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Query: UMLS:C1832526 (PCC)
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P(II) proteins signal the cellular nitrogen status in numerous bacteria, and in cyanobacteria P(II) is subjected to serine phosphorylation when the cells experience a high C to N balance. In the unicellular cyanobacterium Synechococcus sp. PCC 7942, the P(II) protein (glnB gene product) is known to mediate the ammonium-dependent inhibition of nitrate and nitrite uptake. The analysis of gene expression through RNA/DNA hybridization indicated that a P(II)-null mutant was also impaired in the induction of NtcA-dependent, nitrogen assimilation genes amt1 (ammonium permease), glnA (glutamine synthetase) and nir (nitrite reductase), as well as of the N-control gene ntcA, mainly under nitrogen deprivation. This gene expression phenotype of the glnB mutant could be complemented by wild-type P(II) protein or by modified P(II) proteins that cannot be phosphorylated and mimic either the phosphorylated (GlnB(S49D) and GlnB(S49E)) or unphosphorylated (GlnB(S49A)) form of P(II). However, strains carrying the GlnB(S49D) and GlnB(S49E) mutant proteins exhibited higher levels of expression of nitrogen-regulated genes than the strains carrying the wild-type P(II) or the GlnB(S49A) protein.
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PMID:Transcriptional effects of the signal transduction protein P(II) (glnB gene product) on NtcA-dependent genes in Synechococcus sp. PCC 7942. 1275 2

Expression of the glnA gene encoding glutamine synthetase, a key enzyme in nitrogen metabolism, is subject to a variety of regulatory mechanisms in different organisms. In the filamentous, N(2)-fixing cyanobacterium Anabaena sp. strain PCC 7120, glnA is expressed from multiple promoters that generate several transcripts whose abundance is influenced by NtcA, the transcription factor exerting global nitrogen control in cyanobacteria. Whereas RNA(I) originates from a canonical NtcA-dependent promoter (P(1)) and RNA(II) originates from a sigma(70)-type promoter (P(2)), RNA(IV) is influenced by NtcA but the corresponding promoter (P(3)) does not have the structure of NtcA-activated promoters. Using RNA isolated from Anabaena filaments grown under different nitrogen regimens, we observed, in addition to these transcripts, RNA(V), which has previously been detected only in in vitro transcription assays and should originate from P(4). However, in heterocysts, which are differentiated cells specialized in N(2) fixation, RNA(I) was the almost exclusive glnA transcript. Analysis of P(glnA)::lacZ fusions containing different fragments of the glnA upstream region confirmed that fragments carrying P(1), P(2), or P(3) and P(4) have the ability to promote transcription. Mutation of the NtcA-binding site in P(1) eliminated P(1)-directed transcription and allowed increased use of P(2). The NtcA-binding site in the P(1) promoter and binding of NtcA to this site appear to be key factors in determining glnA gene expression in vegetative cells and heterocysts.
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PMID:The NtcA-dependent P1 promoter is utilized for glnA expression in N2-fixing heterocysts of Anabaena sp. strain PCC 7120. 1548 45

The phosphorylated signal transduction protein P(II) (P(II)-P) in the cyanobacterium Synechocystis sp. strain PCC 6803 is dephosphorylated by PphA, a protein phosphatase of the 2C family (PP2C). In this study, the physiological conditions of P(II)-P dephosphorylation were investigated with respect to the in vivo specificity of P(II)-P towards PphA and the cellular abundance of PphA in cells growing under different nitrogen regimes. Furthermore, the consequences of impaired P(II)-P dephosphorylation with respect to short-term inhibition of glutamine synthetase (GS) were studied. With a contribution of approximately 15 % of total Mn(2+)-dependent p-nitrophenyl phosphate hydrolysis activity, PphA has only a minor impact on the total PP2C activity in Synechocystis extracts. Nevertheless, residual P(II)-P dephosphorylation in PphA-deficient cells could only be observed after prolonged incubation in the presence of ammonium. The abundance of PphA correlates with the phosphorylation state of P(II) under nitrogen-replete conditions and is specifically enhanced by nitrite. Regulation of pphA expression operates at the post-transcriptional level. In the presence of nitrate/nitrite, PphA is present in molar excess over P(II)-P, enabling the cells to rapidly dephosphorylate P(II)-P in response to changing environmental conditions. A PphA-deficient mutant is not impaired in short-term inhibition of GS activity following ammonium treatment. Down-regulation of GS occurs by induction of gif genes (encoding GS inactivating factors 7 and 17), which is controlled by NtcA-mediated gene repression. Thus, impaired P(II)-P dephosphorylation does not affect ammonium-prompted inactivation of NtcA.
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PMID:Protein phosphatase PphA from Synechocystis sp. PCC 6803: the physiological framework of PII-P dephosphorylation. 1581 94

