Anabaena sp. strain PCC 7120 bifA gene encoding a sequence-specific DNA-binding protein cloned by in vivo transcriptional interference selection

Marine Synechococcus sp. Strain WH7803 Shows Specific Adaptative Responses to Assimilate Nanomolar Concentrations of Nitrate

Marine Synechococcus, together with Prochlorococcus, contribute to a significant proportion of the primary production on Earth. The spatial distribution of these two groups of marine picocyanobacteria depends on different factors such as nutrient availability and temperature. Some Synechococcus ecotypes thrive in mesotrophic and moderately oligotrophic waters, where they exploit both oxidized and reduced forms of nitrogen. Here, we present a comprehensive study, which includes transcriptomic and proteomic analyses of the response of Synechococcus sp. strain WH7803 to nanomolar concentrations of nitrate, compared to micromolar ammonium or nitrogen starvation. We found that Synechococcus has a specific response to a nanomolar nitrate concentration that differs from the response shown under nitrogen starvation or the presence of standard concentrations of either ammonium or nitrate. This fact suggests that the particular response to the uptake of nanomolar concentrations of nitrate could be an evolutionary advantage for marine Synechococcus against Prochlorococcus in the natural environment. IMPORTANCE Marine Synechococcus are a very abundant group of photosynthetic organisms on our planet. Previous studies have shown blooms of these organisms when nanomolar concentrations of nitrate become available. We have assessed the effect of nanomolar nitrate concentrations by studying the transcriptome and proteome of Synechococcus sp. WH7803, together with some physiological parameters. We found evidence that Synechococcus sp. strain WH7803 does sense and react to nanomolar concentrations of nitrate, suggesting the occurrence of specific adaptive mechanisms to allow their utilization. Thus, very low concentrations of nitrate in the ocean seem to be a significant nitrogen source for marine picocyanobacteria.

Heterocyte production, gene expression and phylogeography in Raphidiopsis ( = Cylindrospermopsis) Raciborskii

Raphidiopsis ( = Cylindrospermopsis) raciborskii was described as a subtropical-tropical cyanobacterium, later reported expanding into temperate regions. Heterocyte presence used to distinguish Cylindrospermopsis from the very similar Raphidiopsis, but recently the two genera were recognized as one and unified. This study aimed to investigate how heterocyte production is related to nitrogen (N) limitation in heterocytous and non-heterocytous strains of R.raciborskii. High N-concentrations did not inhibit heterocyte development in some strains, while prolonged N-starvation periods never stimulated production in others. RT-qPCR was used to examine the genetic background, through the expression patterns of nifH, ntcA and hetR. While gene expression increased under N-restriction, N-sufficiency did not suppress nifH transcripts as previously observed in other diazotrophyc cyanobacteria, suggesting that heterocyte production in R. raciborskii is not regulated by N-availability. Heterocytous and non-heterocytous strains were genotypically characterized to assess their phylogenetic relationships,. In the phylogenetic tree, clusters were intermixed and confirmed Raphidiopsis and Cylindrospermopsis as the same genus. The tree supported previous findings of earlier splitting of American strains, while contesting the African origin hypothesis. The existence of two lines of Chinese strains, with distinct evolutionary patterns, is a significant addition that could lead to new hypotheses of the species biogeography.

The small Ca2+-binding protein CSE links Ca2+ signalling with nitrogen metabolism and filament integrity in Anabaena sp. PCC 7120

Background

Filamentous cyanobacteria represent model organisms for investigating multicellularity. For many species, nitrogen-fixing heterocysts are formed from photosynthetic vegetative cells under nitrogen limitation. Intracellular Ca²⁺ has been implicated in the highly regulated process of heterocyst differentiation but its role remains unclear. Ca²⁺ is known to operate more broadly in metabolic signalling in cyanobacteria, although the signalling mechanisms are virtually unknown. A Ca²⁺-binding protein called the Ca²⁺ Sensor EF-hand (CSE) is found almost exclusively in filamentous cyanobacteria. Expression of asr1131 encoding the CSE protein in Anabaena sp. PCC 7120 was strongly induced by low CO₂ conditions, and rapidly downregulated during nitrogen step-down. A previous study suggests a role for CSE and Ca²⁺ in regulation of photosynthetic activity in response to changes in carbon and nitrogen availability.

Results

In the current study, a mutant Anabaena sp. PCC 7120 strain lacking asr1131 (Δcse) was highly prone to filament fragmentation, leading to a striking phenotype of very short filaments and poor growth under nitrogen-depleted conditions. Transcriptomics analysis under nitrogen-replete conditions revealed that genes involved in heterocyst differentiation and function were downregulated in Δcse, while heterocyst inhibitors were upregulated, compared to the wild-type.

Conclusions

These results indicate that CSE is required for filament integrity and for proper differentiation and function of heterocysts upon changes in the cellular carbon/nitrogen balance. A role for CSE in transmitting Ca²⁺ signals during the first response to changes in metabolic homeostasis is discussed.

Expanding the Natural Products Heterologous Expression Repertoire in the Model Cyanobacterium Anabaena sp. Strain PCC 7120: Production of Pendolmycin and Teleocidin B-4

Cyanobacteria are prolific producers of natural products, and genome mining has shown that many orphan biosynthetic gene clusters can be found in sequenced cyanobacterial genomes. New tools and methodologies are required to investigate these biosynthetic gene clusters, and here we present the use of Anabaena sp. strain PCC 7120 as a host for combinatorial biosynthesis of natural products using the indolactam natural products (lyngbyatoxin A, pendolmycin, and teleocidin B-4) as a test case. We were able to successfully produce all three compounds using codon optimized genes from Actinobacteria. We also introduce a new plasmid backbone based on the native Anabaena 7120 plasmid pCC7120ζ and show that production of teleocidin B-4 can be accomplished using a two-plasmid system, which can be introduced by coconjugation.

Broad-host-range vector system for synthetic biology and biotechnology in cyanobacteria

Inspired by the developments of synthetic biology and the need for improved genetic tools to exploit cyanobacteria for the production of renewable bioproducts, we developed a versatile platform for the construction of broad-host-range vector systems. This platform includes the following features: (i) an efficient assembly strategy in which modules released from 3 to 4 donor plasmids or produced by polymerase chain reaction are assembled by isothermal assembly guided by short GC-rich overlap sequences. (ii) A growing library of molecular devices categorized in three major groups: (a) replication and chromosomal integration; (b) antibiotic resistance; (c) functional modules. These modules can be assembled in different combinations to construct a variety of autonomously replicating plasmids and suicide plasmids for gene knockout and knockin. (iii) A web service, the CYANO-VECTOR assembly portal, which was built to organize the various modules, facilitate the in silico construction of plasmids, and encourage the use of this system. This work also resulted in the construction of an improved broad-host-range replicon derived from RSF1010, which replicates in several phylogenetically distinct strains including a new experimental model strain Synechocystis sp. WHSyn, and the characterization of nine antibiotic cassettes, four reporter genes, four promoters, and a ribozyme-based insulator in several diverse cyanobacterial strains.

