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Query: UMLS:C1832526 (PCC)
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Cell coloration changes from normal blue-green to yellow or yellow-green when the cyanobacterium Synechococcus sp. strain PCC 7942 is deprived of an essential nutrient. We found that this bleaching process (chlorosis) in cells deprived of sulfur (S) was similar to that in cells deprived of nitrogen (N), but that cells deprived of phosphorus (P) bleached differently. Cells divided once after N deprivation, twice after S deprivation, and four times after P deprivation. Chlorophyll (Chl) accumulation stopped almost immediately upon N or S deprivation but continued for several hours after P deprivation. There was no net Chl degradation during N, S, or P deprivation, although cellular Chl content decreased because cell division continued after Chl accumulation ceased. Levels of the light-harvesting phycobiliproteins declined dramatically in a rapid response to N or S deprivation, reflecting an ordered breakdown of the phycobilisomes (PBS). In contrast, P-deprived cultures continued to accumulate PBS for several hours. Whole PBS were not extensively degraded in P-deprived cells, although the PBS contents of P-deprived cells declined because of continued cell division after PBS accumulation ceased. Levels of mRNAs encoding PBS polypeptides declined by 90 to 95% in N- or S-deprived cells and by 80 to 85% in P-deprived cells. These changes in both the synthesis and stability of PBS resulted in a 90% decline in the PC/Chl ratio of N- or S-deprived cells and a 40% decline in the PC/Chl ratio of P-deprived cells. Therefore, although bleaching appears to be a general response to nutrient deprivation, it is not the same under all nutrient-limited conditions and is probably composed of independently controlled subprocesses.
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PMID:Chlorosis induced by nutrient deprivation in Synechococcus sp. strain PCC 7942: not all bleaching is the same. 162 59

Cyanobacteria acclimate to low temperature by desaturating their membrane lipids. Mutant strains of Synechococcus sp. PCC 7002 containing insertionally inactivated desA (Delta12 acyl-lipid desaturase) and desB (omega3 acyl-lipid desaturase) genes were produced, and their low-temperature susceptibility was characterized. The desA mutant synthesized no linoleic acid or alpha-linolenic acid, and the desB mutant did not produce alpha-linolenic acid. The desA mutant grew more slowly than the wild-type at 22 degrees C and could not grow at 15 degrees C. The desB mutant could not continuously grow at 15 degrees C, although no observable phenotype appeared at higher temperatures. It has been shown that expression of the desA gene occurs at 38 degrees C and is up-regulated at 22 degrees C, and that the desB gene is only expressed at 22 degrees C. These results indicate that the expression of the desA and desB genes occurs at higher temperatures than those at which a significant decline in physiological activities is caused by the absence of their products. The temperature dependency of photosynthesis was not affected by these mutations. Since chlorosis and inability to grow at 15 degrees C with nitrate was suppressed by the substitution of urea as a nitrogen source, it is very likely that the chilling susceptibility of the desaturase mutants is attributable to nutrient limitation.
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PMID:Alteration of low-temperature susceptibility of the cyanobacterium Synechococcus sp. PCC 7002 by genetic manipulation of membrane lipid unsaturation. 939 31

When deprived of essential nutrients, the non-diazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 undergoes a proteolytic degradation of the phycobiliproteins, its major light-harvesting pigments. This process is known as chlorosis. This paper presents evidence that the degradation of phycobiliproteins is part of an acclimation process in which growing cells differentiate into non-pigmented cells able to endure long periods of starvation. The time course of degradation processes differs for various photosynthetic pigments, for photosystem I and photosystem II activities and is strongly influenced by the illumination and by the experimental conditions of nutrient deprivation. Under standard experimental conditions of combined nitrogen deprivation, three phases of the differentiation process can be defined. The first phase corresponds to the well-known phycobiliprotein degradation, in phase 2 the cells lose chlorophyll a prior to entering phase 3, the fully differentiated state, in which the cells are still able to regenerate pigmentation after the addition of nitrate to the culture. An analysis of the protein synthesis patterns by two-dimensional gel electrophoresis during nitrogen starvation indicates extensive differential gene expression, suggesting the operation of tight regulatory mechanisms.
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PMID:Nitrogen-starvation-induced chlorosis in Synechococcus PCC 7942: adaptation to long-term survival. 978 92

