation of the cDNA to Southern blots of tobacco genomic DNA indicates the presence of two genes in the haploid genome. Transcription and translation of the cDNA in vitro produced a protein of 41 kDa which was imported by isolated pea chloroplasts and processed to the mature 36 kDa subunit. The gamma subunit precursor was processed to the mature size by a processing peptidase of 180 kDa present in pea stromal extracts.

The Chlorella H+/hexose cotransporter gene

The complete genomic sequence of the inducible Chlorella kessleri H+/hexose cotransporter (HUP1) has been obtained from two overlapping clones isolated from a lambda gt10 library. The HUP1 gene is interrupted by 14 introns with the first intron being located in the 5'-untranslated part of the gene. The average intron length is 220 bp, yielding a very regular intron/exon pattern in the gene. The codon usage in this gene is strongly biased with a clear preference for C and a strong suppression of A. A consensus sequence for a putative algal polyadenylation sequence is shown and compared with other algal cDNA sequences.

The dithiothreitol-stimulated dissociation of the chloroplast coupling factor 1 epsilon-subunit is reversible

The chloroplast coupling factor 1 complex (CF1) contains an epsilon-subunit which inhibits the CF1 ATPase activity. Chloroform treatment of Chlamydomonas reinhardtii thylakoid membranes solubilizes only forms of the enzyme which apparently lack the delta-subunit. Four interrelated observations are described in this paper. (1) The dithiothreitol- (DTT) induced ATPase activation of CF1(-delta) and the DTT-induced formation of a physically resolvable CF1(-delta,epsilon) from the CF1(-delta) precursor are compared. The similar time-courses of these two phenomena suggest that the dissociation of the epsilon-subunit is an obligatory process in the DTT-induced ATPase activation of soluble CF1. (2) The reversible dissociation of the epsilon-subunit of the CF1 is demonstrated by the exchange of subunits between distinguishable oligomers. 35S-labelled chloroplast coupling factor 1 lacking the delta and epsilon subunits [CF1(-delta,epsilon)] was added to a solution of non-radioactive coupling factor 1 lacking only the delta subunit [CF1(-delta)]. After separation of the two enzyme forms, via high resolution anion-exchange chromatography, radioactivity was detected in the chromatographic fractions containing CF1(-delta). (3) epsilon-deficient CF1 can be resolved from DTT pretreated epsilon-containing CF1 for several days after the removal of DTT. On the other hand, brief incubation of the DTT pretreated epsilon-containing CF1 with low concentrations of o-iodosobenzoate results in chromatographs containing only the peak of epsilon-containing CF1. A simple explanation for this phenomenon is that reduction of CF1 with DTT increases the apparent dissociation constant for the epsilon-subunit to an estimated 3.5 x 10(-8) M (+/- 1.0 x 10(-8) M) from a value of less than or equal to 5 x 10(-11) M for the oxidized enzyme. (4) ATPase activity data show that oxidation of the epsilon-deficient enzyme does not completely inhibit its manifest activity, but oxidation of DTT pre-treated CF1 which contains the epsilon-subunit completely inhibits manifest activity. A simple model is proposed for the influence of the oxidation state of the soluble enzyme on the distribution of ATPase-inactive and ATPase-active subunit configurations.

The primary structure of the gamma-subunit of the ATPase from Synechocystis 6803

The nucleotide sequence of the gene coding for the F0F1-ATPase gamma-subunit (atpC) from the transformable cyanobacterium Synchocystis 6083 has been determined. The deduced translation product consists of 314 amino acid residues and is highly homologous (72% identical residues) to the sequences of other cyanobacterial gamma-subunits. The Synechocystis 6803 sequence is also homologous to the chloroplast gamma-sequence. Like in the other cyanobacterial subunits, only the first of the 3 cysteine residues, which are involved in energy-linked functions of the gamma-subunit in spinach chloroplasts, is conserved in Synechocystis 6803.

Chloroplast transit peptides from the green alga Chlamydomonas reinhardtii share features with both mitochondrial and higher plant chloroplast presequences

Chloroplast transit peptides from the green alga Chlamydomonas reinhardtii have been analyzed and compared with chloroplast transit peptides from higher plants and mitochondrial targeting peptides from yeast, Neurospora and higher eukaryotes. In terms of length and amino acid composition, chloroplast transit peptides from C. reinhardtii are more similar to mitochondrial targetting peptides than to chloroplast transit peptides from higher plants. They also contain the potential amphiphilic alpha-helix characteristic of mitochondrial presequences. However, in similarity with chloroplast transit peptides from higher plants, they contain a C-terminal region with the potential to form an amphiphilic beta-strand. As in higher plants, transit peptides that route proteins to the thylakoid lumen consist of an N-terminal domain similar to stroma-targeting transit peptides attached to a C-terminal apolar domain that share many characteristics with secretory signal peptides.

Isolation and characterization of cDNA clones encoding photosystem I subunits with molecular masses 11.0, 10.0 and 8.4 kDa from Chlamydomonas reinhardtii

cDNA clones encoding three photosystem I subunits of Chlamydomonas reinhardtii with apparent molecular masses 13, 5 and 3 kDa (thylakoid polypeptides 28, 35 and 37; P28, P35 and P37, respectively) were isolated using gene specific oligonucleotides as probes. The sequences of these oligonucleotides were deduced from the N-terminal amino acid sequences of the proteins. The cDNAs were sequenced and used to probe Southern and Northern blots. The Southern blot analysis indicates that the proteins are encoded by single-copy genes. The mRNA sizes of the three components are 960 (P28), 1120 (P35) and 790 (P37) nucleotides. Comparison between the open reading frames of the cDNAs and the N-terminal amino acid sequences of the proteins indicates that the nascent polypeptides possess N-terminal transit sequences that are removed to give mature proteins of 11.0 (P28), 10.0 (P35) and 8.4 (P37) kDa. Analysis of the deduced protein sequences suggests that P28 and P35 are extrinsic membrane proteins and that P37 spans the thylakoid membrane. All three proteins have short transit peptides that probably route them to the stromal side of the thylakoid membrane.