E. coli recA protein possesses a strand separating activity on short duplex DNAs.

RecA protein was found to catalyze the dissociation of the strands of a DNA substrate consisting of a 20‐nucleotide primer annealed to circular single‐stranded M13mp DNA. The strand separation reaction requires ATP hydrolysis and the presence of single‐stranded DNA flanking the duplex DNA region to be unwound. RecA‐catalyzed strand separation is effective only for very short duplexes, not exceeding 30 bp, and is not stimulated by single‐stranded DNA‐binding protein. These results are consistent with the ability of recA protein to disrupt regions of secondary structure in single‐stranded DNA and to incorporate large non‐homologies into heteroduplex DNA.