The product of the abrB gene of Bacillus subtilis is an ambiactive repressor and activator of the transcription of genes expressed during the transition state between vegetative growth and the onset of stationary phase and sporulation. Purified AbrB protein binds specifically in a highly co-operative fashion to fragments of DNA containing the promoters it affects. DNase I footprints of the binding regions in these promoters revealed large protected areas of 50-120 nucleotides or more depending on the promoter. Methylation protection experiments gave protected guanine residues on only one face of the DNA helix. A consensus sequence could be deduced around these guanine residues that was not found around non-protected guanine residues in the footprint region. The results suggested that stationary phase functions and sporulation are repressed during active growth by AbrB and other transition state regulators by binding to the affected promoters in a concentration-dependent manner.