Obg, an essential GTP binding protein of Bacillus subtilis, is necessary for stress activation of transcription factor σ(B)

σ(B), the general stress response or factor of Bacillus subtilis, is activated when intracellular ATP levels fall or the bacterium experiences environmental stress. Stress activates σ(B) by means of a collection of regulatory kinases and phosphatases (the Rsb proteins), which catalyze the release of σ(B) from an anti-σ factor inhibitor. By using the yeast dihybrid selection system to identify B. subtilis proteins that could interact with Rsb proteins and act as mediators of stress signaling, we isolated the GTP binding protein, Obg, as an interactor with several of these regulators (RsbT, RsbW, and RsbX). B. subtilis depleted of Obg no longer activated σ(B) in response to environmental stress, but it retained the ability to activate σ(B) by the ATP responsive pathway. Stress pathway components activated σ(B) in the absence of Obg if the pathway's most upstream effector (RsbT) was synthesized in excess to the inhibitor (RsbS) from which it is normally released after stress. Thus, the Rsb proteins can function in the absence of Obg but fail to be triggered by stress. The data demonstrate that Obg, or a process under its control, is necessary to induce the stress-dependent activation of σ(B) and suggest that Obg may directly communicate with one or more σ(B) regulators.