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Query: UMLS:C1832526 (PCC)
 5,967 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A new glutamine synthetase gene, glnN, which encodes a polypeptide of 724 amino acid residues (M(r), 79,416), has been identified in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803; this is the second gene that encodes a glutamine synthetase (GS) in this cyanobacterium. The functionality of this gene was evidenced by its ability to complement an Escherichia coli glnA mutant and to support Synechocystis growth in a strain whose glnA gene was inactivated by insertional mutagenesis. In this mutant (strain SJCR3), as well as in the wild-type strain, the second GS activity was subject to regulation by the nitrogen source, being strongly enhanced in nitrogen-free medium. Transcriptional fusion of a chloramphenicol acetyltransferase (cat) gene with the 5'-upstream region of glnN suggested that synthesis of the second Synechocystis GS is regulated at the transcriptional level. Furthermore, the level of glnN mRNA, a transcript of about 2,300 bases, was found to be strongly increased in nitrogen-free medium. The glnN product is similar to the GS subunits of Bacteroides fragilis and Butyrivibrio fibrisolvens, two obligate anaerobic bacteria whose GSs are markedly different from other prokaryotic and eukaryotic GSs. However, significant similarity is evident in the five regions which are homologous in all of the GSs so far described. The new GS gene was also found in other cyanobacteria but not in N2-fixing filamentous species.
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PMID:A new type of glutamine synthetase in cyanobacteria: the protein encoded by the glnN gene supports nitrogen assimilation in Synechocystis sp. strain PCC 6803. 790 87

A monospecific anti-(glutamine synthetase) antibody raised against glutamine synthetase of the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 immunoreacted with glutamine synthetase from the N2-fixing heterotrophic bacterium Azotobacter chroococcum. In Western-blotting experiments this antibody recognized a single protein of a molecular mass of 59 kDa corresponding to glutamine synthetase subunit. This protein was in vivo-labelled in response to addition of ammonium, both [3H]adenine and H(3)32PO4 preincubation of the cells being equally effective. Nevertheless, the amount of glutamine synthetase present in A. chroococcum was independent of the available nitrogen source. Modified, inactive glutamine synthetase was re-activated by treatment with snake-venom phosphodiesterase but not by alkaline phosphatase. L-Methionine-DL-sulphoximine, an inhibitor of glutamine synthetase, prevented the enzyme from being covalently modified. We conclude that, in A. chroococcum, glutamine synthetase is adenylylated in response to ammonium and that for the modification to take place ammonium must be metabolized.
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PMID:In vivo modification of Azotobacter chroococcum glutamine synthetase. 790 89

The existence in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 of two genes (glnA and glnN) coding for glutamine synthetase (GS) has been recently reported (J.C. Reyes and F.J. Florencio, J. Bacteriol. 176:1260-1267, 1994). In the current work, the regulation of the nitrate assimilation system was studied with a glnA-disrupted Synechocystis mutant (strain SJCR3) in which the only GS activity is that corresponding to the glnN product. This mutant was unable to grow in ammonium-containing medium because of its very low levels of GS activity. In the SJCR3 strain, nitrate and nitrite reductases were not repressed by ammonium, and short-term ammonium-promoted inhibition of nitrate uptake was impaired. In Synechocystis sp. strain PCC 6803, nitrate seems to act as a true inducer of its assimilation system, in a way similar to that proposed for the dinitrogen-fixing cyanobacteria. A spontaneous derivative strain from SJCR3 (SJCR3.1), was able to grow in ammonium-containing medium and exhibited a fourfold-higher level of GS activity than but the same amount of glnN transcript as its parental strain (SJCR3). Taken together, these finding suggest that SJCR3.1 is a mutant affected in the posttranscriptional regulation of the GS encoded by glnN. This strain recovered regulation by ammonium of nitrate assimilation. SJCR3 cells were completely depleted of intracellular glutamine shortly after addition of ammonium to cells growing with nitrate, while SJCR3.1 cells maintained glutamine levels similar to that reached in the wild-type Synechocystis sp. strain PCC 6803. Our results indicate that metabolic signals that control the nitrate assimilation system in Synechocystis sp. strain PCC 6803 require ammonium metabolism through GS.
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PMID:A mutant lacking the glutamine synthetase gene (glnA) is impaired in the regulation of the nitrate assimilation system in the cyanobacterium Synechocystis sp. strain PCC 6803. 800 75

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.
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PMID:Transcription of glutamine synthetase genes (glnA and glnN) from the cyanobacterium Synechocystis sp. strain PCC 6803 is differently regulated in response to nitrogen availability. 909 67

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.
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PMID:Involvement of NtcB, a LysR family transcription factor, in nitrite activation of the nitrate assimilation operon in the cyanobacterium Synechococcus sp. strain PCC 7942. 924 51

The phosphorylation state of the putative signal transduction protein P(II) from the cyanobacterium Synechococcus sp. strain PCC 7942 depends on the cellular state of nitrogen and carbon assimilation. In this study, dephosphorylation of phosphorylated P(II) protein (P[II]-P) was investigated both in vivo and in vitro. The in vivo studies implied that P(II)-P dephosphorylation is regulated by inhibitory metabolites involved in the glutamine synthetase-glutamate synthase pathway of ammonium assimilation. An in vitro assay for P(II)-P dephosphorylation was established that revealed a Mg2+-dependent P(II)-P phosphatase activity. P(II)-P phosphatase and P(II) kinase activities could be separated biochemically. A partially purified P(II)-P phosphatase preparation also catalysed the dephosphorylation of phosphoserine/phosphothreonine residues on other proteins in a Mg2+-dependent manner. However, only dephosphorylation of P(II)-P was regulated by synergistic inhibition by ATP and 2-oxoglutarate. As the same metabolites stimulate the P(II) kinase activity, it appears that the phosphorylation state of P(II) is determined by ATP and 2-oxoglutarate-dependent reciprocal reactivity of P(II) towards its phosphatase and kinase.
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PMID:Dephosphorylation of the phosphoprotein P(II) in Synechococcus PCC 7942: identification of an ATP and 2-oxoglutarate-regulated phosphatase activity. 938 91

