Loss of ribosomal protein L11 blocks stress activation of the Bacillus subtilis transcription factor sigma(B)

sigma (B), the general stress response sigma factor of Bacillus subtilis, i s activated when the cell's energy levels decline or the bacterium is expos ed to environmental stress (e.g,, heat shock, ethanol). Physical stress act ivates sigma (B) through a collection of regulatory kinases and phosphatase s (the Rsb proteins) which catalyze the release of sigma (B) from an anti-s igma (B) factor inhibitor. The means by which diverse stresses communicate with the Rsb proteins is unknown; however, a role for the ribosome in this process was suggested when several of the upstream members of the sigma (B) stress activation cascade (RsbR, -S, and T) were found to cofractionate wi th ribosomes in crude B. subtilis extracts. We now present evidence for the involvement of a ribosome-mediated process in the stress activation of sig ma (B). B. subtilis strains resistant to the antibiotic thiostrepton, doe t o the loss of ribosomal protein L11 (RplK), were found to be blocked in the stress activation of sigma (B). Neither the energy-responsive activation o f sigma (B) nor stress-dependent chaperone gene induction (a sigma (B)-inde pendent stress response) was inhibited by the loss of L11. The Rsb proteins required for stress activation of sigma (B) are shown to be active in the RplK(-) strain but fail to be triggered by stress. The data demonstrate tha t the B. subtilis ribosomes provide an essential input for the stress activ ation of sigma (B) and suggest that the ribosomes may themselves be the sen sors for stress in this system.