Nucleotide sequence of psbC, the gene encoding the CP-43 chlorophyll a-binding protein of Photosystem II, in the cyanobacterium Synechocystis 6803

Chloroplast genomes of Simarouba Aubl., molecular evolution and comparative analyses within Sapindales

Simarouba, a neotropical genus in the family Simaroubaceae, currently lacks comprehensive genomic data in existing databases. This study aims to fill this gap by providing genomic resources for three Simarouba species, S. amara, S. versicolor, and S. glauca. It also aims to perform comparative molecular evolutionary analyses in relation to other species within the order Sapindales. The analysis of these three Simarouba species revealed the presence of the typical quadripartite structure expected in plastomes. However, some pseudogenization events were identified in the psbC, infA, rpl22, and ycf1 genes. In particular, the CDS of the psbC gene in S. amara was reduced from 1422 bp to 584 bp due to a premature stop codon. Nucleotide diversity data pointed to gene and intergenic regions as promising candidates for species and family discrimination within the group, specifically matK, ycf1, ndhF, rpl32, petA-psbJ, and trnS-trnG. Selection signal analyses showed strong evidence for positive selection on the rpl23 gene. Phylogenetic analyses indicated that S. versicolor and S. glauca have a closer phylogenetic relationship than S. amara. We provide chloroplast genomes of three Simaruba species and use them to elucidate plastome evolution, highlight the presence of pseudogenization, and identify potential DNA barcode regions.

Time-resolved comparative molecular evolution of oxygenic photosynthesis

Oxygenic photosynthesis starts with the oxidation of water to O2, a light-driven reaction catalysed by photosystem II. Cyanobacteria are the only prokaryotes capable of water oxidation and therefore, it is assumed that the origin of oxygenic photosynthesis is a late innovation relative to the origin of life and bioenergetics. However, when exactly water oxidation originated remains an unanswered question. Here we use phylogenetic analysis to study a gene duplication event that is unique to photosystem II: the duplication that led to the evolution of the core antenna subunits CP43 and CP47. We compare the changes in the rates of evolution of this duplication with those of some of the oldest well-described events in the history of life: namely, the duplication leading to the Alpha and Beta subunits of the catalytic head of ATP synthase, and the divergence of archaeal and bacterial RNA polymerases and ribosomes. We also compare it with more recent events such as the duplication of Cyanobacteria-specific FtsH metalloprotease subunits and the radiation leading to Margulisbacteria, Sericytochromatia, Vampirovibrionia, and other clades containing anoxygenic phototrophs. We demonstrate that the ancestral core duplication of photosystem II exhibits patterns in the rates of protein evolution through geological time that are nearly identical to those of the ATP synthase, RNA polymerase, or the ribosome. Furthermore, we use ancestral sequence reconstruction in combination with comparative structural biology of photosystem subunits, to provide additional evidence supporting the premise that water oxidation had originated before the ancestral core duplications. Our work suggests that photosynthetic water oxidation originated closer to the origin of life and bioenergetics than can be documented based on phylogenetic or phylogenomic species trees alone.

Whole genome comparison of six Crocosphaera watsonii strains with differing phenotypes

Crocosphaera watsonii, a unicellular nitrogen-fixing cyanobacterium found in oligotrophic oceans, is important in marine carbon and nitrogen cycles. Isolates of C. watsonii can be separated into at least two phenotypes with environmentally important differences, indicating possibly distinct ecological roles and niches. To better understand the evolutionary history and variation in metabolic capabilities among strains and phenotypes, this study compared the genomes of six C. watsonii strains, three from each phenotypic group, which had been isolated over several decades from multiple ocean basins. While a substantial portion of each genome was nearly identical to sequences in the other strains, a few regions were identified as specific to each strain and phenotype, some of which help explain observed phenotypic features. Overall, the small-cell type strains had smaller genomes and a relative loss of genetic capabilities, while the large-cell type strains were characterized by larger genomes, some genetic redundancy, and potentially increased adaptations to iron and phosphorus limitation. As such, strains with shared phenotypes were evolutionarily more closely related than those with the opposite phenotype, regardless of isolation location or date. Unexpectedly, the genome of the type-strain for the species, C. watsonii WH8501, was quite unusual even among strains with a shared phenotype, indicating it may not be an ideal representative of the species. The genome sequences and analyses reported in this study will be important for future investigations of the proposed differences in adaptation of the two phenotypes to nutrient limitation, and to identify phenotype-specific distributions in natural Crocosphaera populations.