The unicellular cyanobacterium Synechocystis sp. strain PCC 6803 has two putative pathways for ammonium assimilation: the glutamine synthetase-glutamate synthase cycle, which is the main one and is finely regulated by the nitrogen source; and a high NADP-dependent glutamate dehydrogenase activity (NADP-GDH) whose contribution to glutamate synthesis is uncertain. To investigate the role of the latter, we used two engineered mutants, one lacking and another overproducing NADP-GDH. No major disturbances in the regulation of nitrogen-assimilating enzymes or in amino acids pools were detected in the null mutant, but phycobiline content, a sensitive indicator of the nutritional state of cyanobacterial cells, was significantly reduced, indicating that NADP-GDH plays an auxiliary role in ammonium assimilation. This effect was already prominent in the initial phase of growth, although differences in growth rate between the wild type and the mutants were observed at this stage only at low light intensities. However, the null mutant was unable to sustain growth at the late stage of the culture at the point when the wild type showed the maximum NADP-GDH activity, and died faster in ammonium-containing medium. Overexpression of NADP-GDH improved culture proliferation under moderate ammonium concentrations. Competition experiments between the wild type and the null mutant confirmed that the presence of NADP-GDH confers a selective advantage to Synechocystis sp. strain PCC 6803 in late stages of growth.
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PMID:The presence of glutamate dehydrogenase is a selective advantage for the Cyanobacterium synechocystis sp. strain PCC 6803 under nonexponential growth conditions. 992 43

Pseudanabaena sp. strain PCC 6903 is the first cyanobacteria lacking the typical prokaryotic glutamine synthetase type I encoded by the glnA gene. The glnN gene product, glutamine synthetase type III, is the only glutamine synthetase activity present in this cyanobacterium. Analysis of glnN expression clearly indicated a nitrogen-dependent regulation. Pseudanabaena glnN gene expression and GSIII activity were upregulated under nitrogen starvation or using nitrate as a nitrogen source, while low levels of transcript and activity were found in ammonium-containing medium. Primer extension analysis showed that the glnN gene promoter structure resembled that of the NtcA-related promoters. Mobility shift assays demonstrated that Synechocystis sp. PCC 6803 NtcA protein, expressed and purified from Escherichia coli, bound to the promoter of the Pseudanabaena 6903 glnN gene. The NtcA control of the glnN gene in this cyanobacterium suggested that, in the absence of a glnA gene, NtcA took control of the only glutamine synthetase gene in a fashion similar to the way the glnA gene is governed in those cyanobacteria harbouring a glnA gene.
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PMID:Nitrogen control of the glnN gene that codes for GS type III, the only glutamine synthetase in the cyanobacterium Pseudanabaena sp. PCC 6903. 998 84

Nitrate assimilation-defective mutants SP7, SP9, and SP17 of the cyanobacterium Anabaena sp. PCC 7120 were isolated by use of transposon mutagenesis and screened on medium containing chlorate. SP7 and SP17 represented nitrate reductase-defective nature, while mutant SP9 appeared to be a regulatory mutant exhibiting pleiotropic behavior. Kinetics of nitrate uptake system exhibited K(s) values of 31-38 &mgr;M for parent, SP7, and SP17 strains; however, mutant SP9 exhibited a high K(s) value of 109.5 &mgr;M. Defective nitrate reductase was apparent in mutant SP7 and SP9, while mutant SP17 exhibited partial defective nature. Methyl viologen-dependent NR activity in parent strain presented a biphasic nature with K(m) values of 0.13 and 2.47 mM, whereas a single K(m) value (2.96 mM) was observed for mutant SP17. Mutant SP9 was also defective in nitrite uptake and reduction. Mutant strains exhibited derepressed nitrogenase activity in the presence of nitrate, while glutamine synthetase activity remained unaltered.http://link.springer-ny. com/link/service/journals/00284/bibs/39n5p237.html</HEA
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PMID:Mutants of the cyanobacterium anabaena sp. PCC 7120 altered in nitrate transport and reduction 1048 30

The nondiazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 responds to nitrogen deprivation by differentiating into nonpigmented resting cells able to survive prolonged periods of starvation. The degradation of photosynthetic pigments, termed chlorosis, proceeds in an ordered manner in which the light-harvesting phycobiliproteins are degraded prior to chlorophyll. Here, we show that the function of the global transcription activator of nitrogen-regulated genes, NtcA, is required for the sequential pigment degradation and cell survival. The P(II) protein, known to signal the nitrogen status of the cells, is most probably not involved in the perception of the nitrogen-starvation-specific signal since in a mutant lacking P(II), chlorosis proceeded in the same manner as in the wild type. Inhibition of glutamine synthetase with l-methionine sulfoximine led to a rapid decrease of apc mRNA and to an increase of nblA mRNA levels, which is characteristic for nitrogen deprivation, suggesting that nitrogen starvation is sensed by a metabolic signal connected to glutamine synthetase activity. However, l-methionine sulfoximine treatment did not induce phycobiliprotein degradation, but led to an immediate cessation of this proteolytic process after its induction by nitrogen deprivation. This suggests that the proteolytic activity elicited by the expression of nblA has to be supported by glutamine synthetase activity.
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PMID:Nitrogen starvation in synechococcus PCC 7942: involvement of glutamine synthetase and NtcA in phycobiliprotein degradation and survival 1052 42


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