Hepatotoxic seafood poisoning (HSP) due to microcystins: a threat from the ocean?

Cyanobacterial blooms are a major and growing problem for freshwater ecosystems worldwide that increasingly concerns public health, with an average of 60% of blooms known to be toxic. The most studied cyanobacterial toxins belong to a family of cyclic heptapeptide hepatotoxins, called microcystins. The microcystins are stable hydrophilic cyclic heptapeptides with a potential to cause cell damage following cellular uptake via organic anion-transporting proteins (OATP). Their intracellular biologic effects presumably involve inhibition of catalytic subunits of protein phosphatases (PP1 and PP2A) and glutathione depletion. The microcystins produced by cyanobacteria pose a serious problem to human health, if they contaminate drinking water or food. These toxins are collectively responsible for human fatalities, as well as continued and widespread poisoning of wild and domestic animals. Although intoxications of aquatic organisms by microcystins have been widely documented for freshwater ecosystems, such poisonings in marine environments have only occasionally been reported. Moreover, these poisonings have been attributed to freshwater cyanobacterial species invading seas of lower salinity (e.g., the Baltic) or to the discharge of freshwater microcystins into the ocean. However, recent data suggest that microcystins are also being produced in the oceans by a number of cosmopolitan marine species, so that Hepatotoxic Seafood Poisoning (HSP) is increasingly recognized as a major health risk that follows consumption of contaminated seafood.

in-silico analysis suggests alterations in the function of XisA protein as a possible mechanism of butachlor toxicity in the nitrogen fixing cyanobacterium Anabaena sp. PCC 7120

Butachlor, a commonly used herbicide adversely affects the nitrogen fixing capability of Anabaena, an acclaimed nitrogen fixer in the Indian paddy fields. The nitrogen fixation in Anabaena is triggered by the excision of nifD element by xisA gene leading to rearrangement of nifD forming nifHDK operon in the heterocyst of Anabaena sp. PCC7120. Functional elucidation adjudged through in-silico analysis revealed that xisA belongs to integrase family of tyrosine recombinase. The predicted functional partners with XisA protein that have shown cooccurence with this protein in a network are mainly hypothetical proteins with unknown functions except psaK1 whose exact function in photosystem I is not yet known. The focus of this study was to find out the relation between XisA and butachlor using in-silico approaches. The XisA protein was modeled and its active sites were identified. Docking studies revealed that butachlor binds at the active site of XisA protein hampering its excision ability vis-à-vis nif genes in Anabaena sp. PCC7120. This study reveals that butachlor is not directly involved in hampering the nitrogen fixing ability of Anabaena sp. PCC7120 but by arresting the excision ability of XisA protein necessary for the functioning of nif gene and nitrogen fixation.

NtcA from Microcystis aeruginosa PCC 7806 is autoregulatory and binds to the microcystin promoter

NtcA is a transcription factor that has been found in a diverse range of cyanobacteria. This nitrogen-controlled factor was focused on as a key component in the yet-to-be-deciphered regulatory network controlling microcystin production. Adaptor-mediated PCR was utilized to isolate the ntcA gene from Microcystis aeruginosa PCC 7806. This gene was cloned, and the recombinant (His-tagged) protein was overexpressed and purified for use in mobility shift assays to analyze NtcA binding to putative sites identified in the microcystin mcyA/D promoter region. Autoregulation of NtcA in M. aeruginosa was shown via NtcA binding in the upstream ntcA promoter region. The observation of binding of NtcA to the mcyA/D promoter region has direct relevance for the regulation of microcystin biosynthesis, as transcription of the mcyABCDEFGHIJ gene cluster appears to be under direct control of nitrogen.

Cyanobacterial heterocysts

Many multicellular cyanobacteria produce specialized nitrogen-fixing heterocysts. During diazotrophic growth of the model organism Anabaena (Nostoc) sp. strain PCC 7120, a regulated developmental pattern of single heterocysts separated by about 10 to 20 photosynthetic vegetative cells is maintained along filaments. Heterocyst structure and metabolic activity function together to accommodate the oxygen-sensitive process of nitrogen fixation. This article focuses on recent research on heterocyst development, including morphogenesis, transport of molecules between cells in a filament, differential gene expression, and pattern formation.

Two genes encoding protein kinases of the HstK family are involved in synthesis of the minor heterocyst-specific glycolipid in the cyanobacterium Anabaena sp. strain PCC 7120

The filamentous cyanobacterium Anabaena sp. strain PCC 7120 can fix N(2) under oxic conditions, and the activity of nitrogen fixation occurs exclusively in heterocysts, cells differentiated from vegetative cells in response to a limitation of a combined-nitrogen source in the growth medium. At the late stages of heterocyst differentiation, an envelope layer composed of two glycolipids is formed to limit the entry of oxygen so that the oxygen-sensitive nitrogenase can function. The genome of Anabaena sp. strain PCC 7120 possesses a family of 13 genes (the hstK family), all encoding proteins with a putative Ser/Thr kinase domain at their N termini and a His-kinase domain at their C termini. In this study, we showed that the double mutant D4.3 strain, in which two members of this gene family, pkn44 (all1625) and pkn30 (all3691), were both inactivated, failed to fix N(2) in the presence of oxygen (Fox(-)). In an environment without oxygen, a low level of nitrogenase activity was detectable (Fix(+)). Heterocyst development in the mutant D4.3 was delayed by 24 h and arrested at a relatively early stage without the formation of the glycolipid layer (Hgl(-)). Only the minor species of the two heterocyst-specific glycolipids (HGLs) was missing in the mutant. We propose that DevH, a putative transcription factor, coordinates the synthesis of both HGLs, while Pkn44/Pkn30 and the previously characterized PrpJ may represent two distinct regulatory pathways involved in the synthesis of the minor HGL and the major HGL, respectively.