Introduction of the 305Arg-->305Ser mutation in the large extrinsic loop E of the CP43 protein of Synechocystis sp. PCC 6803 leads to the loss of cytochrome c(550) binding to Photosystem II

CP43, a component of Photosystem II (PSII) in higher plants, algae and cyanobacteria, is encoded by the psbC gene. Previous work demonstrated that alteration of an arginine residue occurring at position 305 to serine produced a strain (R305S) with altered PSII characteristics including lower oxygen-evolving activity, fewer assembled reaction centers, higher sensitivity to photoinactivation, etc. [Biochemistry 38 (1999) 1582]. Additionally, it was determined that the mutant exhibited an enhanced stability of its S2 state. Recently, we observed a significant chloride effect under chloride-limiting conditions. The mutant essentially lost the ability to grow photoautotrophically, assembled fewer fully functional PSII reaction centers and exhibited a very low rate of oxygen evolution. Thus, the observed phenotype of this mutation is very similar to that observed for the Delta(psb)V mutant, which lacks cytochrome c550 (Biochemistry 37 (1998) 1551). A His-tagged version of the R305S mutant was produced to facilitate the isolation of PSII particles. These particles were analyzed for the presence of cytochrome c550. Reduced minus oxidized difference spectroscopy and chemiluminescence examination of Western blots indicated that cytochrome c550 was absent in these PSII particles. Whole cell extracts from the R305S mutant, however, contained a similar amount of cytochrome c550 to that observed in the control strain. These results indicate that the mutation R305S in CP43 prevents the strong association of cytochrome c550 with the PSII core complex. We hypothesize that this residue is involved in the formation of the binding domain for the cytochrome.

Genome-wide dynamic transcriptional profiling of the light-to-dark transition in Synechocystis sp. strain PCC 6803

We report the results of whole-genome transcriptional profiling of the light-to-dark transition with the model photosynthetic prokaryote Synechocystis sp. strain PCC 6803 (Synechocystis). Experiments were conducted by growing Synechocystis cultures to mid-exponential phase and then exposing them to two cycles of light/dark conditions, during which RNA samples were obtained. These samples were probed with a full-genome DNA microarray (3,169 genes, 20 samples) as well as a partial-genome microarray (88 genes, 29 samples). We concluded that (i) 30-min sampling intervals accurately captured transcriptional dynamics throughout the light/dark transition, (ii) 25% of the Synechocystis genes (783 genes) responded positively to the presence of light, and (iii) the response dynamics varied greatly for individual genes, with a delay of up to 120 to 150 min for some genes. Four classes of genes were identified on the basis of their dynamic gene expression profiles: class I (108 genes, 30-min response time), class II (279 genes, 60 to 90 min), class III (258 genes, 120 to 150 min), and class IV (138 genes, 180 min). The dynamics of several transcripts from genes involved in photosynthesis and primary energy generation are discussed. Finally, we applied Fisher discriminant analysis to better visualize the progression of the overall transcriptional program throughout the light/dark transition and to determine those genes most indicative of the lighting conditions during growth.

Photosystem II: the solid structural era

Understanding the precise role of photosystem II as an element of oxygenic photosynthesis requires knowledge of the molecular structure of this membrane protein complex. The past few years have been particularly exciting because the structural era of the plant photosystem II has begun. Although the atomic structure has yet to be determined, the map obtained at 6 A resolution by electron crystallography allows assignment of the key reaction center subunits with their associated pigment molecules. In the following, we first review the structural details that have recently emerged and then discuss the primary and secondary photochemical reaction pathways. Finally, in an attempt to establish the evolutionary link between the oxygenic and the anoxygenic photosynthesis, a framework structure common to all photosynthetic reaction centers has been defined, and the implications have been described.

