Three Phases in Homologous Pairing: Polymerization of recA Protein on Single-stranded DNA, Synapsis, and Polar Strand Exchange

Purified Escherichia coli recA protein, using the energy of ATP, promotes the pairing of homologous molecules of DNA. The prototype of the reaction is the pairing of a single strand of DNA with duplex DNA; the product is a joint molecule in which the two parental contributions are linked only by the hydrogen bonds of paired, complementary bases. Single-stranded DNA governs this reaction. recA protein will pair a duplex molecule with a molecule that is single-stranded or partially single-stranded. On the other hand, no report has yet described the pairing of two fully duplex molecules. Other ways in which single-stranded DNA governs the pairing reaction are illustrated in Figure 1. When recA protein pairs a single strand of DNA with duplex DNA, the amount of protein that is required is stoichiometrically determined by the amount of single-stranded DNA; 1 molecule of recA protein is required for every 3–5 nucleotide residues...