Ammonium assimilation in cyanobacteria

In cyanobacteria, after transport by specific permeases, ammonium is incorporated into carbon skeletons by the sequential action of glutamine synthetase (GS) and glutamate synthase (GOGAT). Two types of GS (GSI and GSIII) and two types of GOGAT (ferredoxin-GOGAT and NADH-GOGAT) have been characterized in cyanobacteria. The carbon skeleton substrate of the GS-GOGAT pathway is 2-oxoglutarate that is synthesized by the isocitrate dehydrogenase (IDH). In order to maintain the C-N balance and the amino acid pools homeostasis, ammonium assimilation is tightly regulated. The key regulatory point is the GS, which is controlled at transcriptional and posttranscriptional levels. The transcription factor NtcA plays a critical role regulating the expression of the GS and the IDH encoding genes. In the unicellular cyanobacterium Synechocystis sp. PCC 6803, NtcA controls also the expression of two small proteins (IF7 and IF17) that inhibit the activity of GS by direct protein-protein interaction. Cyanobacteria perceive nitrogen status by sensing the intracellular concentration of 2-oxoglutarate, a signaling metabolite that is able to modulate allosterically the function of NtcA, in vitro. In vivo, a functional dependence between NtcA and the signal transduction protein PII in controlling NtcA-dependent genes has been also shown.

Mutagenesis of the cysteine residues in the transcription factor NtcA from Anabaena PCC 7120 and its effects on DNA binding in vitro

NtcA is a transcription factor found in a wide variety of cyanobacteria. It is a key component in the control of the nitrogen metabolism, and regulates genes involved in ammonia assimilation, heterocyst differentiation and nitrogen fixation. NtcA expression is subject to nitrogen control, but there is also evidence that the binding of NtcA to DNA can be regulated by a redox mechanism involving the two cysteine residues in the NtcA protein from Anabaena PCC 7120. In order to investigate this further, the two cysteine residues in NtcA were mutated into alanine to give four variants of the protein: wild-type NtcA, the point-mutated variants Cys157Ala and Cys164Ala, as well as the double mutant Cys157Ala/Cys164Ala. The binding of a DNA probe containing a palindromic NtcA-binding motif was investigated by gel mobility shift analysis under non-reducing and reducing conditions. The experiments show that the DNA binding in vitro is stronger in the presence of the reducing agent DTT than in its absence. However, this effect is not due to breaking of a disulfide bond between the cysteine residues, since the double mutant containing no cysteines was also affected by DTT. A molecular model of a monomer of NtcA, based on the homologous cAMP receptor protein structure, was created in order to locate the positions of the cysteine residues. The NtcA model suggested that the positions of the sulfur atoms are not compatible with formation of a bond between them.

Novel DNA-binding proteins in the cyanobacterium Anabaena sp. strain PCC 7120

As an approach towards elucidation of the biochemical regulation of the progression of heterocyst differentiation in Anabaena sp. strain PCC 7120, we have identified proteins that bind to a 150-bp sequence upstream from hepC, a gene that plays a role in the synthesis of heterocyst envelope polysaccharide. Such proteins were purified in four steps from extracts of vegetative cells of Anabaena sp. Two of these proteins (Abp1 and Abp2) are encoded by neighboring genes in the Anabaena sp. chromosome. The genes that encode the third (Abp3) and fourth (Abp4) proteins are situated at two other loci in that chromosome. Insertional mutagenesis of abp2 and abp3 blocked expression of hepC and hepA and prevented heterocyst maturation and aerobic fixation of N(2).

Establishment of a functional symbiosis between the cyanobacterium Nostoc punctiforme and the bryophyte Anthoceros punctatus requires genes involved in nitrogen control and initiation of heterocyst differentiation

Three mutant strains (ntcA, hetR, hetF) of the cyanobacterium Nostoc punctiforme unable to differentiate heterocysts were characterized and examined for their ability to form a symbiotic association with the bryophyte Anthoceros punctatus. Previously unknown characteristics of the N. punctiforme hetR mutant include differentiation of chilling-resistant akinetes, while vegetative cells of the ntcA mutant randomly lysed, yielding short filaments, following ammonium deprivation. Strains with mutations in hetF and hetR infected A. punctatus with similar frequency to that of wild-type N. punctiforme but did not support growth of the plant partner. These results confirm that the infection of A. punctatus by hormogonia leading to the establishment of an association is physiologically uncoupled from the development of a functional diazotrophic association. They also indicate that heterocyst regulatory elements downstream from HetR and HetF are required in both free-living and symbiotic heterocyst differentiation and nitrogenase expression. A strain with a mutation in the global nitrogen regulator ntcA did not infect A. punctatus despite its ability to differentiate hormogonia at a low frequency. When complemented with one or more copies of ntcA, the mutant strain infected A. punctatus at a similar frequency as the wild-type and supported growth of the plant partner in the absence of combined nitrogen. These results established a connection between the presence of a functional copy of ntcA and the magnitude of hormogonium differentiation, and the behaviour of the formed hormogonia.

NtcA-dependent expression of the devBCA operon, encoding a heterocyst-specific ATP-binding cassette transporter in Anabaena spp

The devBCA operon, encoding subunits of an ATP-binding cassette exporter, is essential for differentiation of N(2)-fixing heterocysts in Anabaena spp. Nitrogen deficiency-dependent transcription of the operon and the use of its transcriptional start point, located 762 (Anabaena variabilis strain ATCC 29413-FD) or 704 (Anabaena sp. strain PCC 7120) bp upstream of the translation start site, were found to require the global nitrogen transcriptional regulator NtcA. Furthermore, NtcA was shown to bind in vitro to the promoter of devBCA.

The hetF gene product is essential to heterocyst differentiation and affects HetR function in the cyanobacterium Nostoc punctiforme

A novel gene, hetF, was identified as essential for heterocyst development in the filamentous cyanobacterium Nostoc punctiforme strain ATCC 29133. In the absence of combined nitrogen, hetF mutants were unable to differentiate heterocysts, whereas extra copies of hetF in trans induced the formation of clusters of heterocysts. Sequences hybridizing to a hetF probe were detected only in heterocyst-forming cyanobacteria. The inactivation and multicopy effects of hetF were similar to those of hetR, which encodes a self-degrading serine protease thought to be a central regulator of heterocyst development. Increased transcription of hetR begins in developing cells 3 to 6 h after deprivation for combined nitrogen (N step-down), and the HetR protein specifically accumulates in heterocysts. In the hetF mutant, this increase in hetR transcription was delayed, and a hetR promoter::green fluorescent protein (GFP) transcriptional reporter indicated that increased transcription of hetR occurred in all cells rather than only in developing heterocysts. When a fully functional HetR-GFP fusion protein was expressed in the hetF mutant from a multicopy plasmid, HetR-GFP accumulated nonspecifically in all cells under nitrogen-replete conditions; when expressed in the wild type, HetR-GFP was observed only in heterocysts after N step-down. HetF therefore appears to cooperate with HetR in a positive regulatory pathway and may be required for the increased transcription of hetR and localization of the HetR protein in differentiating heterocysts.

