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

The P-II protein is encoded by a unique glnB gene in Synechococcus sp. stra in 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 s upply. RNA-DNA hybridization experiments revealed the presence of one trans cript species 680 nucleotides long, whatever the nutrient conditions tested . A second transcript species, 620 nucleotides long, absent in the NtcA nul l 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 C O2 concentration. Primer extension analysis indicated that the two transcri pt species are generated from two tandem promoters, a sigma(70) Escherichia coli-type promoter and an NtcA-dependent promoter, located 120 and 53 nucl eotides, respectively, from the glnB initiation codon. The NtcA-dependent p romoter is up-regulated under the conditions mentioned above, while the sig ma(70) 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 correlat ion between the amounts of the two transcript species and that of the P-II protein was observed, as revealed by immunodetection with specific antibodi es. The phosphorylation level of P-II in the wild type is inversely correla ted with nitrogen availability and directly correlated with higher CO2 conc entration. This regulation is correspondingly less stringent in the NtcA nu ll mutant cells. In contrast, the dephosphorylation of P-II is NtcA indepen dent.