Recent genetic and biochemical studies revealed the mechanisms of late stage of homologous recombination in E. coli. A central intermediate of recombination called "Holliday structure", in which two homologous duplex DNA molecules are linked by a single-stranded crossover, is formed by the functions of RecA and several other proteins. The products of the ruvA and ruvB genes, which constitute an SOS regulated operon, form a functional complex that promotes migration of Holliday junctions by catalyzing strand exchange reaction, thus enlarging the heteroduplex region. RuvA is a DNA-binding protein specific for these junctions, and RuvB is a motor molecule for branch migration providing energy by hydrolyzing ATP. The product of the ruvC gene, which is not regulated by the SOS system, resolves Holiday junctions by introducing nicks at or near the crossover junction in strands with the same polarity at the same sites. The recombination reaction is completed by sealing the nicks with DNA ligase, resulting in spliced or patched recombinants. The product of the recG gene provides an alternative route for resolving Holliday junctions. RecG has been proposed to promote branch migration in the opposite direction to that promoted by RecA protein. The atomic structure of RuvC protein revealed by crystallographic study, when combined with mutational analysis of RuvC, provides mechanistic insights into the interactions of RuvC with Holliday junction.
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