An amino-terminal threonine/serine motif is necessary for activity of the Crp/Fnr homolog, MrpC, and for Myxococcus xanthus developmental robustness

The Crp/Fnr family of transcriptional regulators play central roles in transcriptional control of diverse physiological responses. Activation of individual family members is controlled by a surprising diversity of mechanisms tuned to the particular physiological responses or lifestyles that they regulate. MrpC is a Crp/Fnr homolog that plays an essential role in controlling the Myxococcus xanthus developmental program. A long-standing model proposed that MrpC activity is controlled by the Pkn8/Pkn14 serine/threonine kinase cascade which phosphorylates MrpC on threonine residue(s) located in its extreme amino terminus. In this study, we demonstrate that a stretch of consecutive threonine and serine residues, T21 T22 S23 S24, is necessary for MrpC activity by promoting efficient DNA binding. Mass spectrometry analysis indicated the TTSS motif is not directly phosphorylated by Pkn14 in vitro but is necessary for efficient Pkn14-dependent phosphorylation on several residues in the remainder of the protein. Pkn8 and Pkn14 kinase activities do not play obvious roles in controlling MrpC activity in wild type M. xanthus under laboratory conditions, but likely modulate MrpC DNA binding in response to unknown environmental conditions. Interestingly, mutational analysis of the TTSS motif caused non-robust developmental phenotypes, revealing that MrpC plays a role in developmental buffering.