Fig. 6

Proposed mechanism of stressosome activation. The stressosome is made up of RsbR, RsbS, and RsbT. Stress signals are transduced to the stressosome core from Pril42 to RsbR N-terminal regions (turrets) which could potentially undergo conformational changes. The conserved phosphorylation sites (T715, T209, S56) that activate the stressosome are clustered in one region. In cells that are unstressed, the serine-threonine kinase RsbT phosphorylates T175. During stress the phosphorylation of T175 enhances the activity of the RsbT kinase, which phosphorylates RsbS S56. Following phosphorylation of RsbS, RsbT is released, and binds and activate the phosphatase RsbU. RsbU dephosphorylates a form of RsbV, which releases RsbW. The anti-sigma factor RsbW binds to σB. Sigma B releases RsbW and activates RNA polymerase and the transcription of the stress response genes. During conditions of prolonged stress, RsbT phosphorylates residue T209 whose access is possibly controlled by a disordered loop. In Bacillus phosphorylation of residue T209 leads to mild activation of σB¹²