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Zusammenfassung:
Alternative sigma factors regulate the activity of RNA polymerases under
specific conditions and are regulated through various mechanisms, most
of which depend on anti-sigma factors to regulate their on/off status.
This study reports a new mode of sigma factor regulation that does not
require an anti-sigma factor, but instead sigma factor phosphorylation
in response to the presence of an antibiotic.
A major form of transcriptional regulation in bacteria occurs through
the exchange of the primary sigma factor of RNA polymerase (RNAP) with
an alternative extracytoplasmic function (ECF) sigma factor(1). ECF
sigma factors are generally intrinsically active and are retained in an
inactive state via the sequestration into sigma factor-anti-sigma factor
complexes until their action is warranted(2-20). Here, we report a
previously uncharacterized mechanism of transcriptional regulation that
relies on intrinsically inactive ECF sigma factors, the activation of
which and interaction with the beta '-subunit of RNAP depends on sigma
factor phosphorylation. In Vibrio parahaemolyticus, the threonine kinase
PknT phosphorylates the sigma factor EcfP, which results in EcfP
activation and expression of an essential polymyxin-resistant regulon.
EcfP phosphorylation occurs at a highly conserved threonine residue,
Thr63, positioned within a divergent region in the sigma 2.2 helix. Our
data indicate that EcfP is intrinsically inactive and unable to bind the
beta '-subunit of RNAP due to the absence of a negatively charged DAED
motif in this region. Furthermore, our results indicate that
phosphorylation at residue Thr63 mimics this negative charge and
licenses EcfP to interact with the beta '-subunit in the formation of
the RNAP holoenzyme, which in turn results in target gene expression.
This regulatory mechanism is a previously unrecognized paradigm in
bacterial signal transduction and transcriptional regulation, and our
data suggest that it is widespread in bacteria.