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

Nitrogen (N) limitation in cyanobacteria is well documented: a reduced growth rate is observed, accompanied by a cessation of phycobiliprotein synthesis and an ordered degradation of phycobilisomes (PBS). This leads to a dramatic bleaching phenomenon known as chlorosis. In Synechococcus strain PCC 7942, bleaching due to PBS degradation is also observed under sulfur (S) or phosphorus (P) limitation, and all three are under the control of the nblA gene product, a 59-amino-acid polypeptide which is overexpressed under N, S, and P starvation (J. L. Collier, and A. R. Grossman, EMBO J. 13:1039-1047, 1994). Cyanobase sequence data for Synechocystis strain PCC 6803 indicate the presence of two tandem open reading frames (sll0452 and sll0453) homologous to nblA. We cloned the two genes, identified a unique 5' mRNA end suggestive of a single transcription start site, and studied nblA expression under conditions of N or S starvation by Northern hybridization: transcripts were detected only under N starvation (no signal is detected in replete medium or with S starvation), whether nblA1 or nblA2 was used as a probe. Mutations in nblA1 and nblA2 were constructed by insertion of a kanamycin cassette; both mutations were nonbleaching under N starvation. Synechocystis strain PCC 6803 does not bleach under S starvation, consistent with the absence of nblA induction in these conditions. These results were confirmed by analysis of the PBS components: sequential degradation of phycocyanin and associated linkers was observed only under conditions of N starvation. This indicates differences between Synechocystis strain PCC 6803 and Synechococcus strain PCC 7942 in their regulatory and signaling pathways leading to N- and S-starved phenotypes.
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PMID:Nitrogen or sulfur starvation differentially affects phycobilisome degradation and expression of the nblA gene in Synechocystis strain PCC 6803. 1132 25

Cells of the non-diazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 acclimate to nitrogen deprivation by differentiating into non-pigmented resting cells, which are able to survive prolonged periods of starvation. In this study, the physiological properties of the long-term nitrogen-starved cells are investigated in an attempt to elucidate the mechanisms of maintenance of viability. Preservation of energetic homeostasis is based on a low level of residual photosynthesis; activities of photosystem II and photosystem I were approximately 0.1% of activities of vegetatively growing cells. The low levels of photosystem I activity were measured by a novel colorimetric assay developed from the activity staining of ferredoxin:NADP+ oxidoreductase. Photosystem II reaction centers, as determined by chlorophyll fluorescence measurements, exhibited normal properties, although the efficiency of light harvesting was significantly reduced compared with that of control cells. Long-term chlorotic cells carried out protein synthesis at a very low, but detectable level, as revealed by in vivo [35S]methionine labeling and two-dimensional gel electrophoresis. In conjunction with the very low levels of total cellular protein contents, this implies a continuous protein turnover during chlorosis. Synthesis of components of the photosynthetic apparatus could be detected, whereas factors of the translational machinery were stringently down-regulated. Beyond the massive loss of protein during acclimation to nitrogen deprivation, two proteins that were identified as SomA and SomB accumulated due to an induced expression following nitrogen reduction.
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PMID:Nitrogen starvation-induced chlorosis in Synechococcus PCC 7942. Low-level photosynthesis as a mechanism of long-term survival. 1135 Oct 86

The nblA family of genes encodes for small proteins necessary for the ordered degradation of phycobilisomes under certain stress conditions, a process known as chlorosis. Genes homologous to nblA seem to occur in all phycobilisome-containing organisms. However, to date, no molecular mechanism is known for the action of NblA, nor have the gene products been characterized to understand the physical properties of the molecule and thus help elucidate the mechanism on a structural basis. In this study we report on the first characterization of an NblA-homologous gene product. The chromosomal gene from the cyanobacterium Anabaena sp. PCC 7120 was cloned, heterologously expressed in Escherichia coli and purified to apparent homogeneity. This allowed the protein to be characterized by analytical ultracentrifugation and CD spectroscopy. These experiments show that the NblA protein has a mostly alpha-helical structure, undergoing an association reaction of folded monomers to form trimers in solution. No dimers are detectable.
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PMID:NblA from Anabaena sp. PCC 7120 is a mostly alpha-helical protein undergoing reversible trimerization in solution. 1223 May 74