Identification of iron-responsive, differential gene expression in the cyanobacterium Synechocystis sp. strain PCC 6803 with a customized amplification library

We describe the use of a method called differential expression using customized amplification library (DECAL) to study the global changes in gene expression in iron-deficient versus iron-reconstituting cells of Synechocystis sp. strain PCC 6803. We identified a number of genes, such as isiA, idiA, psbA, cpcG, and slr0374, whose expression either increased or decreased in response to iron availability. Further analysis led to the identification of additional genes related to those identified by DECAL (e.g., psbC, psbO, psaA, apcABC, cpcBAC1C2D, and nblA) that were differentially regulated by iron availability. Expression of cpcG, psbC, psbO, psaA, apcABC, and cpcBAC1C2D increased, whereas that of isiA, idiA, nblA, psbA, and slr0374 decreased, in iron-reconstituting cells. S1 nuclease protection studies showed that increased transcript levels of psbA in iron-deficient cells was due to the increased expression of both psbA2 and psbA3 genes, although the steady-state level of psbA2 remained higher than that of psbA3.

Transcriptional and translational regulation of photosystem I and II genes in light-dark- and continuous-light-grown cultures of the unicellular cyanobacterium Cyanothece sp. strain ATCC 51142

Cyanothece sp. strain ATCC 51142, a unicellular, diazotrophic cyanobacterium, demonstrated extensive metabolic periodicities of photosynthesis, respiration, and nitrogen fixation when grown under N2-fixing conditions. This report describes the relationship of the biosynthesis of photosynthesis genes to changes in the oligomerization state of the photosystems. Transcripts of the psbA gene family, encoding the photosystem II (PSII) reaction center protein D1, accumulated primarily during the light period, and net transcription reached a peak between 2 to 6 h in the light in light-dark (LD) growth and between 4 to 10 h in the subjective light when grown under continuous light (LL). The relative amount of the D1 protein (form 1 versus form 2) appeared to change during this diurnal cycle, along with changes in the PSII monomer/dimer ratio. D1 form 1 accumulated at approximately equal levels throughout the 24-h cycle, whereas D1 form 2 accumulated at significantly higher levels at approximately 8 to 10 h in the light or subjective light. The psbD gene, encoding the reaction center protein D2, also demonstrated differences between the two copies of this gene, with one copy transcribed more heavily around 6 to 8 h in the light. Accumulation of the PSI reaction center proteins PsaA and PsaB was maximal in the dark or subjective-dark periods, a period during which PSI was primarily in the trimeric form. We conclude that photosystem organization changes during the diurnal cycle to favor either noncyclic electron flow, which leads to O2 evolution and CO2 fixation, or cyclic electron flow, which favors ATP synthesis.

Genetic structure of the dnaA region of the cyanobacterium Synechocystis sp. strain PCC6803

We have cloned and sequenced the dnaA region of Synechocystis sp. strain PCC6803, a bacterium with a light-dependent cell cycle. The dnaA gene product, DnaA, is the central factor for replication initiation in bacteria. The deduced amino acid sequence of the protein encoded by the cyanobacterial dnaA gene is 45% identical to DnaA of Bacillus subtilis and fits very well into the homology pattern of the known eubacterial DnaA proteins. The genetic environment of the Synechocystis sp. strain PCC6803 dnaA gene is completely different from the one in other eubacteria. An open reading frame of unknown function, orf134, was detected upstream of dnaA. The purT gene homolog encoding the glycinamide ribonucleotide transformylase T starts about 200 bp away from this open reading frame in the opposite direction. Downstream of the dnaA gene we detected the start of the psbDC operon, which codes for the photosystem II reaction center proteins D2 and CP43 that are involved in the positioning of chlorophyll a.