Redox control of ntcA gene expression in Synechocystis sp. PCC 6803. Nitrogen availability and electron transport regulate the levels of the NtcA protein

In this work we have studied the influence of the cellular redox status in the expression of the Synechocystis sp. PCC 6803 ntcA gene. Two different ntcA transcripts with different 5' ends were detected, depending on the different dark/light or nitrogen availability conditions. Accumulation of a 0.8-kb ntcA message was light and nitrogen dependent, whereas a longer 1.2-kb ntcA transcript was neither light nor nitrogen regulated. NtcA protein levels increased concomitantly with the accumulation of the 0.8-kb ntcA transcript. The light-dependent accumulation of the ntcA gene and the NtcA protein was sensitive to electron transport inhibitors. In addition, Glc-grown Synechocystis sp. cells showed a similar ntcA expression pattern in darkness to that observed under illumination. These data suggested that electron transport, and not light per se may regulate ntcA gene expression. Primer extension analysis, together with gel mobility-shift assays, demonstrated that in vitro, the Synechocystis sp. NtcA protein specifically bound to the putative promoter region from the light/nitrogen-dependent ntcA transcript but not to that from the constitutive 1.2-kb ntcA mRNA. Band-shift experiments carried out in the presence of thiol oxidizing/modifiying agents and different reducing/oxidizing conditions suggested that NtcA binding to its own promoter was under a thiol-dependent redox mechanism. Our results suggest that the cellular redox status plays a central role in the autoregulatory mechanism of the NtcA protein.

Molecular cloning and sequencing of the sodB gene from a heterocystous cyanobacterium Anabaena sp. PCC 7120

Superoxide dismutase (Sod) plays an important role in all aerobic organisms. The sodB gene of a heterocystous cyanobacterium Anabaena sp. PCC 7120 was cloned and sequenced. The Sod protein is predicted to have 199 amino acids and a molecular mass of 22.5 kDa. Sequence comparison among SodB from cyanobacteria and chloroplasts revealed that the sodB gene indeed encodes an iron-Sod. Northern blot analysis showed that the sodB gene of Anabaena sp. PCC 7120 is transcribed as a single gene and its expression was up-regulated when the cells were subjected to a shift from a nitrogen repletion condition to a nitrogen depletion condition.

NtcA represses transcription of gifA and gifB, genes that encode inhibitors of glutamine synthetase type I from Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803 glutamine synthetase type I (GS) activity is controlled by direct interaction with two inactivating factors (IF7 and IF17). IF7 and IF17 are homologous polypeptides encoded by the gifA and gifB genes respectively. We investigated the transcriptional regulation of these genes. Expression of both genes is maximum in the presence of ammonium, when GS is inactivated. Nitrogen starvation attenuates the ammonium-mediated induction of gifA and gifB as well as the ammonium-mediated inactivation of GS. Putative binding sites for the transcription factor NtcA were identified at -7.5 and -30.5 bp upstream of gifB and gifA transcription start points respectively. Synechocystis NtcA protein binding to both promoters was demonstrated by gel electrophoresis mobility shift assays. Constitutive high expression levels of both genes were found in a Synechocystis NtcA non-segregated mutant (SNC1), which showed a fourfold reduction in the ntcA expression. These experiments indicate a repressive role for NtcA on the transcription of gifA and gifB genes. Our results demonstrate that NtcA plays a central role in GS regulation in cyanobacteria, stimulating transcription of the glnA gene (GS structural gene) and suppressing transcription of the GS inactivating factor genes gifA and gifB.

Expression and purification of the transcription factor NtcA from the cyanobacterium Anabaena PCC 7120

The transcription factor NtcA from the cyanobacterium Anabaena PCC 7120 was heterologously expressed in Escherichia coli. In order to optimize the expression of NtcA, random silent mutations were introduced at the 5' end of the DNA encoding the protein. To get as high a yield of pure protein as possible, different strategies of expression as well as purification conditions were used. Under optimal expression conditions, a high-level expression clone of NtcA was coexpressed with GroEL-ES at 37 degrees C. A hexahistidine tag was added to the N-terminus of the protein in order to allow purification on an IMAC affinity column. Expression followed by one purification step using IMAC affinity chromatography gave a yield of 30-40 mg pure NtcA protein per liter of bacterial culture. Gel-shift experiments showed that the recombinant NtcA was active in binding a DNA sequence containing an NtcA-specific site.

The global nitrogen regulator NtcA regulates transcription of the signal transducer PII (GlnB) and influences its phosphorylation level in response to nitrogen and carbon supplies in the Cyanobacterium synechococcus sp. strain PCC 7942

The PII protein is encoded by a unique glnB gene in Synechococcus sp. strain PCC 7942. Its expression has been analyzed in the wild type and in NtcA-null mutant cells grown under different conditions of nitrogen and carbon supply. RNA-DNA hybridization experiments revealed the presence of one transcript species 680 nucleotides long, whatever the nutrient conditions tested. A second transcript species, 620 nucleotides long, absent in the NtcA null mutant, was observed in wild-type cells that were nitrogen starved for 2 h under both high and low CO2 and in the presence of nitrate under a high CO2 concentration. Primer extension analysis indicated that the two transcript species are generated from two tandem promoters, a sigma70 Escherichia coli-type promoter and an NtcA-dependent promoter, located 120 and 53 nucleotides, respectively, from the glnB initiation codon. The NtcA-dependent promoter is up-regulated under the conditions mentioned above, while the sigma70 E. coli-type promoter displays constitutive levels of transcripts in the NtcA null mutant and slightly different levels in the wild-type cells, depending on the nitrogen and carbon supplies. In general, a good correlation between the amounts of the two transcript species and that of the PII protein was observed, as revealed by immunodetection with specific antibodies. The phosphorylation level of PII in the wild type is inversely correlated with nitrogen availability and directly correlated with higher CO2 concentration. This regulation is correspondingly less stringent in the NtcA null mutant cells. In contrast, the dephosphorylation of PII is NtcA independent.

Constitutive and nitrogen-regulated promoters of the petH gene encoding ferredoxin:NADP+ reductase in the heterocyst-forming cyanobacterium Anabaena sp

Determination of the putative transcription start points of the petH gene encoding ferredoxin:NADP+ reductase in the heterocyst-forming cyanobacteria Anabaena sp. PCC 7119 and PCC 7120 showed that this gene is transcribed from two promoters, one constitutively used under different conditions of nitrogen nutrition and the other one used in cells subjected to nitrogen stepdown and in nitrogen-fixing filaments. The latter promoter, whose use was NtcA-dependent but HetR-independent, was functional in heterocysts. The N-control transcriptional regulator NtcA was observed to bind in vitro to this promoter. For the sake of comparison, the transcription start points of the nifHDK operon in strain PCC 7120 and binding of NtcA to the nifHDK promoter were also examined.

