RecBCD

RecBCD is a bacterial DNA repair enzyme complex that initiates the processing of double-strand breaks for homologous recombination. It is a heterotrimer composed of the RecB, RecC, and RecD subunits and is especially well characterized in Escherichia coli. The complex binds to DNA ends generated by double-strand breaks and functions as an ATP-dependent motor with both helicase and nuclease activities to resect the DNA ends, producing single-stranded DNA tails required for recombination.

Mechanistically, RecB and RecD provide helicase activities with different polarities, while RecB also carries nuclease activity. RecC acts as a recognition subunit that contains a channel for detecting a short DNA sequence called chi. As RecBCD degrades and unwinds DNA, encountering a chi site modulates the nuclease activities: the 3' end is spared to generate a long 3' single-stranded overhang, while degradation of the opposite strand is favored. This processing creates the appropriate substrate for loading of the RecA protein, which forms a nucleoprotein filament on the 3' tail and catalyzes homology search and strand invasion during repair. The formation of the RecA-ssDNA filament also contributes to the SOS response by promoting derepression of SOS genes.

Role and variation: RecBCD is central to double-strand-break repair by homologous recombination in many bacteria. Some species employ functionally analogous complexes, such as AddAB, which perform similar end-resection tasks. RecBCD has been a model system for understanding DNA repair, recombination, and the regulation of nuclease activity in response to chi sequences.