Binding of σAand σBto Core RNA Polymerase after Environmental Stress inBacillus subtilis

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Abstract
In Bacillus subtilis, the alternative sigma factor σB is activated in response to environmental stress or energy depletion. The general stress regulon under the control of σB provides the cell with multiple stress resistance. Experiments were designed to determine how activated σB replaces σA as a constituent of the RNA polymerase holoenzyme. Studies of the transcription of the σA-dependent stress gene clpE under σB-inducing conditions showed that expression was higher in a sigB mutant background than in the wild type. The relative affinities of σA and σB for binding to the core RNA polymerase (E) were determined by means of indirect surface plasmon resonance. The results showed that the affinity of σB for E was 60-fold lower than that of σA. Western blot analyses with antibodies against σA, σB, and E showed that, after exposure to ethanol stress, the concentration of σB was only twofold higher than those of σA and E. Thus, the concentration of σB 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 σB to E after stress.

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