Regulation of the sol locus genes for butanol and acetone formation in Clostridium acetobutylicum ATCC 824 by a putative transcriptional repressor

A gene (orf1, now designated solR) previously identified upstream of the aldehyde/alcohol dehydrogenase gene aad (R. V. Nair, G. N. Bennett, and E. T. Papoutsakis, J. Bacteriol. 176:871-885, 1994) was found to encode a repressor of the sol locus (aad, ctfA, ctfB and adc) genes for butanol and acetone formation in Clostridium acetobutylicum ATCC 824. Primer extension analysis identified a transcriptional start site 35 bp upstream of the solR start codon. Amino acid comparisons of SolR identified a potential helix-turn-helix DNA-binding motif in the C-terminal half towards the center of the protein, suggesting a regulatory role. Overexpression of SolR in strain ATCC 824(pCO1) resulted in a solvent-negative phenotype owing to its deleterious effect on the transcription of the sol locus genes. Inactivation of solR in C. acetobutylicum via homologous recombination yielded mutants B and H (ATCC 824 solR::pO1X) which exhibited deregulated solvent production characterized by increased flux towards butanol and acetone formation, earlier induction of aad, lower overall acid production, markedly improved yields of solvents on glucose, a prolonged solvent production phase, and increased biomass accumulation compared to those of the wild-type strain.

Molecular genetic analysis of enoyl-acyl carrier protein reductase inhibition by diazaborine

Diazaborine and isoniazid are, at first sight, unrelated anti-bacterial agents that inhibit the enoyl-ACP reductase (ENR) of Escherichia coli and Mycobacterium tuberculosis respectively. The crystal structures of these enzymes including that of the diazaborine-inhibited E. coli ENR have been obtained at high resolution. Site-directed mutagenesis was used to study the importance of amino acid residues in diazaborine susceptibility and enzyme function. The results show that drug binding and inhibition require the presence of a glycine residue at position 93 of E. coli ENR or at the structurally equivalent position in the plant homologue, which is naturally resistant to the drug. The data confirm the hypothesis that any amino acid side-chain other than hydrogen at this position within the three-dimensional structure of these enzymes will affect diazaborine resistance by encroaching into the drug binding site. Substitutions of Gly-93 by amino acids with small side-chains, such as serine, alanine, cysteine and valine, hardly affected the catalytic parameters and rendered the bacterial host resistant to the drug. Larger amino acid side-chains, such as that of arginine, histidine, lysine and glutamine, completely inactivated the activity of the enzyme.

Heterocyst formation in Anabaena

Heterocystous cyanobacteria grow as multicellular organisms with a distinct one-dimensional developmental pattern of single nitrogen-fixing heterocysts separated by approximately ten vegetative cells. Several genes have been identified that are required for heterocyst development and pattern formation. A key regulator, HetR, has been recently shown to be aserine-type protease.

Evolution of the structure and chromosomal distribution of histidine biosynthetic genes

A database of more than 100 histidine biosynthetic genes from different organisms belonging to the three primary domains has been analyzed, including those found in the now completely sequenced genomes of Haemophilus influenzae, Mycoplasma genitalium, Synechocystis sp., Methanococcus jannaschii, and Saccharomyces cerevisiae. The ubiquity of his genes suggests that it is a highly conserved pathway that was probably already present in the last common ancestor of all extant life. The chromosomal distribution of the his genes shows that the enterobacterial histidine operon structure is not the only possible organization, and that there is a diversity of gene arrays for the his pathway. Analysis of the available sequences shows that gene fusions (like those involved in the origin of the Escherichia coli and Salmonella typhimurium hisIE and hisB gene structures) are not universal. In contrast, the elongation event that led to the extant hisA gene from two homologous ancestral modules, as well as the subsequent paralogous duplication that originated hisF, appear to be irreversible and are conserved in all known organisms. The available evidence supports the hypothesis that histidine biosynthesis was assembled by a gene recruitment process.

Nitrogen availability and electron transport control the expression of glnB gene (encoding PII protein) in the cyanobacterium Synechocystis sp. PCC 6803

The glnB gene from Synechocystis sp. PCC 6803 that encodes the PII protein has been cloned by heterologous hybridization using the corresponding glnB gene from Synechococcus sp. PCC 7942. An ORF of 336 nucleotides appeared that potentially coded for a protein of 112 amino acid residues (M(r) 12,397). The deduced amino acid sequence revealed a high identity (higher than 80%) with its cyanobacterial counterparts and a basal level of identity (close to 60%) with other PII proteins. A single mRNA of about 680 nucleotides was found under all growth conditions studied. glnB gene expression was specifically activated under nitrogen deprivation (a 10-fold increase respect to nitrogen-replete conditions). No differences in glnB mRNA levels were observed when using nitrate or ammonium as nitrogen sources. Amount of glnB mRNA decreased to undetectable levels when transferring cells to the dark, but effect was avoided by adding glucose to the culture medium. Primer extension analysis and band-shift assays indicated that expression of the glnB gene, elevated under nitrogen deprivation, might lie under the control of the nitrogen transcriptional regulator NtcA, although constitutive levels of expression were also detected from a sigma 70-dependent Escherichia coli-like promoter.

Nitrate assimilation by bacteria

Nitrate is a significant nitrogen source for plants and microorganisms. Recent molecular genetic analyses of representative bacterial species have revealed structural and regulatory genes responsible for the nitrate-assimilation phenotype. Together with results from physiological and biochemical studies, this information has unveiled fundamental aspects of bacterial nitrate assimilation and provides the foundation for further investigations. Well-studied genera are: the cyanobacteria, including the unicellular Synechococcus and the filamentous Anabaena; the gamma-proteobacteria Klebsiella and Azotobacter; and a Gram-positive bacterium, Bacillus. Nitrate uptake in most of these groups seems to involve a periplasmic binding protein-dependent system that presumably is energized by ATP hydrolysis (ATP-binding cassette transporters). However, Bacillus may, like fungi and plants, utilize electrogenic uptake through a representative of the major facilitator superfamily of transport proteins. Nitrate reductase contains both molybdenum cofactor and an iron-sulfur cluster. Electron donors for the enzymes from cyanobacteria and Azotobacter are ferredoxin and flavodoxin, respectively, whereas the Klebsiella and Bacillus enzymes apparently accept electrons from a specific NAD(P)H-reducing subunit. These subunits share sequence similarity with the reductase components of bacterial aromatic ring-hydroxylating dehydrogenases such as toluene dioxygenase. Nitrite reductase contains sirohaem and an iron-sulfur cluster. The enzymes from cyanobacteria and plants use ferredoxin as the electron donor, whereas the larger enzymes from other bacteria and fungi contain FAD and NAD(P)H binding sites. Nevertheless, the two forms of nitrite reductase share recognizable sequence and structural similarity. Synthesis of nitrate assimilation enzymes and uptake systems is controlled by nitrogen limitation in all bacteria examined, but the relevant regulatory proteins exhibit considerable structural and mechanistic diversity in different bacterial groups. A second level of control, pathway-specific induction by nitrate and nitrite in Klebsiella, involves transcription antitermination. Several issues await further experimentation, including the mechanism and energetics of nitrate uptake, the pathway(s) for nitrite uptake, the nature of electron flow during nitrate reduction, and the action of transcriptional regulatory circuits. Fundamental knowledge of nitrate assimilation physiology should also enhance the study of nitrate metabolism in soil, water and other natural environments, a challenging topic of considerable interest and importance.

