Role of phosphorylated metabolic intermediates in the regulation of glutamine synthetase synthesis in Escherichia coli

Transcription of the Ntr regulon is controlled by the two-component system consisting of the response regulator NR(I) (NtrC) and the kinase/phosphatase NR(II) (NtrB), which both phosphorylates and dephosphorylates NR(I). Even though in vitro transcription from nitrogen-regulated promoters requires phosphorylated NR(I), NR(II)-independent activation of NR(I) also occurs in vivo. We show here that this activation likely involves acetyl phosphate; it is eliminated by mutations that reduce synthesis of acetyl phosphate and is elevated by a mutation expected to cause accumulation of acetyl phosphate. With purified components, we investigated the mechanism by which acetyl phosphate stimulates glutamine synthetase synthesis. Acetyl phosphate, carbamyl phosphate, and phosphoramidate but not ATP or phosphoenolpyruvate acted as substrates for the autophosphorylation of NR(I) in vitro. Phosphorylated NR(I) produced by this mechanism exhibited the properties associated with NR(I) phosphorylated by NR(II), including the activated ATPase activity of the central domain of NR(I) and the ability to activate transcription from the nitrogen-regulated glutamine synthetase glnAp₂ promoter.