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. Here, we show that the three spontaneous pipX point mutations pipX-92delT, pipX160C>T and pipX194T>A, initially found in different glnB strains, are indeed suppressor mutations. When these mutations were reconstructed in the wild-type background, the glnB gene could be efficiently inactivated. Furthermore, the point mutations have different effects on PipX levels, coactivation of NtcA-dependent genes and protein-protein interactions. Further support for an in vivo role of PipX-P(II) complexes is provided by interaction analysis with the in vivo-generated P(II)(T-loop+7) protein, a P(II) derivative unable to interact with its regulatory target N-acetyl-l-glutamate kinase, but which retains the ability to bind to PipX. The implications of these results are discussed.