“...the authors propose that NepR acts as an anti-sigma factor for σEcfG1 and that PhyR acts as an anti-antisigma factor.”
The Alphaproteobacterium
Methylobacterium extorquens
colonizes the surface of plant leaves, where it encounters various stresses, including temperature changes, oxidative stress and desiccation. Response to these stresses requires the regulator PhyR. Unusually, PhyR contains both a receiver domain, which is normally found in proteins in two-component signalling cascades and is often fused to a DNA-binding domain, and a domain that resembles an ECF sigma factor. Using complementation experiments, the authors showed that the sigma factor-like domain of PhyR mediates this response and is regulated by the receiver domain in a phosphorylation-dependent manner. Despite the similarity to ECF sigma factors, no DNA-binding activity or interaction with RNA polymerase could be established for PhyR. However, in many Alphaproteobacteria, nepR orthologues are located upstream of a gene encoding an ECF sigma factor, and NepR orthologues have been proposed to regulate ECF sigma factor activity. Overexpression of nepR made the bacterium more sensitive to various stresses, an effect that could be partially overcome by concomitant overexpression of PhyR. In vitro, both full-length phosphorylated PhyR and the PhyR ECF domain interacted with NepR. In searching for an ECF sigma factor that controls PhyR-dependent genes, the authors identified σEcfG1 as a possible target for NepR. In vitro studies showed that a truncated version of σEcfG1 and NepR could form a heterodimer, and NepR was found to bind both PhyR and σEcfG1 in vivo.
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