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At least 649 records · Page 36Linked to original sources
PubMed1994

Reciprocal light-dark transcriptional control of nif and rbc expression and light-dependent posttranslational control of nitrogenase activity in Synechococcus sp. strain RF-1.

Synechococcus sp. strain RF-1 exhibits a circadian rhythm of N2 fixation when cells are grown under a light-dark cycle, with nitrogenase activity observed only during the dark period. This dark-dependent activity correlated with nif gene transcription in strain RF-1. By using antibodies against dinitrogenase reductase (the Fe protein of the nitrogenase complex), it was found that there was a distinct shift in the mobility of this protein on sodium dodecyl sulfate gels during the light-dark cycle. The Fe protein was present only when cells were incubated in the dark. Upon illumination, there was a conversion of all Fe protein to a modified form, after which it rapidly disappeared from extracts. These studies indicated that all nitrogenase activity present during the dark cycle resulted from de novo synthesis of nitrogenase. Upon entering the light phase, cells appeared to quickly degrade the modified form of Fe protein, perhaps as a result of activating or inducing a protease. By contrast, transcription of the rbcL gene, which encodes the catalytic subunit of the key enzyme of CO2 fixation (a light-dependent process), was enhanced in the light.

Bacterial Proteins↗
PubMed1994

Adaptation to high light intensity in Synechococcus sp. strain PCC 7942: regulation of three psbA genes and two forms of the D1 protein.

The three psbA genes in the cyanobacterium Synechococcus sp. strain PCC 7942 encode two distinct forms of the D1 protein of photosystem II. The psbAI message, which encodes form I, dominates the psbA transcript pool at low to moderate light intensities; however, exposure to high light triggers a response in which the psbAI message is actively degraded while psbAII and psbAIII, which encode form II, are transcriptionally induced. We addressed whether these changes result from a generalized stress response and examined the consequence of light-responsive psbA regulation on the composition of D1 in thylakoid membranes. Heat shock and oxidative stress had some effect on levels of the three psbA transcripts but did not produce the responses generated by an increase in light intensity. Prolonged exposure to high light (24-h time course) was characterized by elevated levels of all psbA transcripts through maintenance of high levels of psbAII and psbAIII messages and a rebound of the psbAI transcript after its initial decline. Form II-encoding transcripts were enriched relative to those encoding form I at all high-light time points. Form II replaced form I in the thylakoid membrane at high light despite an abundance of psbAI transcript at later time points; this may be explained by the observed faster turnover of form I than form II in the membrane. We propose that form II is less susceptible to damage at high light and that this qualitative alteration, coupled with increased turnover of D1, protects the cells from photoinhibition.

Adaptation, Physiological↗
PubMed1995

Identification and characterization of two nitrogen-regulated genes of the cyanobacterium Synechococcus sp. strain PCC7942 required for maximum efficiency of nitrogen assimilation.

Two nitrogen-regulated genes were found in the genomic DNA region upstream of the nirA operon involved in uptake and utilization of nitrate in Synechococcus sp. strain PCC7942. The two genes (nirB and ntcB) are transcribed divergently from nirA and encode proteins of 349 and 309 amino acid residues, respectively. The levels of nirB and ntcB transcripts were low in cells growing on ammonium and increased upon transfer of ammonium-grown cells to nitrate-containing medium. The deduced NirB protein sequence has no similarities to other known proteins, whereas the deduced NtcB protein sequence is homologous to bacterial transcriptional activators of the LysR family. Defined mutants constructed by interrupting nirB or ntcB with a drug resistance marker grew as fast as the wild-type strain on ammonium but grew slower than the wild-type strain on nitrate or nitrite. The nirB mutant had higher activities of nitrate reductase, glutamine synthetase, and glutamate synthase than the wild-type strain, but its nitrite reductase activity was 40% of the wild-type levels. The mutant excreted nitrite into the medium during growth on nitrate, showing that nitrite reductase limits nitrate assimilation. These findings suggested that nirB is required for expression of maximum nitrite reductase activity. When grown on ammonium, the nirB mutant grew normally but cultures of the ntcB mutant still showed a yellowish-green color typical of nitrogen-limited cells. NtcB seems to regulate utilization of fixed nitrogen by controlling the expression of a certain gene(s) involved in nitrogen metabolism.

