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PubMed1997

Photosystem I of Synechococcus elongatus at 4 A resolution: comprehensive structure analysis.

An improved structural model of the photosystem I complex from the thermophilic cyanobacterium Synechococcus elongatus is described at 4 A resolution. This represents the most complete model of a photosystem presently available, uniting both a photosynthetic reaction centre domain and a core antenna system. Most constituent elements of the electron transfer system have been located and their relative centre-to-centre distances determined at an accuracy of approximately 1 A. These include three pseudosymmetric pairs of Chla and three iron-sulphur centres, FX, FA and FB. The first pair, a Chla dimer, has been assigned to the primary electron donor P700. One or both Chla of the second pair, eC2 and eC'2, presumably functionally link P700 to the corresponding Chla of the third pair, eC3 and eC'3, which is assumed to constitute the spectroscopically-identified primary electron acceptor(s), A0, of PSI. A likely location of the subsequent phylloquinone electron acceptor, QK, in relation to the properties of the spectroscopically identified electron acceptor A1 is discussed. The positions of a total of 89 Chla, 83 of which constitute the core antenna system, are presented. The maximal centre-to-centre distance between antenna Chla is < or = 16 A; 81 Chla are grouped into four clusters comprising 21, 23, 17 and 20 Chla, respectively. Two "connecting" Chla are positioned to structurally (and possibly functionally) link the Chla of the core antenna to those of the electron transfer system. Thus the second and third Chla pairs of the electron transfer system may have a dual function both in energy transfer and electron transport. A total of 34 transmembrane and nine surface alpha-helices have been identified and assigned to the 11 subunits of the PSI complex. The connectivity of the nine C-terminal (seven transmembrane, two "surface") alpha-helices of each of the large core subunits PsaA and PsaB is described. The assignment of the amino acid sequence to the transmembrane alpha-helices is proposed and likely residues involved in co-ordinating the Chla of the electron transfer system discussed.

Bacterial Proteins↗
PubMed1994

Sequence of a 2.6-kb cryptic plasmid from a marine cyanobacterium Synechococcus sp.

We have shown previously that the copy number of plasmid pSY10 from the marine cyanobacterium Synechococcus sp. NKBG 042902 is dependent on the salinity of the growth medium. We report here the complete nucleotide sequence (2561 bp) of this plasmid. The longest open reading frame, ORF-B (1.08 kb), occurs on a 1.6-kb EcoRI fragment. This ORF encodes a putative protein which is 360 aa residues in length and is 37.8% homologous to the replication protein of plasmid pCA2.4 from Synechocystis sp. strain PCC 6803, 35.8% homologous to an ORF from the Nostoc plasmid pGL2, and 33.2% homologous to the ORF of a plasmid from Lactobacillus plantarum, pC30il. Highly conserved regions of amino acid sequence were also found between ORF-B and other bacterial plasmids.

Amino Acid Sequence↗
PubMed1994

A new Synechococcus cyanophage from a reservoir in Korea.

A unicellular cyanobacterium (Synechococcus) and its cyanophage were both isolated from a reservoir in Korea. Although morphologically similar to AS-1, the cyanophage differs from cyanophage AS-1 in some respects. The burst size in the light is approximately 100 plaque-forming units (PFU)/cell. Replication of the virus also occurs in the dark, releasing about 10% of the virus particles observed in the light. Na+ is not necessary for adsorption.

Bacteriophages↗
PubMed1995

Transformation of Synechococcus with a gene for choline oxidase enhances tolerance to salt stress.

Choline oxidase, isolated from the soil bacterium Arthrobacter globiformis, converts choline to glycinebetaine (N-trimethylglycine) without a requirement for any cofactors. The gene for this enzyme, designated codA, was cloned and introduced into the cyanobacterium Synechococcus sp. PCC 7942. The codA gene was expressed under the control of a strong constitutive promoter, and the transformed cells accumulated glycinebetaine at intracellular levels of 60-80 mM. Consequently the cells acquired tolerance to salt stress, as evaluated in terms of growth, accumulation of chlorophyll and photosynthetic activity.

