The recombination hotspot χ is a regulatory sequence that acts by attenuating the nuclease activity of the E. coli RecBCD enzyme

The RecBCD enzyme is a multifunctional enzyme that is essential for homologous recombination in E. coli. In vitro, the RecBCD enzyme degrades linear double-stranded DNA nonspecifically during the process of unwinding the double-stranded DNA. Here we demonstrate that this DNA degradation is asymmetric, with the strand that is 3′ terminal at the entry site of RecBCD enzyme being degraded much more vigorously than the 5′ terminal strand. Furthermore, interaction with the recombination hotspot χ causes an attenuation of the nuclease activity but not of the hellcase activity and is accompanied by a pause of RecBCD enzyme at the χ site. These results demonstrate that χ is a unique regulatory element that acts by controlling the degradative function of RecBCD enzyme and, thereby, enhancing its recombination function.