Required for homologous recombination (HR) and the bypass of mutagenic DNA lesions (double strand breaks, DSB) by the SOS response. 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. Numerous X-ray crystals have been resolved under different conditions which indicate the flexibility of the protein, essential to its function. Gln-196 contributes to this plasticity by acting as a switch residue, which transmits the effect of nucleotide binding to the DNA-binding region. Polymerizes non-specifically on ssDNA to form filaments. Interacts with and activates LexA leading to autocatalytic cleavage of LexA, which derepresses the SOS regulon and activates DNA repair (By similarity). Not essential in the absence of DNA damage, its deletion renders cells much more sensitive to DNA damaging agents such as UV light, ionizing radiation, alkylating agents and DNA gyrase inhibitors and desiccation-induced DSBs. 5-fold decrease in survival, loss of HR, no change in NHEJ (non-homologous end-joining) or single-strand annealing DSB repair. Belongs to the RecA family.