Physical and genetic map of the chromosome of the unicellular cyanobacterium Synechocystis sp. strain PCC 6803

A combined physical and genetic map of the cyanobacterium Synechocystis sp. strain PCC 6803 chromosome was constructed. An estimated genome size of 3.82 Mb was obtained by summing the sizes of 25 MluI or 40 NotI fragments seen by pulsed-field electrophoresis. The order of the restriction fragments was determined by using two independent experimental approaches: pulsed-field fragment hybridization and linking clone analysis. The relative positions of 30 known genes or gene clusters were localized.

Molecular topology of the Photosystem II chlorophyll a binding protein, CP 43: Topology of a thylakoid membrane protein

We have used antibodies generated against synthetic peptides to determine the topology of the 43 kD chlorophyll a binding protein (CP 43) of Photosystem II. Based on the pattern of proteolytic fragments detected (on western blots) by peptide specific antibodies, a six transmembrane span topological model, with the amino and carboxyl termini located on the stromal membrane surface, is predicted. This structure is similar to that predicted for CP 47, a PS II chlorophyll a binding protein (Bricker T (1990) Photosynth Res 24: 1-13). The model is discussed in reference to the possible location of chlorophyll binding sites.

Photosystem II genes isiA, psbDI and psbC in Anabaena sp. PCC 7120: cloning, sequencing and the transcriptional regulation in iron-stressed and iron-repleted cells

Under conditions of iron deprivation cyanobacteria produce flavodoxin to replace ferredoxin as the terminal electron acceptor of photosynthesis. In unicellular cyanobacteria, the gene for flavodoxin is the second open reading frame in a dicistronic operon whose transcription is tightly regulated by iron. The first gene, isiA, produces a protein that is very similar to CP43, a chlorophyll-binding, antenna protein of the photosystem II reaction center. In the filamentous, heterocystous cyanobacterium Anabaena sp. PCC 7120, isiA and the gene for flavodoxin are located in separate operons with independent promoters. In this paper, we report on the sequence of isiA and show that it is found in a monocistronic operon that is transcriptionally regulated to be expressed under iron stress but does not produce detectable transcripts under conditions of iron repletion. We also report on the sequence, organization and expression of the gene that codes for CP43, psbC. In Anabaena sp. PCC 7120, psbC has a genetic organization similar to that of other cyanobacteria and higher plants; the 5' end of psbC overlaps the 3' end of psbDI. Transcriptional analysis of the psbDC operon showed that it is constitutively expressed in both iron-repleted and iron-stressed conditions; however, a new monocistronic transcript was detected that contains psbC and is preferentially expressed under iron stress conditions.

psbD sequences of Bumilleriopsis filiformis (Heterokontophyta, Xanthophyceae) and Porphyridium purpureum (Rhodophyta, Bangiophycidae): evidence for polyphyletic origins of plastids

The nucleotide sequences of the plastidal psbD genes of Bumilleriopsis filiformis and Porphyridium purpureum (encoding the D2 protein of photosystem II) are reported in this paper. The Bumilleriopsis sequence clusters together with Porphyridium when a most parsimonious protein tree of D2 sequences is constructed. A composite D1/D2 protein-similarity network reveals that neither the three red algal sequences nor the two heterokontophyte sequences (Bumilleriopsis, xanthophytes and Ectocarpus, phaeophytes) group together. Therefore, the Heterokontophyta and Rhodophyta may be heterogeneous groups. Instead, it emerges that the D1/D2 proteins of Porphyridium and Bumilleriopsis clearly form a tight cluster. D1 and D2 proteins apparently do not provide a reliable molecular clock. These results fit into hypotheses proposing a polyphyletic origin for complex plastids, even among the supposedly "natural" group of heterokontophytes.

