Transcriptional effects of the signal transduction protein PII (glnB gene product) on NtcA‐dependent genes in Synechococcus sp. PCC 7942

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.