Regulation of an alternative sigma factor σI by a partner switching mechanism with an anti-sigma factor PrsI and an anti-anti-sigma factor ArsI in Streptomyces coelicolor A3(2).

Two genes, prsI and arsI, are located divergently next to the sigI gene encoding alternative sigma factor σI of Streptomyces coelicolor A3(2). The similarity of PrsI and ArsI to anti-sigma and anti-anti-sigma factors, respectively, suggests that both putative regulators may be involved in regulation of σI. By using a combination of several approaches including bacterial two-hybrid assays, pull-down assay and visualization of the complex by native polyacrylamide electrophoresis, we demonstrated that PrsI specifically interacts with σI and ArsI. In vitro phosphorylation demonstrated that PrsI serves as a specific kinase for its putative partner, ArsI, and negatively regulates its activity. The sigI gene was deleted in the S. coelicolor M145 strain without obvious effect on growth, stress response and differentiation. Complex transcriptional analyses of sigI, prsI, and arsI revealed that sigI is directed by a single promoter induced by osmotic stress, arsI is directed by a single constitutive promoter, and prsI is directed by two tandem promoters, one constitutive and the second one induced by osmotic stress. None of the determined promoters was dependent upon σI, σB and σH. These data suggested a role of σI in the osmotic stress response and its regulation by a partner switching mechanism through the anti-sigma factor PrsI and its antagonist, anti-anti-sigma factor ArsI.