Binding of σ ^A and σ ^B to Core RNA Polymerase after Environmental Stress in Bacillus subtilis

In Bacillus subtilis , the alternative sigma factor σ B is activated in response to environmental stress or energy depletion. The general stress regulon under the control of σ B provides the cell with multiple stress resistance. Experiments were designed to determine how activated σ B replaces σ A as a constituent of the RNA polymerase holoenzyme. Studies of the transcription of the σ A -dependent stress gene clpE under σ B -inducing conditions showed that expression was higher in a sigB mutant background than in the wild type. The relative affinities of σ A and σ B for binding to the core RNA polymerase (E) were determined by means of indirect surface plasmon resonance. The results showed that the affinity of σ B for E was 60-fold lower than that of σ A . Western blot analyses with antibodies against σ A , σ B , and E showed that, after exposure to ethanol stress, the concentration of σ B was only twofold higher than those of σ A and E. Thus, the concentration of σ B after stress is not high enough to compensate for its relatively low affinity for E, and it seems that additional mechanisms must be invoked to account for the binding of σ B to E after stress.