ROLE OF THE GLNK SIGNAL-TRANSDUCTION PROTEIN IN THE REGULATION OF NITROGEN ASSIMILATION IN ESCHERICHIA-COLI

Two structurally similar but functionally distinct PII-like proteins, PII and GlnK, regulate nitrogen assimilation in Escherichia coil, Stud ies with cells indicated that both Prl (the glnB product) and GlnK (th e glnK product) acted through the kinase/phosphatase NRII [NtrB, the g lnL (ntrB) product] to reduce transcription initiation from Ntr promot ers, apparently by regulating the phosphorylation state of the transcr iptional activator NRI similar to P (NtrC similar to P, the phosphoryl ated form of the glnG (ntrC) product). Both GlnK and PII also acted th rough adenylyltransferase (ATase, the glnE product) to regulate the ad enylylation state of glutamine synthetase (GS), The activity of both G lnK and PII was regulated by the signal-transducing uridylyltransferas e/uridylyl-removing enzyme (UTase/UR, glnD product). Our experiments i ndicate that either PII or GlnK could effectively regulate ATase, but that PII was required far the efficient regulation of NRII required to prevent expression of glnA, which encodes GS, Yet, GlnK also particip ated in regulation of NRII, Although calls that lack either PII or Gln K grew well, cells lacking both of these proteins were defective for g rowth on nitrogen-rich minimal media. This defect was alleviated by th e loss of NRII, and was apparently due to unregulated expression of th e Ntr regulon. Also, mutations in glnK, designated glnK, were obtaine d as suppressors of the Ntr(-) phenotype of a double mutant lacking PI I and the UTase/UR, These suppressors appeared to reduce, but not elim inate, the ability of GlnK to prevent Ntr gene expression by acting th rough NRII. We hypothesize that one role of GlnK is to regulate the ex pression of the level of NRI similar to P during conditions of severe nitrogen starvation, and by so doing to contribute to the regulation o f certain Ntr genes.