Adaptation, Physiological↗
PubMed1995

Cloning and sequencing of the nitrate transport system from the thermophilic, filamentous cyanobacterium Phormidium laminosum: comparative analysis with the homologous system from Synechococcus sp. PCC 7942.

A genomic region from the filamentous, thermophilic non-N2-fixing cyanobacterium Phormidium laminosum was cloned and sequenced. It includes the nitrite reductase gene (nirA) and three other genes (nrtA, B and C) located downstream of nirA, which are related to the nitrate transport system on the basis of a comparison with the homologous system from Synechococcus sp. PCC 7942. No additional nitrate assimilation-related genes were identified in about 5 kb sequenced downstream of nrtC. All four genes are arranged as an operon with a promoter-like region upstream of the nirA gene. Transcripts of these nitrate assimilation genes accumulated after long periods of nitrogen starvation. This operon also contains inverted repeat sequences in the intercistronic regions which might be involved in mRNA processing or stability.

Adenosine Triphosphate↗
PubMed1993

A conjugative plasmid vector for promoter analysis in several cyanobacteria of the genera Synechococcus and Synechocystis.

A promoter-probe vector, pSB2A, based on the plasmid RSF1010 and the promoterless chloramphenicol acetyl transferase (cat) reporter gene, has been constructed. pSB2A appeared to be most efficiently transferred by conjugation to the widely used cyanobacteria Synechocystis strains PCC6803 (S.6803) and PCC6714 (S.6714) and Synechococcus strains PCC7942 (S.7942) and PCC6301 (S.6301), where it replicates stably even though it contains no cyanobacterial DNA. Using pSB2A we found that (1) a light-regulated promoter from S.6803 remains controlled by light intensity in S.7942 while it is silent in Escherichia coli, and (2) the E. coli tac promoter behaves as a strong and light-independent promoter in the four cyanobacterial hosts tested.

Conjugation, Genetic↗
PubMed1992

Cloning and sequencing of the petBD operon from the cyanobacterium Synechococcus sp. PCC 7002.

The genes encoding the photosynthetic cytochrome b6 (petB) and subunit 4 (petD) have been cloned and sequenced from the unicellular, photoheterotrophic, transformable cyanobacterium Synechococcus sp. PCC 7002, formerly designated Agmenellum quadruplicatum. The gene arrangement was found to be similar to that reported in the cyanobacterium Nostoc PCC 7906. The DNA and derived protein sequences were compared to chloroplast and the other cyanobacterial sequences. By pulsed-field electrophoresis, the petBD operon and the petCA operon, encoding the Rieske iron-sulfur protein and cytochrome f, were found to be located on separate, unlinked, Not I-digested DNA fragments. The petBD operon was found on the third largest Not I fragment (NC-325) while the petCA operon was found on the second largest Not I fragment (NB-370). These results suggest the two operons are not in proximity. The 1.35 kb transcript was shown to be light-regulated. Transcripts from cells grown under constant illumination showed a decrease in petB transcript levels to undetectable levels within 2 h after the cells were placed in the dark. Upon reillumination, transcript levels rose to three-fold over that seen initially under constant illumination.

Amino Acid Sequence↗
PubMed1996

Over-production of the D1:2 protein makes Synechococcus cells more tolerant to photoinhibition of photosystem II.

Over-expression of the psbAIII gene encoding for the D1 protein (form II; D1:2) of the photosystem II reaction centre in the Synechococcus sp. PCC 7942 was studied using a tac promoter and the lacIQ system. Over-expression was induced with 40 microgram/ml IPTG in the growth medium for either 6 or 12 h at growth irradiance (50 mumol photons m-2 s-1). This treatment doubled the amount of psbAII/III mRNA and the D1:2 protein in membranes but decreased the amount of psbAI messages and the D1:1 protein. The total amount of both heterodimeric reaction centre proteins, D1 and D2, remained constant under growth light conditions, indicating that the number of PSII centres in the membranes was not affected, only the form of the D1 protein was changed from D1:1 to D1:2 in most centres. When the cells were photoinhibited either at 500 or 1000 mumol photons m-2 s-1, in the presence or absence of the protein synthesis inhibitor lincomycin, the D1:2 protein remained at a higher level in cells in which over-expression had been induced by IPTG. These cells were also less prone to photoinhibition of PSII. It is suggested that the tolerance of cells to photoinhibition increases when most PSII reaction centres contain the D1:2 protein at the beginning of high irradiance. This tolerance is further strengthened by maintaining psbAIII gene over-expression during the photoinhibitory treatment.

