Bacillus subtilis cells entering stationary phase due to nutrient deprivation have a number of options. Complex interconnected regulatory circuits govern differential gene expression patterns that channel the cell along the path it has sensed is most advantageous for survival in the environment. The actual choice depends upon the activity of an elaborate signal transduction network (the phosphorelay) that ultimately affects the activity of two key transcription factors, SpoOA and AbrB. Should the cell commit to sporulation, a temporally and spatially controlled cascade of RNA polymerase sigma factors leads to the development and release of an endospore from within the terminally differentiated, apoptotic mother cell.

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.