Amino Acid Sequence↗
PubMed1995

Specific binding of Synechococcus sp. strain PCC 7942 proteins to the enhancer element of psbAII required for high-light-induced expression.

The psbAII gene of the cyanobacterium Synechococcus sp. strain PCC 7942 is a member of a three-gene family that encodes the D1 protein of the photosystem II reaction center. Transcription of psbAII is rapidly induced when the light intensity reaching the culture increases from 125 microE.m-2.s-1 (low light) to 750 microE.m-2.s-1 (high light). The DNA segment upstream of psbAII that corresponds to the untranslated leader of its major transcript has enhancer activity and confers high-light induction. We show that one or more soluble proteins from PCC 7942 specifically bind to this region of psbAII (designated the enhancer element). In vivo footprinting showed protein binding to the enhancer element in high-light-exposed cell samples but not in those maintained at low light, even though in vitro mobility shifts were detectable with extracts from low- or high-light-grown cells. When 12 bp were deleted from the psbAII enhancer element, protein binding was impaired and high-light induction of both transcriptional and translational psbAII-lacZ reporters was significantly reduced. This finding indicates that protein binding to this region is required for high-light induction of psbAII. The mutant element also showed impaired enhancer activity when combined with a heterologous promoter.

Bacterial Proteins↗
PubMed1995

Functional analysis of the phosphoprotein PII (glnB gene product) in the cyanobacterium Synechococcus sp. strain PCC 7942.

The PII protein (glnB gene product) in the cyanobacterium Synechococcus sp. strain PCC 7942 signals the cellular N status by being phosphorylated or dephosphorylated at a seryl residue. Here we show that the PII-modifying system responds to the activity of ammonium assimilation via the glutamine synthase-glutamate synthase pathway and to the state of CO2 fixation. To identify possible functions of PII in this microorganism, a PII-deficient mutant was created and its general phenotype was characterized. The analysis shows that the PII protein interferes with the regulation of enzymes required for nitrogen assimilation, although ammonium repression is still detectable in the PII-deficient mutant. We suggest that the phosphorylation and dephosphorylation of PII are part of a complex signal transduction network involved in global nitrogen control in cyanobacteria. In this regulatory process, PII might be involved in mediating the tight coordination between carbon and nitrogen assimilation.

Bacterial Proteins↗
PubMed1995

Characterization of a zwf mutant of Synechococcus sp. strain PCC 7942.

A mutant of the cyanobacterium Synechococcus sp. strain PCC 7942 carrying a disrupted gene encoding glucose-6-phosphate dehydrogenase (zwf) produced no detectable glucose-6-phosphate dehydrogenase as assessed by enzyme assay and Western blot (immunoblot) analysis. This mutant exhibited significantly impaired dark viability.

Cyanobacteria↗
PubMed1996

Positive regulation by nitrite of the nitrate assimilation operon in the cyanobacteria Synechococcus sp. strain PCC 7942 and Plectonema boryanum.

In the absence of fixation of ammonium to glutamine, nitrate and nitrite activated transcription of the nitrate assimilation (nirA-nrtABCD-narB) operon of Synechococcus sp. strain PCC 7942. In a nitrate reductase-deficient mutant, only nitrite activated transcription, indicating that nitrite is the actual activator of the operon. Nitrate and nitrite were also found to activate the transcription of a nitrate assimilation operon in the filamentous nonheterocystous nitrogen-fixing cyanobacterium Plectonema boryanum.

