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

sigma(B), the general stress response sigma factor of Bacillus subtilis, is activated when intracellular ATP levels fall or the bacterium experiences environmental stress. Stress activates sigma(B) by means of a collection of regulatory kinases and phosphatases (the Rsb proteins), which catalyze the release of sigma(B) from an anti-sigma factor inhibitor. By using the yeas t dihybrid selection system to identify B. subtilis proteins that could int eract with Rsb proteins and act as mediators of stress signaling, we isolat ed the GTP binding protein, Obg, as an interactor with several of these reg ulators (RsbT, RsbW, and RsbX). B. subtilis depleted of Obg no longer activ ated sigma(B) in response to environmental stress, but it retained the abil ity to activate sigma(B) by the ATP responsive pathway. Stress pathway comp onents activated sigma(B) in the absence of Obg if the pathway's most upstr eam effector (RsbT) was synthesized in excess to the inhibitor (RsbS) from which it is normally released after stress. Thus, the Rsb proteins can func tion in the absence of Obg but fail to be triggered by stress. The data dem onstrate that Obg, or a process under its control, is necessary to induce t he stress-dependent activation of sigma(B) and suggest that Obg may directl y communicate with one or more sigma(B) regulators.