Control of crossing over

The Holliday junction is a central intermediate in homologous recombination . It consists of a four-way structure that can be resolved by cleavage to g ive either the crossover or noncrossover products observed. We show here th at the formation of these products is controlled by the E. coli resolvasome (RuvABC) in such way that double-strand break repair (DSBR) leads to cross ing over and single-strand gap repair (SSGR) does not lead to crossing over . We argue that the positioning of the RuvABC complex and its consequent di rection of junction-cleavage is not random. In fact, the action of the RuvA BC complex avoids crossing over in the most commonly predicted situations w here Holliday junctions are encountered in DNA replication and repair. Our observations suggest that the positioning of the resolvasome may provide a general biochemical mechanism by which cells can control crossing over in r ecombination.