Regulatory Roles of the Structural Homologues P_II and P_z Proteins in Azospirillum brasilense

Abstract

Glutamine synthetase (GS), encoded by glnA, is a key enzyme in ammonia assimilation in diverse species. In enteric bacteria, four proteins are involved in the nitrogen regulation of glnA transcription. The uridylyltransferase (UTase) is the primary nitrogen sensor. The small trimeric protein P_II, encoded by glnB, is an intracellular signal transmitter which coordinates the response of the Ntr system to the N-status of the cell. Under conditions of nitrogen limitation the UTase covalently modifies P_II by uridylylation. The signal which stimulates the sensing system is based on the concentration ratio of regulatory metabolites. UTase activity is inhibited by glutamine, whereas a-ketoglutarate and ATP bound to P_II protein, modify its allosteric properties facilitating its uridylylation. The two other proteins, NtrB, a histidine kinase, and NtrC the transcriptional activator of glnA, belong to a two component regulatory system. In addition of the regulation of GS biosynthesis, the activity of GS is also modified by reversible adenylylation according to the cell-N status (reviewed by Magasanik, 1996).