Analysis of abrB mutations, mutant proteins, and why abrB does not utilize a perfect consensus in the -35 region of its sigma A promoter.

The Bacillus subtilis global regulator AbrB is a DNA-binding protein composed of six identical monomers of 96 amino acids that shows specificity to the promoter regions of its target genes including its own. We have sequenced thirteen previously uncharacterized abrB mutations. Four mutant AbrB proteins were purified, and their DNA-binding properties and multimeric structures were examined. AbrB23 (R25S) had no appreciable DNA binding activity but retained a hexameric structure, indicating that Arg25 is important in DNA interactions. Three other mutant proteins, AbrB1 (C56Y), AbrB19 (Gln83-->termination codon), and AbrB100 (L69P), showed decreased DNA binding and altered multimeric interactions. Analysis of the expression and AbrB binding affinities of mutant abrB promoters demonstrated that a consensus -35 region is incompatible with proper autoregulation of the abrB gene.