cis-Acting sequences required for light-responsive expression of the psbDII gene in Synechococcus sp. strain PCC 7942

We analyzed the sequences required for promoter activity and high-light responsiveness of the psbDII gene in the cyanobacterium Synechococcus sp. strain PCC 7942 by using transcriptional fusions to a lacZ reporter gene. The basal promoter drives high constitutive expression, although no canonical -35 element is evident. The smallest fragment that showed clear light-responsive expression extends from -38 to +160, which includes 52 bp of the psbDII open reading frame. Sequences downstream from the promoter, within the untranslated leader region from +11 to +24, were required for high-light induction.

Evidence for redox regulation of the transcription factor NtcA, acting both as an activator and a repressor, in the cyanobacterium Anabaena PCC 7120

NtcA has been identified as a nitrogen-responsive regulatory protein required for nitrogen assimilation and heterocyst differentiation in cyanobacteria. It is proposed that NtcA functions through the formation of DNA-protein complexes with its specific target sequence within the promoter regions of the regulated genes. In vitro, NtcA of Anabaena PCC 7120 binds to upstream regions of the genes whose products are involved in nitrogen assimilation, but also to the upstream region of rbcLS (carbon-fixation gene), xisA (encoding a site-specific recombinase expressed during heterocyst differentiation) and ntcA (encoding NtcA itself). However, the mechanism by which NtcA serves as a critical regulator for such diverse processes is not understood. With the use of electrophoretic mobility shift assays, NtcA from Anabaena PCC 7120 was here shown to interact with the promoter sequence of the gor gene, encoding glutathione reductase, thereby providing a novel example of NtcA's acting as a repressor, previously found only for the rbcLS gene. Furthermore we demonstrate that the binding of DNA by NtcA is regulated in vitro by a redox-dependent mechanism involving cysteine residues of the NtcA protein. These findings suggest that NtcA is a transcriptional regulator that responds not only to the nitrogen status but also to the cellular redox status, a function that might be particularly significant during heterocyst differentiation.

Cloning, sequencing, and regulation of the global nitrogen regulator gene ntcA in the unicellular diazotrophic cyanobacterium Cyanothece sp. strain BH68K

In cyanobacteria, ammonium represses expression of proteins involved in nitrogen fixation and assimilation. The global nitrogen regulator gene ntcA encodes a DNA-binding protein, NtcA, that is a transcriptional activator of genes subject to nitrogen control. We report the cloning and sequencing of the ntcA gene from a nitrogen-fixing unicellular cyanobacterium, Cyanothece sp. strain BH68K. The gene comprises 678 nucleotides, and the deduced NtcA protein contains 226 amino acids with a predicted molecular weight of 25,026. In addition, ntcA mRNA levels were measured in cells grown under different nitrogen regimes. Under nitrogen-fixing conditions, ntcA transcripts were weakly expressed. Furthermore, ntcA expression was diminished or inversely proportional to nifHDK expression. Conversely, ntcA expression increased in nitrate-grown cells, and a concentration-dependent increase was seen in ammonium-grown cells up to 1 mM NH4Cl. These results indicate that ntcA is involved more in nitrogen assimilation than in nitrogen fixation and also imply that the rhythmic expression of ntcA and nifHDK transcription may be under the control of a circadian clock.

Transcription of glutamine synthetase genes (glnA and glnN) from the cyanobacterium Synechocystis sp. strain PCC 6803 is differently regulated in response to nitrogen availability

In the cyanobacterium Synechocystis sp. strain PCC 6803 we have previously reported the presence of two different proteins with glutamine synthetase activity: GSI, encoded by the glnA gene, and GSIII, encoded by the glnN gene. In this work we show that expression of both the glnA and glnN genes is subjected to transcriptional regulation in response to changes in nitrogen availability. Northern blot experiments and transcriptional fusions demonstrated that the glnA gene is highly transcribed in nitrate- or ammonium-grown cells and exhibits two- to fourfold-higher expression in nitrogen-starved cells. In contrast, the glnN gene is highly expressed only under nitrogen deficiency. Half-lives of both mRNAs, calculated after addition of rifampin or ammonium to nitrogen-starved cells, were not significantly different (2.5 or 3.4 min, respectively, for glnA mRNA; 1.9 or 1.4 min, respectively, for glnN mRNA), suggesting that changes in transcript stability are not involved in the regulation of the expression of both genes. Deletions of the glnA and glnN upstream regions were used to delimit the promoter and the regulatory sequences of both genes. Primer extension analysis showed that structure of the glnA gene promoter resembles those of the NtcA-regulated promoters. In addition, mobility shift assays demonstrated that purified, Escherichia coli-expressed Synechocystis NtcA protein binds to the promoter of the glnA gene. Primer extension also revealed the existence of a sequence related to the NtcA binding site upstream from the glnN promoter. However, E. coli-expressed NtcA failed to bind to this site. These findings suggest that an additional modification of NtcA or an additional factor is required for the regulation of glnN gene expression.

Nitrate assimilation gene cluster from the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120

A region of the genome of the filamentous, nitrogen-fixing, heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 that contains a cluster of genes involved in nitrate assimilation has been identified. The genes nir, encoding nitrite reductase, and nrtABC, encoding elements of a nitrate permease, have been cloned. Insertion of a gene cassette into the nir-nrtA region impaired expression of narB, the nitrate reductase structural gene which together with nrtD is found downstream from nrtC in the gene cluster. This indicates that the nir-nrtABCD-narB genes are cotranscribed, thus constituting an operon. Expression of the nir operon in strain PCC 7120 is subjected to ammonium-promoted repression and takes place from an NtcA-activated promoter located 460 bp upstream from the start of the nir gene. In the absence of ammonium, cellular levels of the products of the nir operon are higher in the presence of nitrate than in the absence of combined nitrogen.

