The sequences near Chi sites allow the RecBCD pathway to avoid genomic rearrangements

Bacterial recombinational repair is initiated by RecBCD, which creates a 3' single-stranded DNA (ssDNA) tail on each side of a double strand break (DSB). Each tail terminates in a Chi site sequence that is usually distant from the break. Once an ssDNA-RecA filament forms on a tail, the tail searches for homologous double-stranded DNA (dsDNA) to use as template for DSB repair. Here we show that the nucleoprotein filaments rarely trigger sufficient synthesis to form an irreversible repair unless a long strand exchange product forms at the 3' end of the filament. Our experimental data and modeling suggest that terminating both filaments with Chi sites allows recombinational repair to strongly suppress fatal genomic rearrangements resulting from mistakenly joining different copies of a repeated sequence after a DSB has occurred within a repeat. Taken together our evidence highlights cellular safe fail mechanisms that bacteria use to avoid potentially lethal situations.