Anion Transport Proteins↗
PubMed1996

Regulation by cyanate of the genes involved in carbon and nitrogen assimilation in the cyanobacterium Synechococcus sp. strain PCC 7942.

A mutant (M45) of the cyanobacterium Synechococcus sp. strain PCC 7942, which is defective in active transport of nitrate, was used for the studies of the nitrogen regulation of the genes involved in nitrate and CO2 assimilation. In a medium containing 30 mM nitrate as the nitrogen source, M45 grew under constant stress of nitrogen deficiency and accumulated a five-times-larger amount of the transcript of nirA, the gene for nitrite reductase, compared with nitrate-grown wild-type cells. By contrast, the level of the transcript of rbcL, the gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, was 40% of the wild-type level. Addition of ammonium to the culture of M45 abolished the accumulation of the nirA transcript and stimulated the accumulation of the rbcL transcript, showing that ammonium repressed and activated the transcription of nirA and rbcL, respectively. Glutamine, the initial product of ammonium fixation, also showed negative and positive effects on nirA and rbcL, respectively. One of the metabolites of glutamine, carbamoylphosphate, and its decomposition product, cyanate, were found to repress nirA and also to markedly activate rbcL. Cyanate negatively regulated another ammonium-repressible gene, glnA, but had no effect on the psbAI and rps1 genes. The effects of cyanate were not ascribable to the ammonium and CO, resulting from its decomposition. These findings suggested that cyanate may act as a regulator of the ammonium-responsive genes involved in carbon and nitrogen assimilation in the cyanobacterium.

Base Sequence↗
PubMed1997

Involvement of NtcB, a LysR family transcription factor, in nitrite activation of the nitrate assimilation operon in the cyanobacterium Synechococcus sp. strain PCC 7942.

Nitrite, either exogenously supplied or endogenously generated by nitrate reduction, activates transcription of the nitrate assimilation operon (nirA-nrtABCD-narB) in Synechococcus sp. strain PCC 7942 cells treated with L-methionine-DL-sulfoximine (an inhibitor of glutamine synthetase), in which there is no negative feedback resulting from fixation of the ammonium generated by nitrite reduction (Kikuchi et al., J. Bacteriol. 178:5822-5825, 1996). Other transcription units related to nitrogen assimilation, i.e., the nirB-ntcB operon, glnA, and ntcA, were not activated by nitrite. Nitrite did not activate nirA operon transcription in a mutant with a deletion of ntcB, an ammonium-repressible gene encoding a LysR-type DNA-binding protein. Introduction of plasmid-borne ntcB into the ntcB deletion mutant restored the response of the cells to nitrite, indicating that NtcB activates the nirA operon in response to nitrite. Supplementation of nitrite or nitrate to nitrogen-starved cultures of the wild-type strain, but not of the ntcB deletion mutant, caused activation of the nirA operon without L-methionine-DL-sulfoximine treatment of the cells. The results suggested that the positive-regulation mechanism of nirA operon transcription plays a role in rapid adaptation of nitrogen-starved cells to changing availability of nitrate and nitrite.

Bacterial Proteins↗
PubMed1997

Identification and nitrogen regulation of the cyanase gene from the cyanobacteria Synechocystis sp. strain PCC 6803 and Synechococcus sp. strain PCC 7942.