Nitrate reductase activity and heterocyst suppression on nitrate in Anabaena sp. strain PCC 7120 require moeA

Mutants of Anabaena sp. strain PCC 7120 that form heterocysts when grown on nitrate-containing media were isolated following nitrosoguanidine mutagenesis. Six independent mutants were isolated, and the characterization of one mutant, strain AMC260, which forms 6 to 8% heterocysts in the presence of nitrate, is presented. A 1.8-kb chromosomal fragment that complemented the AMC260 mutant was sequenced, and a 1.2-kb open reading frame, named moeA, was identified. The deduced amino acid sequence of the predicted Anabaena sp. strain PCC 7120 MoeA polypeptide shows 37% identity to MoeA from Escherichia coli, which is required for the synthesis of molybdopterin cofactor. Molybdopterin is required by various molybdoenzymes, such as nitrate reductase. Interruption of the moeA gene in Anabaena sp. strain PCC 7120 resulted in a strain, AMC364, that showed a phenotype similar to that of AMC260. We show that AMC260 and AMC364 lack methyl viologen-supported nitrate reductase activity. We conclude that the inability of the moeA mutants to metabolize nitrate results in heterocyst formation on nitrate-containing media. Northern (RNA) analysis detected a 1.5-kb moeA transcript in wild-type cells grown in the presence or absence of a combined nitrogen source.

The NADP+-isocitrate dehydrogenase gene (icd) is nitrogen regulated in cyanobacteria

NADP+-isocitrate dehydrogenase (NADP+-IDH) activity and protein levels in crude extracts from the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 and the filamentous, dinitrogen-fixing Anabaena sp. strain PCC 7120 were determined under different nitrogen conditions. The highest NADP+-IDH activity and protein accumulation were found under dinitrogen-fixing conditions for the Anabaena strain and under nitrogen starvation for Synechocystis sp. PCC 6803. The icd gene that encodes the NADP+-IDH from Synechocystis sp. strain PCC 6803 was cloned by heterologous hybridization with the previously isolated icd gene from Anabaena sp. strain PCC 7120. The two cyanobacterial icd genes show 81% sequence identity and share a typical 44-amino-acid region different from all the other icd genes sequenced so far. The icd gene seems to be essential for Synechocystis growth since attempts to generate a completely segregated icd mutant were unsuccessful. Transcripts of 2.0 and 1.6 kb were detected by Northern (RNA) blot analysis, for the Anabaena and Synecho-cystis icd genes, respectively. Maximal icd mRNA accumulation was reached after 5 It of nitrogen starvation in Synechocystis cells and under dinitrogen-fixing conditions in Anabaena cells. Primer extension analysis showed that the structure of the Synechocystis icd gene promoter resembles those of the NtcA-regulated promoters. In addition, mobility shift assays demonstrated that purified Synechocystis NtcA protein binds to the promoter of the icd gene. All these data suggest that the expression of the icd gene from Synechocystis sp. strain PCC 6803 may be subjected to nitrogen control mediated by the positively acting regulatory protein NtcA.

Transcription of the Anabaena sp. strain PCC 7120 ntcA gene: multiple transcripts and NtcA binding

The Anabaena sp. strain PCC 7120 ntcA gene showed multiple transcripts with different 5' ends. The relative abundance of transcripts varied in response to nitrogen availability. The ntcA product, NtcA, showed binding to the promoter region of its own gene. The binding site mapped to a region between the transcription start site used under nitrogen-replete conditions and the start sites used under nitrogen-limiting conditions, suggesting that NtcA regulates its own expression.

Nitrogen control in bacteria

Nitrogen metabolism in prokaryotes involves the coordinated expression of a large number of enzymes concerned with both utilization of extracellular nitrogen sources and intracellular biosynthesis of nitrogen-containing compounds. The control of this expression is determined by the availability of fixed nitrogen to the cell and is effected by complex regulatory networks involving regulation at both the transcriptional and posttranslational levels. While the most detailed studies to date have been carried out with enteric bacteria, there is a considerable body of evidence to show that the nitrogen regulation (ntr) systems described in the enterics extend to many other genera. Furthermore, as the range of bacteria in which the phenomenon of nitrogen control is examined is being extended, new regulatory mechanisms are also being discovered. In this review, we have attempted to summarize recent research in prokaryotic nitrogen control; to show the ubiquity of the ntr system, at least in gram-negative organisms; and to identify those areas and groups of organisms about which there is much still to learn.

Isolation of the Anabaena sp. strain PCC 7120 sigA gene in a transcriptional-interference selection

A transcriptional-interference selection was performed to identify genes of Anabaena sp. strain PCC 7120 that encode DNA-binding proteins able to bind to the rbcL promoter. Unexpectedly, the selection yielded the previously identified sigA gene, which encodes the principal sigma factor. Protein extracts from Escherichia coli containing the sigA gene bound the rbcL promoter fragment in mobility shift assays, and competition experiments indicated binding to rbcL and glnA but not xisA or nifH upstream regions.

Cloning, sequencing, and regulation of the glutathione reductase gene from the cyanobacterium Anabaena PCC 7120

Glutathione reductase (GR) was purified from the cyanobacterium Anabaena PCC 7120. A 3-kilobase genomic DNA fragment containing the coding sequence for the GR gene (gor) was identified and cloned by polymerase chain reaction based on sequences of selected peptides isolated from proteolyzed GR. The coding sequence encompassing 458 amino acid residues, as well as 360 base pairs of the 5'-flanking region and 430 base pairs of the 3'-flanking region, were determined. Genomic Southern analysis indicates that gor is a single-copy gene. A gor antisense RNA probe hybridized with a 1.4-kilobase transcript, suggesting that the gene is not part of an operon including additional genes. The deduced GR amino acid sequence shows 41 to 48% identity with those of human, Escherichia coli, Pseudomonas aeruginosa, pea, and Arabidopsis thaliana GR. The coding sequence of GR was overexpressed in a GR-deficient E. coli strain, SG5, and the recombinant protein was purified. Anabaena GR is NADPH-linked, but a Lys residue replaces an Arg residue involved in NADPH binding in GR from other species. In addition, Anabaena GR carries the GXGXXG "fingerprint" motif which otherwise characterizes NAD(H)-dependent enzymes. These differences may contribute to the lack of affinity for 2',5'-ADP-Sepharose 4B of Anabaena GR. Three E. coli-type promoter sequences and a BifA/NtcA binding motif were found upstream of the open reading frame. The middle and the proximal promoters were shown to be active. However, the use of the middle promoter was dependent on the nitrogen source in the culture medium. Both GR activity and GR protein concentration increased in ammonium grown cultures in which both the middle and proximal promoters were used for transcriptional initiation. The BifA/NtcA-binding site overlaps the middle promoter sequence and may thus be involved in regulation of differential transcription.

