Mutations at pipX Suppress Lethality of P II -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. In Synechococcus elongatus PCC 7942, where P II forms complexes with the NtcA coactivator PipX, attempts to engineer P II -deficient strains failed in a wild-type background but were successful in pipX null mutants. Consistent with the idea that P II is essential to counteract the activity of PipX, four different spontaneous mutations in the pipX gene were found in cultures in which glnB had been genetically inactivated.