An open reading frame (slr0899) on the genome of Synechocystis sp. strain PCC 6803 encodes a polypeptide of 149 amino acid residues, the sequence of which is 40% identical to that of cyanase from Escherichia coli. Introduction into a cyanase-deficient E. coli strain of a plasmid-borne slr0899 resulted in expression of low but significant activity of cyanase. Targeted interruption of a homolog of slr0899 from Synechococcus sp. strain PCC 7942, encoding a protein 77% identical to that encoded by slr0899, resulted in loss of cellular cyanase activity. These results indicated that slr0899 and its homolog in the strain PCC 7942 represent the cyanobacterial cyanase gene (designated cynS). While cynS of strain PCC 6803 is tightly clustered with the four putative molybdenum cofactor biosynthesis genes located downstream, cynS of strain PCC 7942 was found to be tightly clustered with the two genes located upstream, which encode proteins similar to the subunits of the cyanobacterial nitrate-nitrite transporter. In both strains, cynS was transcribed as a part of a large transcription unit and the transcription was negatively regulated by ammonium. Cyanase activity was low in ammonium-grown cells and was induced 7- to 13-fold by inhibition of ammonium fixation or by transfer of the cells to ammonium-free media. These findings indicated that cyanase is an ammonium-repressible enzyme in cyanobacteria, the expression of which is regulated at the level of transcription. Similar to other ammonium-repressible genes in cyanobacteria, expression of cynS required NtcA, a global nitrogen regulator of cyanobacteria.

Amino Acid Sequence↗
PubMed1997

The action in vivo of glycine betaine in enhancement of tolerance of Synechococcus sp. strain PCC 7942 to low temperature.

The cyanobacterium Synechococcus sp. strain PCC 7942 was transformed with the codA gene for choline oxidase from Arthrobacter globiformis under the control of a constitutive promoter. This transformation allowed the cyanobacterial cells to accumulate glycine betaine at 60 to 80 mM in the cytoplasm. The transformed cells could grow at 20 degrees C, the temperature at which the growth of control cells was markedly suppressed. Photosynthesis of the transformed cells at 20 degrees C was more tolerant to light than that of the control cells. This was caused by the enhanced ability of the photosynthetic machinery in the transformed cells to recover from low-temperature photoinhibition. In darkness, photosynthesis of the transformed cells was more tolerant to low temperature such as 0 to 10 degrees C than that of the control cells. In parallel with the improvement in the ability of the transformed cells to tolerate low temperature, the lipid phase transition of plasma membranes from the liquid-crystalline state to the gel state shifted toward lower temperatures, although the level of unsaturation of the membrane lipids was unaffected by the transformation. These findings suggest that glycine betaine enhances the tolerance of photosynthesis to low temperature.

Alcohol Oxidoreductases↗
PubMed1997

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.

Amino Acid Sequence↗
PubMed1997

Calcium is required for swimming by the nonflagellated cyanobacterium Synechococcus strain WH8113.

The marine cyanobacterium Synechococcus strain WH8113 swims in the absence of any recognizable organelles of locomotion. We have found that calcium is required for this motility. Cells deprived of calcium stopped swimming, while addition of calcium completely restored motility. No other divalent ions tested could replace calcium. Terbium, a lanthanide ion, blocked motility even when calcium was present at 10(5)-fold-higher concentrations, presumably by occupying calcium binding sites. Calcium chelators, EGTA or EDTA, blocked motility, even when calcium was present at 25-fold-higher concentrations, presumably by acting as calcium ionophores. Finally, motility was blocked by verapamil and nitrendipine, molecules known to block voltage-gated calcium channels of eukaryotic cells by an allosteric mechanism. These results suggest that a calcium potential is involved in the mechanism of motility.

Calcium↗
PubMed1998

Cloning, characterization, and transcriptional analysis of a gene encoding an alpha-crystallin-related, small heat shock protein from the thermophilic cyanobacterium Synechococcus vulcanus.

hspA, a gene encoding a 16-kDa heat-induced protein from the thermophilic cyanobacterium Synechococcus vulcanus, has been cloned and sequenced. The deduced amino acid sequence of the gene product showed significant homology to sequences of the family of alpha-crystallin-related, small heat shock proteins. A monocistronic mRNA of hspA increased transiently in response to heat shock. The heat shock induction occurred at a vegetative promoter but without the CIRCE (controlling inverted repeat of chaperone expression) element.

Amino Acid Sequence↗
PubMed1998

cis-acting sequences required for NtcB-dependent, nitrite-responsive positive regulation of the nitrate assimilation operon in the cyanobacterium Synechococcus sp. strain PCC 7942.

