The RuvAB and RecG proteins of Escherichia coli promote branch migration of Holliday junction intermediates in genetic recombination. Both are structure-specific helicases that unwind and rewind DNA at the junction point. The helicase activities of these proteins were investigated using RNA:DNA hybrid molecules. RuvAB catalyses the unwinding of RNA:DNA partial duplexes of at least 218 bp in a reaction that requires both RuvA and RuvB, ATP and Mg2+. RecG failed to unwind these substrates even when the duplex region was reduced to 35 bp. In contrast, RecG rapidly removes a 218 nt RNA from an R-loop substrate, whereas RuvAB does not. RecG's ability to dissociate R-loops is correlated with an ability to reduce the copy number of pUC plasmids and other constructs based on the ColE1 replicon. Copy number is reduced severely when the plasmid carries recG+. RecG is assumed to reduce copy number by interfering with RNA II's ability to form an R-loop at the plasmid origin of replication and prime DNA synthesis. The dissociation of R-loops by RecG is discussed in terms of the functions needed to promote recombination and to prime DNA replication at D-loops formed during the early stages of RecA-mediated recombination.