BRANCH MIGRATION OF 3-STRAND RECOMBINATION INTERMEDIATES BY RECG, A POSSIBLE PATHWAY FOR SECURING EXCHANGES INITIATED BY 3'-TAILED DUPLEX DNA

RecG protein is required for normal levels of recombination and DNA re pair in Escherichia coli, This 76 kDa polypeptide is a junction-specif ic DNA helicase that acts post-synaptically to drive branch migration of Holliday junction intermediates made by RecA during the strand exch ange stage of recombination, To gain further insight into the role of RecG, we studied its activity on three-strand intermediates formed by RecA between circular single-stranded and linear duplex DNAs, Once Rec A is removed, RecG drives branch migration of these intermediates by a junction-targeted activity that depends on hydrolysis of ATP, RuvAB h as a similar activity. However, when RecG is added to a RecA strand ex change reaction it severely reduces the accumulation of joint molecule intermediates by driving branch migration of junctions in the reverse direction to that catalysed by RecA strand exchange, In comparison, R uvAB has little effect on the reaction. We discuss how reverse branch migration by RecG, which acts counter to the 5'-->3' polarity of RecA binding and strand exchange, could serve to promote or abort the early stages of recombination, depending on the orientation of the single D NA strand initiating the exchange relative to the adjacent duplex regi on.