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.
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PMID:Ammonium assimilation in cyanobacteria. 1614 48

The Synechocystis sp. strain PCC 6803 mutant deficient in PII protein (the glnB gene product) was found to express glutamine synthetase activity at levels several times higher than the wild-type strain. There was no significant difference in nitrate reductase activity levels between the two strains, and the nitrite reductase levels were somewhat lower in the mutant than in the wild-type strain. The higher glutamine synthetase activity in the mutant was ascribed to higher expression levels of the glutamine synthetase genes (glnA and glnN), which belong to the regulon controlled by NtcA, a Crp-family transcription regulator. Examination of the effects of PII deficiency on other NtcA-regulated genes revealed that the transcript levels of amt1 (encoding an ammonium permease) and gifB (encoding an inhibitor of glutamine synthetase) were increased, whereas that of gifA (a homolog of gifB, encoding another glutamine synthetase inhibitor) was decreased, with those of nirA, nrtC, icd, sigE (rpoD2-V), nblA and ntcA being unaffected. Unlike the Synechococcus elongatus strain PCC 7942, induction or repression of the NtcA-regulated genes proceeded normally in the PII-deficient mutant upon nitrogen depletion. The altered steady-state expression levels of glnA, glnN, amt1, gifA and gifB in the PII-deficient mutant suggested that Synechocystis sp. strain PCC 6803 has a mechanism for regulation of the subset of the NtcA-regulated genes related directly to ammonium assimilation.
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PMID:Effects of PII deficiency on expression of the genes involved in ammonium utilization in the cyanobacterium Synechocystis sp. Strain PCC 6803. 1654 96

The Synechococcus sp. PCC 7942 nitrogen regulator PipX interacts in a 2-oxoglutarate-dependent manner with the global nitrogen transcription factor NtcA and the signal transduction protein P(II). In vivo, PipX is involved in the NtcA-dependent induction of glnB and glnN genes. To further investigate the extent to which PipX is involved in global nitrogen control, the effect of pipX inactivation on various nitrogen-regulated processes was determined. The PipX-deficient mutant was able to use nitrate as a nitrogen source and to efficiently inhibit the nitrate transport upon ammonium addition but showed decreased nitrate and nitrite reductase activities and a delay in the induction of nitrate utilization after transfer of cultures from ammonium- to nitrate-containing media. In contrast to the wild-type, glutamine synthetase activity was not upregulated upon depletion of combined nitrogen from cultures of the mutant strain. Inactivation of pipX impaired induction of nblA and delayed phycobilisome degradation, but did not affect recovery of nitrogen-deprived cultures. Taken together, the results indicate that PipX interacts with NtcA to facilitate efficient acclimation of cyanobacteria to conditions of nitrogen limitation.
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PMID:Role of the Synechococcus PCC 7942 nitrogen regulator protein PipX in NtcA-controlled processes. 1732 91

The Synechocystis sp. PCC 6803 glutamine synthetase type I (GS) activity is controlled by a process that involves protein-protein interaction with two inactivating factors (IF7 and IF17). Following addition of ammonium, the genes encoding these proteins, gifA and gifB, respectively, are derepressed, leading to the synthesis of IF7 and IF17 and consequently GS is inactivated. Upon ammonium removal, the GS activity rapidly returns to the initial level within 20 min. In this study, we analyse the mechanism underlying GS reactivation and find that this process involves IF7 and IF17 degradation. We show that the presence of ammonium as nitrogen source enhances IF17 but not IF7 stability independently of gif gene transcription. Studies with Synechocystis crude extracts under different conditions revealed that IF7 and IF17 display different stabilities in vitro. We found that IF7 is degraded in vitro by the activity of metalloproteases. Furthermore, the involvement of soluble processing metallopeptidases in IF7 degradation has also been demonstrated in vivo, by analysing Synechocystis mutant strains devoid of genes of the prp family. Finally, using a Synechocystis strain lacking GS type I, we establish the crucial role of the target protein GS for in vivo IF7 and IF17 stability.
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PMID:The ammonium-inactivated cyanobacterial glutamine synthetase I is reactivated in vivo by a mechanism involving proteolytic removal of its inactivating factors. 1758 Nov 27

