Can catalyze the hydrolysis of ATP in the presence of single-stranded DNA, the ATP-dependent uptake of single-stranded DNA by duplex DNA, and the ATP-dependent hybridization of homologous single-stranded DNAs. It interacts with LexA causing its activation and leading to its autocatalytic cleavage. Expressed in light grown cells, strongly repressed post-transcriptionally by UV-C light, repressed by H(2)O(2) and SeO(4) but not by SeO(3) (PubMed:15225304). This bacterium is considerably more resistant to UV and gamma irradiation than E.coli; the E.coli-like SOS regulon model is not an appropriate model for DNA repair in this cyanobacterium. Belongs to the RecA family.