The RecA protein is a key enzyme involved in DNA recombination in bacteria. Using a polymerase chain reaction (PCR) amplification we cloned a recA homolog from Helicobacter pylori. The gene revealed an open reading frame (ORF) encoding a putative protein of 37.6 kDa showing closest homology to the Campylobacter jejuni RecA (75.5% identity). A putative ribosome binding site and a near-consensus sigma 70 promoter sequence was found upstream of recA. A second ORF, encoding a putative protein with N-terminal sequence homology to prokaryotic and eukaryotic enolases, is located directly downstream of recA. Compared to the wild-type strains, isogenic H. pylori recA deletion mutants of strains 69A and NCTC11637 displayed increased sensitivity to ultraviolet light and abolished general homologous recombination. The recombinant H. pylori RecA protein produced in Escherichia coli strain GC6 (recA-) was 38 kDa in size but inactive in DNA repair, whereas the corresponding protein in H. pylori 69A migrated at the greater apparent molecular weight of approx. 40 kDa in SDS-polyacrylamide gels. However, complementation of the H. pylori mutant using the cloned recA gene on a shuttle vector resulted in a RecA protein of the original size and fully restored the general functions of the enzyme. These data can be best explained by a modification of RecA in H. pylori which is crucial for its function. The potential modification seems not to occur when the protein is produced in E. coli, giving rise to a smaller but inactive protein.