PII is an important signal protein for regulation of nitrogen metabolism in bacteria and plants. We constructed a mutant of glnB, encoding PII, in a heterocystous cyanobacterium, Anabaena sp. PCC 7120, with a cre-loxP system. The mutant (MP2alpha) grew more slowly than the wild type under all nitrogen regimens. It excreted a large amount of ammonium when grown on nitrate due to altered activities of glutamine synthetase and nitrate reductase. MP2alpha had a low nitrogenase activity but was able to form heterocysts under diazotrophic conditions, suggesting that PII is not required for heterocyst differentiation. Analysis of the PII with mass spectroscopy found tyrosine nitration at Tyr-51 under diazotrophic conditions while no phosphorylation at Ser-49 was detected. The strains 51F and 49A, which have PII with mutations of Y51F and S49A, respectively, were constructed to analyze the functions of the two key residues on the T-loop. Like MP2alpha, they had low nitrogenase activity and grew slowly under diazotrophic conditions. 49A was also impaired in nitrate uptake and formed heterocysts in the presence of nitrate. The up-regulation of ntcA after nitrogen step-down, which was present in the wild type, was not observed in 51F and 49A. While our results showed that the Ser-49 residue is important to the function of PII in Anabaena sp. PCC 7120, evidence from the PII pattern of the wild type and 49A in non-denaturing gel electrophoresis suggested that Ser-49 is not modified. The possible physiological roles of tyrosine nitration of PII are discussed.
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PMID:PII is important in regulation of nitrogen metabolism but not required for heterocyst formation in the Cyanobacterium Anabaena sp. PCC 7120. 1787 43

Glutaminase is widely distributed among microorganisms and mammals with important functions. Little is known regarding the biochemical properties and functions of the deamidating enzyme glutaminase in cyanobacteria. In this study a putative glutaminase encoded by gene slr2079 in Synechocystis sp. PCC 6803 was investigated. The slr2079 was expressed as histidine-tagged fusion protein in Escherichia coli. The purified protein possessed glutaminase activity, validating the functional assignment of the genomic annotation. The apparent K (m) value of the recombinant protein for glutamine was 26.6 +/- 0.9 mmol/L, which was comparable to that for some of other microbial glutaminases. Analysis of the purified protein revealed a two-fold increase in catalytic activity in the presence of 1 mol/L Na(+). Moreover, the K (m) value was decreased to 12.2 +/- 1.9 mmol/L in the presence of Na(+). These data demonstrate that the recombinant protein Slr2079 is a glutaminase which is regulated by Na(+) through increasing its affinity for substrate glutamine. The slr2079 gene was successfully disrupted in Synechocystis by targeted mutagenesis and the Deltaslr2079 mutant strain was analyzed. No differences in cell growth and oxygen evolution rate were observed between Deltaslr2079 and the wild type under standard growth conditions, demonstrating slr2079 is not essential in Synechocystis. Under high salt stress condition, however, Deltaslr2079 cells grew 1.25-fold faster than wild-type cells. Moreover, the photosynthetic oxygen evolution rate of Deltaslr2079 cells was higher than that of the wild-type. To further characterize this phenotype, a number of salt stress-related genes were analyzed by semi-quantitative RT-PCR. Expression of gdhB and prc was enhanced and expression of desD and guaA was repressed in Deltaslr2079 compared to the wild type. In addition, expression of two key enzymes of ammonium assimilation in cyanobacteria, glutamine synthetase (GS) and glutamate synthase (GOGAT) was examined by semi-quantitative RT-PCR. Expression of GOGAT was enhanced in Deltaslr2079 compared to the wild type while GS expression was unchanged. The results indicate that slr2079 functions in the salt stress response by regulating the expression of salt stress related genes and might not play a major role in glutamine breakdown in Synechocystis.
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PMID:Characterization of a sodium-regulated glutaminase from cyanobacterium Synechocystis sp. PCC 6803. 1909 79

Genes homologous to those implicated in glutamine synthetase (GS) regulation by protein-protein interaction in the cyanobacterium Synechocystis sp. strain PCC 6803 are conserved in several cyanobacterial sequenced genomes. We investigated this GS regulatory mechanism in Anabaena sp. strain PCC 7120. In this strain the system operates with only one GS inactivation factor (inactivation factor 7A [IF7A]), encoded by open reading frame (ORF) asl2329 (gifA). Following addition of ammonium, expression of gifA is derepressed, leading to the synthesis of IF7A, and consequently, GS is inactivated. Upon ammonium removal, the GS activity returns to the initial level and IF7A becomes undetectable. The global nitrogen control protein NtcA binds to the gifA promoter. Constitutive high expression levels of gifA were found in an Anabaena ntcA mutant (CSE2), indicating a repressor role for NtcA. In vitro studies demonstrate that Anabaena GS is not inactivated by Synechocystis IFs (IF7 and IF17), indicating the specificity of the system. We constructed an Anabaena strain expressing a second inactivating factor, containing the amino-terminal part of IF17 from Synechocystis fused to IF7A. GS inactivation in this strain is more effective than that in the wild type (WT) and resembles that observed in Synechocystis. Finally we found differential expression of the IF system between heterocysts and vegetative cells of Anabaena.
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PMID:Posttranscriptional regulation of glutamine synthetase in the filamentous Cyanobacterium Anabaena sp. PCC 7120: differential expression between vegetative cells and heterocysts. 2063 19


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