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. (C) 2003 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.