The Signal Transduction Protein PII Controls the Levels of the Cyanobacterial Protein PipX

Cyanobacteria, microorganisms performing oxygenic photosynthesis, must adapt their metabolic processes to environmental challenges such as day and night changes. PipX, a unique regulatory protein from cyanobacteria, provides mechanistic link between the signalling PII, widely conserved (in bacteria plants) transducer of carbon/nitrogen/energy richness, transcriptional regulator NtcA, which controls large regulon involved in nitrogen assimilation. PipX is also translational regulation through interaction with ribosome-assembly GTPase EngA. However, increases PipX/PII ratio are toxic, presumably due abnormally increased binding other partner(s). Here, we present mutational structural analyses reported PipX-PII PipX-NtcA complexes, leading identification single amino acid changes that decrease or abolish toxicity. Notably, 4 out 11 mutations decreasing toxicity did not levels, suggesting targeted residues (F12, D23, L36, R54) provide determinants. In addition, one those four (D23A) argued against over-activation NtcA cause Most at contacting PII decreased indicating stability would depend on its ability bind conclusion supported by light-induced levels Synechococcus elongatus PCC7942 (hereafter S. elongatus).