Deletion mutations in a long hydrophilic loop in the photosystem II chlorophyll-binding protein CP43 in the cyanobacterium Synechocystis sp. PCC 6803

In order to investigate the role and function of the hydrophilic region between transmembrane regions V and CI in the photosystem II core antenna protein CP43, we introduced eight different deletions in psbC of Synechocystis sp; PCC 6803 resulting in a loss of 7-11 codons in evolutionary conserved domains in this region. All deletions resulted in an obligate photoheterotrophic phenotype (requirement of glucose for cell growth) and the absence of any detectable oxygen evolution activity. The various deletion mutations showed a different impact on the amount of CP43 in the thylakoid, ranging from wild-type levels of (a now slightly smaller) CP43 to no detectable CP43 at all. All deletions led to a decrease in the amount of the D1 and D2 proteins in the thylakoids with a larger effect on D2 than on D1. CP47, the other major chlorophyll-binding protein, was present in reduced but significant amounts in the thylakoid. Herbicide binding (diuron) was lost in all but one mutant indicating the PSII components are not assembled into functionally intact complexes. Fluorescence-emission spectra confirmed this notion. This indicates that the large hydrophilic loop of CP43 plays an important role in photosystem II, and even though a shortened CP43 is present in thylakoids of most mutants, functional characteristics resembled that of a mutant with interrupted psbC.

Analysis of chimeric spinach/cyanobacterial CP43 mutants of Synechocystis sp. PCC 6803: the chlorophyll-protein CP43 affects the water-splitting system of Photosystem II

Mutants of the cyanobacterium Synechocystis sp. PCC 6803 have been generated in which parts of psbC (the gene encoding the Photosystem II chlorophyll-protein CP43) have been replaced with the homologous gene fragment from spinach. Upon the replacement of all but the 3' 84 bp of the cyanobacterial psbC gene with the homologous fragment from spinach, an obligate photoheterotrophic mutant was generated. Two photoautotrophic derivatives of this mutant were made reincorporating 3' cyanobacterial sequences back into the spinach psbC gene of the mutant. These two mutants are similar to each other, carrying a chimeric CP43 with the N-terminal half from spinach. These mutants are photosynthetically active at a rate of about half that of wild type, which correlates with a decreased Photosystem II/chlorophyll ratio in these mutants. Thylakoids from the chimeric mutants contain a CP43 protein which migrates slightly more slowly on SDS-polyacrylmide gels than the native Synechocystis CP43. Interestingly, these mutants show significant shifts in thermoluminescence peaks, reflecting altered thermodynamic properties of the back reaction between the acceptor side and the water-splitting system. On the basis of the oscillations of these shifts with number of flashes, we conclude that S2 is stabilized and S3 is destabilized in these mutants. This represents evidence for an involvement of CP43 in events associated with water splitting.

Light-regulated expression of the psbD gene family in Synecbococcus sp. strain PCC 7942: evidence for the role of duplicated psbD genes in cyanobacteria

The genome of the cyanobacterium Synechococcus sp. strain PCC 7942 contains two psbD genes encoding the D2 protein of the photosystem II reaction center: psbDI, which is cotranscribed as a discistronic message with psbC (the gene encoding CP43, a chlorophyll-a binding protein), and psbDII, which is monocistronic. Northern blot analysis of psbD transcripts showed that the two genes responded differently when wild-type cells were shifted from moderate to high light intensity. Whereas psbDII transcripts increased 500% relative to unshifted control cells, psbDI-psbC transcripts remained unchanged. The beta-galactosidase activities expressed from translational fusions between the psbD genes and the Escherichia coli lacZ reporter gene displayed responses similar to those seen in the RNA. D2 protein levels in thylakoid membranes from wild-type cells increased to 250% of those of the unshifted control cells 12 h after a shift to high light intensities. In contrast, in a mutant strain (AMC016) that carries an inactive psbDII gene, D2 levels decreased by 50% under identical conditions. These results suggested that induction of psbDII gene expression by light can serve as a supplementary system for maintaining a functional photosystem II reaction center at high light intensity. This hypothesis was corroborated by mixed-culture experiments, in which AMC016 cells competed poorly with wild-type cells at high light intensity. These data suggest for the first time that differential expression of members of a cyanobacterial gene family serves to maintain a functional PSII reaction center under diverse environmental conditions.

