Interactions of Bacillus subtilis RNA polymerase with subunits determining the specificity of initiation.  Sigma and delta peptides can bind simultaneously to core. 
The Bacillus subtilis RNA polymerase sigma 43 subunit and the phage SP82 encoded 28-kDa peptide are responsible for the binding of RNA polymerase to early and middle SP82 promoters, respectively.  The delta peptide enhances the specificity of the interaction of B. subtilis RNA polymerase with these promoters.  We have used sedimentation experiments to determine the effect of each of the three specificity factors, delta, sigma, and the 28-kDa peptide, on the binding of the other two factors to RNA polymerase core and the effect of NaCl on these binding equilibria.  We show that sigma 43 and the 28-kDa peptide can each bind to RNA polymerase core at the same time as delta.  Sigma 43 and the 28-kDa peptide have similar affinities to core at 0.1 M NaCl, but the 28-kDa peptide binds to core-delta more strongly than sigma 43.  The implications of these findings with respect to the replacement of sigma 43 by the 28-kDa peptide and the mechanism of promoter search by B. subtilis RNA polymerase are discussed.