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Abstract:
The Crp/Fnr family of transcriptional regulators play central roles in
transcriptional control of diverse physiological responses, and are
activated by a surprising diversity of mechanisms. MrpC is a Crp/Fnr
homolog that controls 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, T-21 T-22 S-23 S-24,S- 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. In an important correction to
a long-standing model, we show Pkn8 and Pkn14 kinase activities do not
play obvious roles in controlling MrpC activity in wild-type M. xanthus
under laboratory conditions. Instead, we propose Pkn14 modulates MrpC
DNA binding in response to unknown environmental conditions.
Interestingly, substitutions in the TTSS motif caused developmental
defects that varied between biological replicates, revealing that MrpC
plays a role in promoting a robust developmental phenotype.
