Role of Cys54 in AbrB multimerization and DNA-binding activity

The DNA-binding, global regulatory protein AbrB from Bacillus subtilis is homotetrameric in solution. Mutation of the lone cysteine present in the protomers (C54), to either a serine, tyrosine or tryptophan, abolishes DNA-binding activity in vitro and regulatory activity in vivo. The effect of these changes is not due to abrogation of disulfide bond formation since it can be shown biochemically that none of the C54 residues participates in disulfide bond formation. It is unlikely that C54 is involved in direct contact with DNA targets. Rather, it appears that the role of C54 is to provide a nucleophilic center required for proper spatial orientation of the polypeptide subunits.