RecG helicase activity at three- and four-strand DNA structures

The RecG helicase of Escherichia coil is necessary for efficient recombinat ion and repair of DNA in vivo and has been shown to catalyse the unwinding of DNA junctions in vitro. Despite these findings, the precise role of RecG remains elusive. However, models have been proposed in which RecG promotes the resolution of linked duplexes by targeting three-strand junctions pres ent at D-loops, One such model postulates that RecG catalyses the formation of four-strand (Holliday) junctions from three-strand junctions, To test t his model, the DNA binding and unwinding activities of RecG were analysed u sing synthetic three- and four-strand junctions. The substrate specificity of RecG was found to depend critically on the concentrations of ATP and MgC l2 and under certain conditions RecG preferentially unwound three-strand ju nction DNA, This was at least partly due to the larger inhibitory effect of MgCl2 on the binding of four-strand as opposed to three-strand junctions b y RecG, Thus RecG may be targeted to three-strand junctions in vivo whilst still being able to branch migrate the four-strand junctions formed as a re sult of the initial helicase reaction, The increase in the dissociation con stant of RecG on conversion of a three-strand into a four-strand junction m ay also facilitate resolution of the four-strand junction by the RuvABC com plex.