Effects of amino acid substitutions in the -10 binding region of sigma E from Bacillus subtilis

The sigma subunit of bacterial RNA polymerase is required for specific binding to promoters. One region in most sigma factors makes sequence-specific contacts at the -10 region of its cognate promoters. To test the role of the amino acids in this -10 binding region, we examined the effects of 49 single-amino-acid substitutions in sigma E from Bacillus subtilis. We assayed the effect of each amino acid substitution on spore formation because sigma E is essential for endospore formation in B. subtilis. Our results showed that substitutions at several positions, including the highly conserved aromatic amino acid at position 102, had little or no detectable effect. Substitutions at another position, position 117, produced dominant negative mutations; we suggest that these mutations allow RNA polymerase containing the mutant sigma factor to bind specifically to promoters but prevent transcription initiation. Of the recessive defective alleles, those that produced substitutions at positions 113, 115, and 120 produced the most defective sigma factors. These results suggest that the residues at or near these positions in wild-type sigma E play important roles in sigma E function.