The expression of the cyanobacterial recA gene, isolated from Anabaena variabilis, has been examined at the levels of transcript and protein abundance. Exposure of the cyanobacterium to a variety of DNA-damaging agents, including mitomycin C, methyl methanesulfonate, and UV irradiation, results in a rapid increase in the abundance of the recA transcript above basal levels as determined by Northern (RNA) blot analysis. A concomitant increase in the abundance of a 37- to 38-kilodalton polypeptide was also detected by Western (immuno-) blot analysis of soluble cyanobacterial polypeptides using polyclonal antiserum directed against the Escherichia coli recA protein. The cyanobacterial polypeptide is of the same molecular mass as that synthesized by an in vitro, DNA-directed procaryotic transcription-translation system primed with an A. variabilis genomic fragment containing the recA gene. Nucleotide sequence analysis of the cyanobacterial gene revealed a protein of 358 amino acids with a molecular weight of 38,403 daltons. The A. variabilis and E. coli recA genes share similarity at 58% of the amino acid residues; however, an E. coli-like lexA repressor-binding site is not present in the A. variabilis promoter region. The similarities of A. variabilis and E. coli recA expression and gene sequence are discussed.