Double-stranded DNA break repair and homologous recombination in E. coli are initiated by the RecBCD enzyme, which unwinds and simultaneously degrades DNA from a double-stranded DNA end. This process is stimulated by cis-acting DNA elements, known as χ sites. Using both in vitro pairing and nuclease protection assays, we demonstrate that the translocating RecBCD enzyme, which has been activated by χ, coordinates the preferential loading of the homologous pairing protein, RecA, onto the resultant single-stranded DNA downstream of χ. This facilitated loading of RecA protein results in a substantial increase in both the efficiency and rate of in vitro recombination reactions and offers an explanation for stimulation of recombination and repair in vivo by χ.