Nucleotide sequence and expression of the two genes encoding D2 protein and the single gene encoding the CP43 protein of Photosystem II in the cyanobacterium synechococcus sp. PCC 7002

The unicellular photoheterotrophic cyanobacterium Synechococcus sp. PCC 7002 was shown to encode two genes for the Photosystem II reaction center core protein D2 and one gene for the reaction center chlorophyhll-binding protein CP43. These three genes were cloned and their DNA sequences determined along with their flanking DNA sequences. Northern hybridization experiments show that both genes which encode D2, psbD1 and psbD2, are expressed at roughly equivalent levels. For each of the two psbD genes, there are 18 nucleotide differences among the 1059 nucleotides which are translated. The DNA sequences surrounding the coding sequences are nearly 70% divergent. Despite the DNA sequence differences in the genes, the proteins encoded by the two genes are predicted to be identical. The proteins encoded by psbD1 and psbD2 are ∼92% homologous to other sequenced cyanobacterial psbD genes and ∼86% homologous to sequenced chloroplast-encoded psbD genes.The single gene for CP43, psbC, overlaps the 3' end of psbD1 and is co-transcribed with it. Results from previous sequencing of psbC genes encoded by chloroplasts suggest that the 5' end of the psbC gene overlaps the 3' end of the coding sequence of psbD by ∼50 nucleotides. In Synechococcus sp. PCC 7002, the methionine codon previously proposed to be the start codon for psbC is replaced by an ACG (threonine) codon. We propose an alternative start for the psbC gene at a GTG codon 36 nucleotides downstream from the threonine codon. This GTG codon is preceded by a consensus E. coli-like ribosome binding sequence. Both the GTG start codon and its preceding ribosome binding sequence are conserved in all psbC genes sequenced from cyanobacteria and chloroplasts. This suggests that all psbC genes start at this alternative GTG codon. Based on this alternative start codon, the gene product is ∼85% identical to other cyanobacterial psbC gene products and ∼77% identical to eucaryotic chloroplast-encoded psbC gene products.

The structure and function of CPa-1 and CPa-2 in Photosystem II

This review presents a summary of recent investigations examining the structure and function of the chlorophyll-proteins CPa-1 (CP47) and CPa-2 (CP43). Comparisons of the derived amino acid sequences of these proteins suggest sites for chlorophyll binding and for interactions between these chlorophyll-proteins and other Photosystem II components. Hydropathy plot analysis of these proteins allows the formulation fo testable hypotheses concerning their topology and orientation within the photosynthetic membrane. The role of these chlorophyll-proteins as interior light-harvesting chlorophyll-a antennae for Photosystem II is examined and other possible additional roles for these important Photosystem II components are discussed.

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.

The psbC start codon in Synechocystis sp. PCC 6803

The translation start codon for psbC, the gene encoding CP43, a chlorophyll-binding protein of photosystem II, has been identified for the cyanobacterium Synechosystis sp. PCC 6803 using site-directed mutagenesis. An AUG codon, about 50 bases upstream from the end of psbD-I had previously been assumed to be the translation start site of psbC. However, the fact that the AUG codon is not present in psbC from several other organisms, whereas a GUG codon 14 bases upstream from the end of psbD-I is strictly conserved suggests that CP43 translation starts at the latter codon. Mutation of GUG, but not of AUG, led to a loss of CP43 and photoautotrophic growth, indicating that the GUG codon is the sole initiation site for translation of the CP43 protein in Synechocystis sp. PCC 6803.