Amino Acid Sequence↗
PubMed1992

Conjugative gene transfer in marine cyanobacteria: Synechococcus sp., Synechocystis sp. and Pseudanabaena sp.

Versatility of gene transfer by transconjugation in marine cyanobacteria was demonstrated. In this study, seven different marine cyanobacteria were used as recipient cells. First, transconjugation was carried out using the mobilizable transposon (Tn5) carrying plasmid pSUP1021. Transconjugates were observed in all marine cyanobacteria tested. Second, the broad-host-range vector pKT0230 (IncQ) was tested for transconjugation. pKT230 has been successfully transferred in a marine cyanobacterium Synechococcus sp. NKBG15041C, and replicated as an autonomous replicon without alteration in the restriction enzyme pattern. A maximum transfer efficiency of 5.2 x 10(-4) transconjugants/recipient cell was observed, when mating was performed on agar plates containing low salinity (0.015 M NaCl) medium. This is the first study to demonstrate gene transfer in marine cyanobacteria via transconjugation.

Biotechnology↗
PubMed1994

Isolation and characterisation of porin from the outer membrane of Synechococcus PCC 6301.

Pore-forming protein (porin) was isolated from N,N-dimethyl-dodecylaminoxid (LDAO)-extracted outer membranes of Synechococcus PCC 6301 and purified by ion exchange chromatography on DEAE-Sephacel column. The apparent molecular mass on SDS-PAGE was determined to be about 52,000. The native porin was reconstituted into black lipid bilayer membranes and showed a single-channel conductance of 5.5 nS in 1 M KCl. The porin was found to be N-terminally blocked. The C-terminal amino acid sequence was identified as Phe-Thr-Phe. Amino acid analysis suggested that the porin protein consists of about 420 amino acid residues, yielding a polarity of 43.6% and a molecular mass of 45,000 in contrast to the mobility on SDS-PAGE.

Amino Acid Sequence↗
PubMed1993

Identification and characterization of a gene cluster involved in nitrate transport in the cyanobacterium Synechococcus sp. PCC7942.

The nrtA gene, which has been proposed to be involved in nitrate transport of Synechococcus sp. PCC7942 (Anacystis nidulans R2), was mapped at 3.9 kb upstream of the nitrate reductase gene, narB. Three closely linked genes (designated nrtB, nrtC, and nrtD), which encode proteins of 279, 659, and 274 amino acids, respectively, were found between the nrtA and narB genes. NrtB is a hydrophobic protein having structural similarity to the integral membrane components of bacterial transport systems that are dependent on periplasmic substrate-binding proteins. The N-terminal portion of NrtC (amino acid residues 1-254) and NrtD are 58% identical to each other in their amino acid sequences, and resemble the ATP-binding components of binding protein-dependent transport systems. The C-terminal portion of NrtC is 30% identical to NrtA. Mutants constructed by interrupting each of nrtB and nrtC were unable to grow on nitrate, and the nrtD mutant required high concentration of nitrate for growth. The rate of nitrate-dependent O2 evolution (photosynthetic O2 evolution coupled to nitrate reduction) in wild-type cells measured in the presence of L-methionine D,L-sulfoximine and glycolaldehyde showed a dual-phase relationship with nitrate concentration. It followed saturation kinetics up to 10 mM nitrate (the concentration required for half-saturation = 1 microM), and the reaction rate then increased above the saturation level of the first phase as the nitrate concentration increased. The high-affinity phase of nitrate-dependent O2 evolution was absent in the nrtD mutant. The results suggest that there are two independent mechanisms of nitrate uptake and that the nrtB-nrtC-nrtD cluster encodes a high-affinity nitrate transport system.

