Unexpected DNA context-dependence identifies a new determinant of Chi recombination hotspots.

Homologous recombination occurs especially frequently near special chromosomal sites called hotspots. In Escherichia coli, Chi hotspots control RecBCD enzyme, a protein machine essential for the major pathway of DNA break-repair and recombination. RecBCD generates recombinogenic single-stranded DNA ends by unwinding DNA and cutting it a few nucleotides to the 3' side of 5' GCTGGTGG 3', the sequence historically equated with Chi. To test if sequence context affects Chi activity, we deep-sequenced the products of a DNA library containing 10 random base-pairs on each side of the Chi sequence and cut by purified RecBCD. We found strongly enhanced cutting at Chi with certain preferred sequences, such as A or G at nucleotides 4-7, on the 3' flank of the Chi octamer. These sequences also strongly increased Chi hotspot activity in E. coli cells. Our combined enzymatic and genetic results redefine the Chi hotspot sequence, implicate the nuclease domain in Chi recognition, indicate that nicking of one strand at Chi is RecBCD's biologically important reaction in living cells, and enable more precise analysis of Chi's role in recombination and genome evolution.