Fig. 1: Nitrogen regulation of nitrogen fixation in Klebsiella oxytoca.

The upper part of the diagram shows how the intracellular concentration of glutamine is synthesized by GS which could be modified by the bifunctional enzyme GlnE, and regulates the activity of the PII signal transduction proteins (GlnB and GlnK) as a consequence of post-translational modification by the bifunctional enzyme GlnD (UTase/UR). This has downstream consequences for transcriptional regulation of genes meditated by the nitrogen regulatory NtrBC two-component regulatory system. Under excess nitrogen conditions, when the intracellular concentration of glutamine is relatively high, the PII protein GlnB interacts with NtrB, favoring dephosphorylation of NtrC, restricting its ability to activate transcription (left side of the diagram). Under nitrogen deficient conditions, when the glutamine concentration is low, GlnD uridylylates the PII proteins, preventing the interaction of GlnB with NtrB. As a consequence, phosphorylation of NtrC is favored, enabling activation of Ntr-regulated genes (right side of diagram). The lower part of the figure depicts the influence of nitrogen status on the regulation of nitrogen fixation by the NifL and NifA regulatory proteins. Under nitrogen-limiting conditions (right), expression of the glnK-amtB and nifL-nifA operons is activated by phosphorylated NtrC. As a consequence, GlnK is highly expressed (and also uridylylated by GlnD), preventing the formation of an inhibitory complex between NifL and NifA, thus enabling NifA to activate nif transcription. Under nitrogen excess conditions (left) expression of the glnK-amtB and nifL-nifA operons is limited and there is insufficient GlnK to prevent NifL from inactivating NifA.