Physical and genetic maps of the genome of the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120

A restriction map of the chromosome of the cyanobacterium Anabaena sp. strain PCC 7120 was generated by the determination of the order of restriction fragments of the infrequently cleaving restriction endonucleases AvrII, SalI, and PstI. These restriction fragments were resolved by the pulsed homogeneous orthogonal field gel electrophoresis system of pulsed-field gel electrophoresis (I. Bancroft and C. P. Wolk, Nucleic Acids Res. 16:7405-7418, 1988). Other infrequently cutting restriction endonucleases (AhaII, Asp718, AsuII, BanII, BglII, BssHII, FspI, NcoI, NruI, SphI, SplI, SstII, and StuI) were identified that could prove useful for higher-resolution mapping. The chromosome was found to be 6.4 megabases in size and circular. Three apparently circular large plasmids (410, 190, and 110 kilobases) were also identified. A genetic map was constructed by hybridization with gene-specific probes. Genes encoding components of the photosynthetic electron transport chain were not within a single tight cluster.

Evolutionary relationship of psbA genes from cyanobacteria, cyanelles and plastids

The psbA gene is part of the reaction center of photosystem II in cyanobacteria and the plastids of higher plants. Its primary sequence is highly conserved among all species investigated so far and its sequence shows homologies with the L and M subunits of the reaction center of photosynthetic bacteria. We have analyzed the psbA homolog from a eukaryotic alga, Cyanophora paradoxa, where the gene is encoded on cyanelle DNA. These cyanelles are surrounded by a murein sacculus and resemble cyanobacteria in many other characteristics, although they are genuine organelles that functionally replace plastids. Analysis of the gene revealed a psbA protein identical in length (360 codons) with the cyanobacterial counterpart. The overall sequence identity is, however, more pronounced between cyanelle psbA and the shorter (353 amino acids) psbA product found in higher plants. These data strongly support the postulated bridge position of cyanelles between chloroplasts and free-living cyanobacteria.

Mutagenesis by random cloning of an Escherichia coli kanamycin resistance gene into the genome of the cyanobacterium Synechocystis PCC 6803: selection of mutants defective in photosynthesis

Photosynthetic mutants of the cyanobacterium Synechocystis PCC 6803 were produced by a random cartridge mutagenesis method leading to gene inactivation. This procedure relies on random ligation of an Escherichia coli kanamycin resistance (Kmr) gene to restriction fragments of genomic DNA from the host. Then recombination occurring during transformation promotes integration of the marker gene into the genome of the recipient cells. Several mutants impaired in photosynthesis were obtained by this procedure. All are partially or totally defective in photosystem II activity and some of them also harbour a functionally modified photosystem I. Restriction and recombination data showed that one mutant (AK1) is best explained as an insertion of the Kmr gene into an AvaII restriction site of the gene psbD-1. All others harbour a deletion, ranging from at least 1.15 kb (AK3) to more than 50 kb (AK9), which partly or fully overlaps the genes psbB and/or psbD-1, depending on the mutant. A genetic-physical map of the more than 60 kb region of the cyanobacterial genome harbouring the genes psbB, psbC and psbD-1 was constructed by combining published sequence data on these genes with the results of recombination and restriction mapping.

Nucleotide sequence and transcript analysis of three photosystem II genes from the cyanobacterium Synechococcus sp. PCC7942

The genome of the cyanobacterium Synechococcus sp. PCC7942 contains two genes encoding the D2 polypeptide of photosystem II (PSII), which are designated here as psbDI and psbDII. The psbDI gene, like the psbD gene of plant chloroplasts, is cotranscribed with and overlaps the open reading frame of the psbC gene, encoding the PSII protein CP43. The psbDII gene is not linked to psbC, and appears to be transcribed as a monocistronic message. The two psbD genes encode identical polypeptides of 352 amino acids, which are 86% conserved with the D2 polypeptide of spinach. In plants, the translational start codon of the psbC gene has been reported to be an ATG codon 50 bp upstream from the end of the psbD gene. This triplet is not present in the psbDI sequence of Synechococcus sp., but is replaced by ACG, a codon which is very unlikely to initiate translation. Translation of the psbC gene may begin at a GTG codon which overlap the psbDI open reading frame by 14 bp and is preceded by a block of homology to the 3' end of the 16S ribosomal RNA, a potential ribosome-binding site. There are only two bp differences between the sequences of the two psbD genes; one of these results in substitution in psbDII of GCG for the presumed GTG start codon in psbDI.