The DNA mismatch repair gene hexA was identified in Lactococcus lactis by PCR amplification by using a pair of primers homologous to the DNA-binding Dps protein. The gene in its entirety, including the regulatory regions, was sequenced, by using a strategy of chromosomal walking based on two PCR protocols. The open reading frame of 2526 bp was preceded by a strong ribosome-binding site (AGGAAG) and was followed by a potential transcription terminator (hairpin loop structure). The 5 ' terminus of the hexA mRNA was located 135 bp upstream of the start codon, and putative - 10 and - 35 regions were identified. The deduced amino acid sequence revealed two motifs, the ATP/GTP-binding site (P-loop) and the "MutS family signature". The hexA promoter was cloned into pMU1327, which contained a promoter-less CAT reporter gene, and the promoter activity was examined under oxidative-stress conditions. It appears that the promoter activity is down-shifted by H2O2 at 4 mm.