Abstract

DNA double-strand breaks threaten the stability of the genome, and yet are induced deliberately during meiosis in order to provoke homologous recombination and generate the crossovers needed to promote faithful chromosome transmission. Crossovers are secured via biased resolution of the double Holliday junction intermediates formed when both ends of the broken chromosome engage an intact homologue. To investigate whether the enzymes catalysing branch migration and resolution of Holliday junctions are directed to favour production of either crossover or noncrossover products, we engineered a genetic system based on DNA breakage induced by the I-SceI endonuclease to detect analogous exchanges in Escherichia coli where the enzymology of recombination is more fully understood. Analysis of the recombinants generated revealed approximately equal numbers of crossover and noncrossover products, regardless of whether repair is mediated via RecG, RuvABC, or the RusA resolvase. We conclude that there little or no control of crossing over at the level of Holliday junction resolution. Thus, if similar resolvases function during meiosis, additional factors must act to bias recombination in favour of crossover progeny.