The recA gene was isolated from Streptomyces ambofaciens DSM40697. Its nucleotide sequence predicted a protein of 372 residues. Two recA mutants, NSAR1001 and NSAR57, obtained by gene disruption encoded a RecA protein lacking respectively 30 and at least 62 amino acids from the C-terminal end. NSAR1001 showed a wild-type sensitivity to UV light and oxolinic acid. In contrast, NSAR57 was highly sensitive to these agents and the loss of the inserted DNA restored the wild-type phenotype. Western blot analysis using antiserum to Escherichia coli RecA showed that overproduction of RecA was correlated with overtranscription of recA in an S. ambofaciens amplified mutant derived from genetic instability.