Recombination Pathways

Abstract

Homologous recombination is a complex process that has several important roles: repair of broken DNA, repair of disintegrated replication forks, and generation of genetic diversity. Two independent recombination pathways, RecBCD and RecF(OR), operate in Escherichia coli. Both pathways are involved in the initial stages of recombination, that is, in preparation of 3′-terminated single-stranded (ss) DNA covered with RecA protein, a RecA–ssDNA filament. RecBCD and RecF pathways use different enzymes to make this intermediate, but the enzymatic activities are similar: helicase activity to unwind DNA, a nuclease activity to degrade 5′ strand leaving a 3′ ssDNA, and RecA loading activity to cover ssDNA with RecA. In the next step of recombination pathways, synapsis, RecA mediates homologous pairing, strand invasion, and formation of a Holliday junction. In the postsynapsis, branch migration by RuvAB or RecG helicases extends heteroduplex intermediate and the final recombination products are generated by resolution of heteroduplexes by RuvC nuclease which is a part of a complex with RuvAB. RecA and single-stranded binding (SSB) proteins that promote homologous DNA pairing and the RuvABC complex or RecG that resolves Holliday junctions are shared by both recombination pathways.