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

AbstractPII proteins signal the cellular nitrogen status in numerous bacteria, and in cyanobacteria PII is subjected to serine phosphorylation when the cells experience a high C to N balance. In the unicellular cyanobacterium Synechococcus sp. PCC 7942, the PII 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 PII-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 PII protein or by modified PII proteins that cannot be phosphorylated and mimic either the phosphorylated (GlnBS49D and GlnBS49E) or unphosphorylated (GlnBS49A) form of PII. However, strains carrying the GlnBS49D and GlnBS49E mutant proteins exhibited higher levels of expression of nitrogen-regulated genes than the strains carrying the wild-type PII or the GlnBS49A protein