Alternative Sigma Factor of Interacts with the Cognate Antisigma Factor Primarily Using Its Domain 3.

σ, an alternative sigma factor, is usually employed to tackle the general stress response in and other Gram-positive bacteria. This protein, involved in -mediated pathogenesis, is typically blocked by RsbW, an antisigma factor having serine kinase activity. σ, a σ-like sigma factor, harbors three conserved domains designated σ, σ, and σ. To better understand the interaction between RsbW and σ or its domains, we have studied their recombinant forms, rRsbW, rσ, rσ, rσ, and rσ, using different probes. The results show that none of the rσ domains, unlike rσ, showed binding to a cognate DNA in the presence of a core RNA polymerase. However, both rσ and rσ, like rσ, interacted with rRsbW, and the order of their rRsbW binding affinity looks like rσ > rσ > rσ. Furthermore, the reaction between rRsbW and rσ or rσ was exothermic and occurred spontaneously. rRsbW and rσ also associate with each other at a stoichiometry of 2:1, and different types of noncovalent bonds might be responsible for their interaction. A structural model of the RsbW-σ complex that has supported our experimental results indicated the binding of rσ at the putative dimeric interface of RsbW. A genetic study shows that the tentative dimer-forming region of RsbW is crucial for preserving its rσ binding ability, serine kinase activity, and dimerization ability. Additionally, a urea-induced equilibrium unfolding study indicated a notable thermodynamic stabilization of σ in the presence of RsbW. Possible implications of the stabilization data in drug discovery were discussed at length.