Postreplication repair in E. coli: strand exchange reactions of gapped DNA by RecA protein.

We have used a sensitive gel electrophoresis assay to detect the products of Escherichia coli RecA protein catalysed strand exchange reactions between gapped and duplex DNA molecules. We identify structures that correspond to joint molecules formed by homologous pairing, and show that joint molecules are converted by RecA protein into heteroduplex monomers by reciprocal strand exchanges. However, strand exchanges only occur when there is a 3'-terminus complementary to the single stranded DNA in the gap. In the absence of a complementary free end, the two DNA molecules pair and short heteroduplex regions are formed by localised interwinding.