Enoyl-acyl-carrier-protein reductase and Mycobacterium tuberculosis InhA do not conserve the Tyr-Xaa-Xaa-Xaa-Lys motif in mammalian 11 beta- and 17 beta-hydroxysteroid dehydrogenases and Drosophila alcohol dehydrogenase

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Vectors for determining the differential expression of genes in heterocysts and vegetative cells of Anabaena sp. strain PCC 7120

Plasmid vectors were constructed to study promoters of the cyanobacterium Anabaena sp. strain PCC 7120. Plasmid pCCBSelect contains the promoterless reporter genes in the order cat-nifHDK. In pCCBSelect/a, the nifHDK operon precedes the cat gene. Putative promoter sequences were cloned into a polylinker region upstream of the reporter genes. Activity in heterocysts was determined by complementation of a strain containing a deletion of the nifH gene. Activity in vegetative cells was determined by measuring resistance to chloramphenicol. The promoter of the nifHDK operon was active only in heterocysts; the promoter of the nifJ gene was active only in iron-depleted medium; and the promoters of the psbB gene, the ntcA gene, and a newly found transcription factor gene were all active in both cell types.

Genes encoded on a cyanobacterial plasmid are transcriptionally regulated by sulfur availability and CysR

A cyanobacterial sulfur-regulated gene (cysR), which encodes a protein with similarity to the Crp family of prokaryotic regulatory proteins, has recently been isolated and characterized. Polyacrylamide gel electrophoresis of periplasmic protein extracts reveals that a cysR mutant fails to synthesize a 36-kDa polypeptide that is normally induced in wild-type cells that have been grown under sulfur-deficient conditions. The amino-terminal sequence of this protein was obtained, and a synthetic oligonucleotide was used to isolated a clone containing a 1.9-kb NruI-KpnI fragment from a Synechococcus sp. strain PCC 7942 genomic library. RNA blot analysis indicates that this fragment encodes a transcript that is detectable in wild-type but not cysR mutant cells that have been starved for sulfur. DNA blot analysis revealed that the 1.9-kb NruI-KpnI fragment is contained within the Ba4 BamHI fragment of the endogenous 50-kb plasmid pANL. RNA blot studies indicate that the accumulation of a large number of pANL transcripts is regulated by sulfur levels and CysR. DNA sequence analysis confirmed that the gene encoding the sulfur-regulated 36-kDa periplasmic protein is encoded on the Ba4 fragment of pANL. The sequence of the 36-kDa protein displays sequence similarity to the enzyme catalase, and two downstream proteins exhibit 25 and 62% identity to a subunit of a P-type ATPase complex involved in Mg2+ transport and a chromate resistance determinant, respectively. Surprisingly, a strain in which the putative chromate resistance gene was interrupted by a drug resistance marker exhibited increased resistance to chromate when grown in media containing low sulfate concentrations. The possible role of this protein in the acclimation of cyanobacteria to conditions of low sulfur availability is discussed.

Carbon monoxide-induced activation of gene expression in Rhodospirillum rubrum requires the product of cooA, a member of the cyclic AMP receptor protein family of transcriptional regulators

Induction of the CO-oxidizing system of the photosynthetic bacterium Rhodospirillum rubrum is regulated at the level of gene expression by the presence of CO. In this paper, we describe the identification of a gene that is required for CO-induced gene expression. An 11-kb deletion of the region adjacent to the previously characterized cooFSCTJ region resulted in a mutant unable to synthesize CO dehydrogenase in response to CO and unable to grow utilizing CO as an energy source. A 2.5-kb region that corresponded to a portion of the deleted region complemented this mutant for its CO regulation defect, restoring its ability to grow utilizing CO as an energy source. When the 2.5-kb region was sequenced, one open reading frame, designated cooA, predicted a product showing similarity to members of the cyclic AMP receptor protein (CRP) family of transcriptional regulators. The product, CooA, is 28% identical (51% similar) to CRP and 18% identical (45% similar) to FNR from Escherichia coli. The insertion of a drug resistance cassette into cooA resulted in a mutant that could not grow utilizing CO as an energy source. CooA contains a number of cysteine residues substituted at, or adjacent to, positions that correspond to residues that contact cyclic AMP in the crystal structure of CRP. A model based on this observation is proposed for the recognition of CO by Cooa. Adjacent to cooA are two genes, nadB and nadC, with predicted products similar to proteins in other bacteria that catalyze reactions in the de novo synthesis of NAD.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron transport controls transcription of the glutamine synthetase gene (glnA) from the cyanobacterium Synechocystis sp. PCC 6803

The glnA gene, encoding type I glutamine synthetase (GS) in Synechocystis sp. PCC 6803, showed a high sequence similarity with other cyanobacterial glnA genes. A dramatic decrease in the amount of glnA mRNA, a single transcript of about 1.6 kb, was observed after transfer to darkness, or after incubation with the electron transport inhibitors DCMU or DBMIB. The levels of glnA transcript were fully recovered after 5 min of reillumination. The glnA mRNA was found to be equally stable both in the light and the dark (half-life about 2.5 min). Unlike the glnA messenger, the amount of GS protein was not reduced in the dark. Synthesis of the glnA transcript in the dark required the presence of glucose. In addition, glnA transcription in a Synechocystis psbE-psbF mutant lacking photosystem II required the presence of glucose even when grown in the light. These observations indicate that glnA transcription is under the control of the redox state of the cell. Finally, nitrogen starvation provoked a delay in the decrease of glnA transcript in darkness, suggesting a connection between nitrogen and redox controls of glnA transcript levels.

Characterization of devA, a gene required for the maturation of proheterocysts in the cyanobacterium Anabaena sp. strain PCC 7120

Mutant M7, obtained by transposon mutagenesis of the cyanobacterium Anabaena sp. strain PCC 7120, is impaired in the development of mature heterocysts. Under aerobic conditions, the mutant is unable to fix N2 because of a deficiency of at least two components of the oxygen-protective mechanisms: a hemoprotein-coupled oxidative reaction and heterocyst-specific glycolipids. DNA contiguous with the inserted transposon was recovered from the mutant and sequenced. The transposon had inserted itself within a 732-bp open reading frame designated devA. The wild-type form of devA, obtained from a lambda-EMBL3 library of Anabaena sp. DNA, had the identical sequence. Directed mutagenesis of devA in the wild-type strain showed that the phenotype of the mutant was caused by insertion of the transposon. The wild-type form of devA on a shuttle vector complemented the mutation in M7. Expression of devA by whole filaments, monitored following nitrogen stepdown by using luxAB as the reporter, increased ca. eightfold during differentiation; the increase within differentiating cells was much greater. The deduced sequence of the DevA protein shows strong similarity to the ATP-binding subunit of binding protein-dependent transport systems. The product of devA may, therefore, be a component of a periplasmic permease that is required for the transition from a proheterocyst to a mature, nitrogen-fixing heterocyst.