1. Regulation of nitrogen assimilation.
Regulation of nitrogen assimilation. Uridylyltransferase (UTase) and uridylyl-removing enzyme (UR) sense the relative amount of nitrogen. Under limiting conditions, UTase uridylylates PII to PII-UMP. In excess nitrogen, the UR deuridylylates PII-UMP to PII. PII favors adenylyltransferase (ATase) activity. ATase inactivates GS by adenylylating it to GS-AMP. PII (or its ortholog GlnK) also enhances the phosphatase activity of the histidine kinase/phosphatase NRII, which then dephosphorylates NRI∼P. This diminishes transcription from the nitrogen-regulated promoters such as ntr, nac, glnK, and glnALG. In contrast, PII-UMP enhances the deadenylylation of GS-AMP, hence activating the enzyme. PII-UMP exerts no direct effect on NRII; however, the lack of PII favors the kinase activity of NRII, which donates its phosphoryl group to NRI. Acetyl∼P also donates its phosphoryl group to NRI. Transcription from the glnALG promoter requires low amounts of NRI∼P (thin arrow), whereas transcription from the other promoters requires high amounts of NRI∼P (thick arrows).
Alan J. Wolfe Microbiol. Mol. Biol. Rev. 2005; doi: /MMBR