There are three binding sites for NtcA (nirI, nirII, and nirIII), the global nitrogen regulator of cyanobacteria, in the DNA region between the two divergently transcribed operons (nirA and nirB operons) involved in nitrate assimilation in Synechococcus sp. strain PCC 7942. Using the luxAB reporter system, we showed that nirI and nirIII, which are located 23 bp upstream from the -10 promoter element of nirA and nirB, respectively, are required for induction by nitrogen depletion of the nirA and nirB operons, respectively. The induction of nirA operon transcription was a prerequisite for the nitrite-responsive positive regulation of the transcription by NtcB, a LysR-type protein. The NtcA-binding site nirII, located in the middle of the nirA-nirB intergenic region, and a potential binding site for a LysR-type protein (TGCAN5TGCA; designated L1), located between nirI and nirII, were required for the nitrite-responsive, NtcB-dependent enhancement of nirA operon transcription. Although the requirement for the L1 site was consistent with the involvement of the LysR family protein NtcB in transcriptional regulation, NtcB did not bind to the nirA regulatory region in vitro in the presence of nitrite and NtcA, suggesting the involvement of some additional factor(s) in the regulation. An L1-like inverted repeat with the consensus sequence TGCN7GCA was conserved in the nirA promoter region of cyanobacteria, being centered at position -23 with respect to the NtcA-binding site corresponding to nirI, which suggested the common occurrence of nitrite-responsive regulation of the nitrate assimilation operon among cyanobacteria.

Bacterial Proteins↗
PubMed1998

Nitrite-specific active transport system of the cyanobacterium Synechococcus sp. strain PCC 7942.

Studies on the nitrite uptake capability of a mutant of Synechococcus sp. strain PCC 7942 lacking the ATP-binding cassette-type nitrate-nitrite-bispecific transporter revealed the occurrence of a nitrite-specific active transport system with an apparent Km (NO2-) of about 20 microM. Similar to the nitrate-nitrite-bispecific transporter, the nitrite-specific transporter was reversibly inhibited by ammonium in the medium.

Biological Transport, Active↗
PubMed1998

Regulation of ntcA expression and nitrite uptake in the marine Synechococcus sp. strain WH 7803.

NtcA is a transcriptional activator involved in global nitrogen control in cyanobacteria. In the absence of ammonium it regulates the transcription of a series of genes encoding proteins required for the uptake and assimilation of alternative nitrogen sources (I. Luque, E. Flores, and A. Herrero, EMBO J. 13:2862-2869, 1994). ntcA, present in a single copy in the marine Synechococcus sp. strain WH 7803, was cloned and sequenced. The putative amino acid sequence shows a high degree of identity to NtcA from freshwater cyanobacteria in two functional domains. The expression of ntcA was negatively regulated by ammonium from a putative transcription start point located downstream of an NtcA consensus recognition sequence. Addition of either rifampin or ammonium led to a rapid decline in ntcA transcript levels with half-lives of less than 2 min in both cases. Nitrate-grown cells showed high ntcA transcript levels, as well as the capacity for active nitrite uptake. However, ammonium-grown cells showed low levels of the ntcA transcript and did not utilize nitrite. The addition of ammonium to nitrite uptake-active cells resulted in a gradual decline in the rate of uptake over a 24-h period. Active nitrite uptake was not induced in cells transferred to medium lacking a nitrogen source despite evidence of elevated expression of ntcA, indicating that ntcA expression is not sufficient for uptake capacity to develop. Nitrate and nitrite addition led to the development of nitrite uptake, whereas the addition of leucine did not. Furthermore, nitrite addition triggered the de novo protein synthesis required for uptake capacity to develop. These data suggest that nitrite and nitrate act as specific inducers for the synthesis of proteins required for nitrite uptake.

Amino Acid Sequence↗