Mutational analysis of the cyanobacterial nitrogen regulator PipX.

PipX provides a functional link between the cyanobacterial global transcriptional regulator NtcA and the signal transduction protein P(II), a protein found in all three domains of life as integrators of signals of the nitrogen and carbon balance. PipX, which is toxic in the absence of P(II), can form alternative complexes with NtcA and P(II) and these interactions are respectively stimulated and inhibited by 2-oxoglutarate, providing a mechanism by which P(II) can modulate expression at the NtcA regulon. Structural information on PipX-NtcA complexes suggests that PipX coactivates NtcA controlled genes by stabilizing the active conformation of NtcA bound to 2-oxoglutarate and by possibly helping recruit RNA polymerase.

The nitrogen interaction network in Synechococcus WH5701, a cyanobacterium with two PipX and two P(II)-like proteins.

Nitrogen regulation involves the formation of different types of protein complexes between signal transducers and their transcriptional or metabolic targets. In oxygenic phototrophs, the signal integrator P(II) activates the enzyme N-acetyl-l-glutamate kinase (NAGK) by complex formation. P(II) also interacts with PipX, a protein with a tudor-like domain that mediates contacts with P(II) and with the transcriptional regulator NtcA, to which it binds to increase its activity.

Effects of spontaneous mutations in PipX functions and regulatory complexes on the cyanobacterium Synechococcus elongatus strain PCC 7942.

In Synechococcus elongatus sp. PCC 7942, PipX forms complexes with P(II), a protein found in all three domains of life as an integrator of signals of the nitrogen and carbon balance, and with the cyanobacterial nitrogen regulator NtcA. We recently showed that previous inactivation of pipX facilitates subsequent inactivation of the glnB gene.

Mutations at pipX suppress lethality of PII-deficient mutants of Synechococcus elongatus PCC 7942.

The P(II) proteins are found in all three domains of life as key integrators of signals reflecting the balance of nitrogen and carbon. Genetic inactivation of P(II) proteins is typically associated with severe growth defects or death. However, the molecular basis of these defects depends on the specific functions of the proteins with which P(II) proteins interact to regulate nitrogen metabolism in different organisms.