The hrcA gene of Bacillus subtilis encodes a transcriptional repressor protein which negatively controls the heat shock operons dnaK and groESL. Alignment of the HrcA protein with repressor proteins from the NCBI database revealed that it exhibits a striking homology near its N-terminal part with proteins of the DeoR family. This region contains a helix-turn-helix motif and has been shown to be involved in DNA binding. To investigate whether this is also true for the HrcA protein, three critical amino acid residues were changed within or adjacent to the recognition helix. While single amino acid replacements barely influenced the binding activity, alteration of two consecutive amino acid residues within the recognition helix completely abolished the binding activity. When this mutant hrcA allele was expressed together with the wild-type allele within the same cell, it conferred a dominant-negative phenotype to the cells underlining that these amino acid residues are crucial for specific DNA binding and that HrcA binds to DNA in an oligomeric form.