The PhyR homolog RSP_1274 of Rhodobacter sphaeroides is involved in defense of membrane stress and has a moderate effect on RpoE (RSP_1092) activity

Background: A major role of the PhyR-NepR-sigma(EcfG) cascade in the general stress response was demonstrated for some bacterial species and considered as conserved in Alphaproteobacteria. The sigma(EcfG) factor activates its target genes in response to diverse stresses and NepR represents its anti-sigma factor. PhyR comprises a response regulator domain and a sigma factor domain and acts as anti-sigma factor antagonist. The facultative phototrophic alphaproteobacterium Rhodobacter sphaeroides harbours a PhyR homolog in the same genomic context as found in other members of this class. Results: Our study reveals increased expression of the phyR gene in response to superoxide, singlet oxygen, and diamide and also an effect of PhyR on rpoE expression. RpoE has a central role in mounting the response to singlet oxygen in R. sphaeroides. Despite these findings a mutant lacking PhyR was not significantly impeded in resistance to oxidative stress, heat stress or osmotic stress. However a role of PhyR in membrane stress is demonstrated. Conclusion: These results support the view that the effect of the PhyR-NepR-sigma(EcfG) cascade on diverse stress responses varies among members of the Alphaproteobacteria. In the facultative phototroph Rhodobacter sphaeroides PhyR plays no major role in the general stress or the oxidative stress response but rather has a more specialized role in defense of membrane stress.