Cyanobacterial light-harvesting complexes, the phycobilisomes, are proteolytically degraded when the organisms are starved for combined nitrogen, a process referred to as chlorosis or bleaching. Gene nblA, present in all phycobilisome-containing organisms, encodes a protein of about 7 kDa that plays a key role in phycobilisome degradation. The mode of action of NblA in this degradation process is poorly understood. Here we presented the 1.8-A crystal structure of NblA from Anabaena sp. PCC 7120. In the crystal, NblA is present as a four-helix bundle formed by dimers, the basic structural units. By using pull-down assays with immobilized NblA and peptide scanning, we showed that NblA specifically binds to the alpha-subunits of phycocyanin and phycoerythrocyanin, the main building blocks of the phycobilisome rod structure. By site-directed mutagenesis, we identified amino acid residues in NblA that are involved in phycobilisome binding. The results provided evidence that NblA is directly involved in phycobilisome degradation, and the results allowed us to present a model that gives insight into the interaction of this small protein with the phycobilisomes.
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PMID:Crystal structure of NblA from Anabaena sp. PCC 7120, a small protein playing a key role in phycobilisome degradation. 1635 35

SufA, IscA, and Nfu have been proposed to function as scaffolds in the assembly of Fe/S clusters in bacteria. To investigate the roles of these proteins further, single and double null-mutant strains of Synechococcus sp. strain PCC 7002 were constructed by insertional inactivation of genes homologous to sufA, iscA, and nfu. Demonstrating the nonessential nature of their products, the sufA, iscA, and sufA iscA mutants grew photoautotrophically with doubling times that were similar to the wild type under standard growth conditions. In contrast, attempts to inactivate the nfu gene only resulted in stable merodiploids. These results imply that Nfu, but not SufA or IscA, is the essential Fe/S scaffold protein in cyanobacteria. When cells were grown under iron-limiting conditions, the iscA and sufA mutant strains exhibited less chlorosis than the wild type. Under iron-sufficient growth conditions, isiA transcript levels, a marker for iron limitation in cyanobacteria, as well as transcript levels of genes in both the suf and isc regulons were significantly higher in the iscA mutant than in the wild type. Under photosynthesis-induced redox stress conditions, the transcript levels of the suf genes are notably higher in the sufA and the sufA iscA mutants than in the wild type. The growth phenotypes and mRNA abundance patterns of the mutant strains contradict the proposed scaffold function for the SufA and IscA proteins in generalized Fe/S cluster assembly and instead suggest that they play regulatory roles in iron homeostasis and the sensing of redox stress in cyanobacteria.
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PMID:Regulatory roles for IscA and SufA in iron homeostasis and redox stress responses in the cyanobacterium Synechococcus sp. strain PCC 7002. 1662 10

Cells of Anabaena PCC 7119 and of a mutant strain of Nostoc muscorum unable to fix dinitrogen, grown at pH 8 and under low CO(2) tension (air), showed a reduced capacity for photosynthesis when cultured in the absence of sodium, this inhibition being followed by symptoms of photooxidation, such as chlorosis, oxygen consumption in the light, and decrease of superoxide dismutase activity. The impairment of photosynthesis preceded that of nitrogenase activity, indicating that the requirement for sodium in photosynthesis was independent of its effects on nitrogen metabolism. However, when cyanobacteria were grown at pH 6.3 or under high CO(2) tensions, sodium was not required for photosynthesis and no symptoms of photooxidation were observed.
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PMID:Sodium Requirement for Photosynthesis and Its Relationship with Dinitrogen Fixation and the External CO(2) Concentration in Cyanobacteria. 1666 40


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