Amino Acid Sequence↗
PubMed1992

Light-regulated expression of the psbD gene family in Synechococcus 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.

Blotting, Northern↗
PubMed1995

Factors regulating cryIVB expression in the cyanobacterium--Synechococcus PCC 7942.

The expression of the larvicidal Bacillus thuringiensis subsp. israelensis cryIVB gene in cyanobacteria has been suggested to be an effective means of controlling mosquito populations. Using a variety of cryIVB constructs, in this study we have examined the effect of Synechococcus PCC 7942 culture age on intracellular toxin levels and have attempted to determine the mechanisms by which cryIVB gene expression is regulated. The data suggest that specific degradation of the cryIVB mRNA limits toxin production; however, the addition of cyanobacterial 3' untranslated DNA sequences to the cryIVB gene did not improve mRNA stability or toxin levels. An analysis of the cryIVB sequence and comparison of codon usage patterns with highly expressed cyanobacterial genes suggest that inefficient translation and intragenic ribosomal binding sites impede protein synthesis and result in rapid turnover of the toxin mRNA.

Bacillus thuringiensis↗
PubMed1992

Clustering of genes involved in nitrate assimilation in the cyanobacterium Synechococcus.

A region of the genome of the cyanobacterium Synechococcus R2, that bears a cluster of genes involved in nitrate assimilation, has been cloned and the relative positions of some of the genes in the region have been determined. Mutations generated by insertion of an antibiotic-resistance gene cassette into the gene encoding nitrite reductase are associated with reduced expression of nitrate reductase; cotranscription of nitrate assimilation genes in the cluster is inferred from this finding.

Blotting, Southern↗
PubMed1983

Transformation in the cyanobacterium Synechococcus R2: improvement of efficiency; role of the pUH24 plasmid.

Systematic studies of the parameters influencing transformation efficiencies in Synechococcus R2 and its spontaneous pUH24 plasmid-minus variant S60, have led to substantial increases in these frequencies (up to 70 times for R2 and 9 times for S60) compared to those obtained by using previously published procedures. Important factors are the physiological state of the host cells (competence occurs at the transition from first to second exponential phases of growth) and the conditions of expression of the transferred characters (they must be exposed to progressively increasing concentrations of the selective agent). Consistently lower (about 30 times) efficiencies were obtained with the S60 strain, the only difference from the wild-type R2 that could be detected. Physiological functions of the pUH24 plasmid are discussed. The interest of the S60 strain as a host in gene cloning work is discussed in the light of published work suggesting inter-vector recombination in R2.

Cyanobacteria↗
PubMed1977

Effect of carbon dioxide on pigment and membrane content in Synechococcus lividus.

The effect of carbon dioxide on pigment and membrane content in Synechococcus lividus was studied by depriving cells of CO2 and examining cell populations biochemically and by electron microscopy. After 120 h of CO2 deprivation, S. lividus lost all detectable chlorophyll a and C-phycocyanin. Such bleached cultures were "mustard yellow", the result of approximately 1.8 times more carotenoid per cell than green control cultures. Although cells from beached cultures appeared morphologically identical to control green cells when examined by light microscopy, electron microscopic examination revealed them to be devoid of detectable thylakoid membrane. Thylakoid membrane could not be recovered by physical isolation or revealed by freeze etching of bleached S. lividus. In addition, inclusion bodies characteristically found in S. lividus were also absent. Reintroduction of CO2 into bleached cultures resulted in a rapid resynthesis of both chlorophyll a and C-phycocyanin. Electron microscopic examination of these regreening cultures revealed that thylakoid membrane was also rapidly resynthesized. Growth of regreened cultures did not occur until there was the synthesis of a full complement of chlorophyll a, C-phycocyanin, and thylakoid membrane. A time course study of the cytological events occurring during bleaching and regreening is presented.

Carbon Dioxide↗