RecB

RecB is a protein subunit of the bacterial RecBCD enzyme complex, which plays a central role in the repair of double-strand DNA breaks by homologous recombination. In Escherichia coli and many related bacteria, RecBCD is a heterotrimer composed of RecB, RecC, and RecD subunits. RecB contributes a DNA helicase activity that moves along the duplex and a nuclease activity that processes DNA ends, helping to generate single-stranded DNA required for recombination.

The RecBCD complex binds to DNA ends at double-strand breaks and unwinds the duplex while degrading the ends. When the complex encounters a Chi site, the activity is modulated: RecC recognizes the Chi sequence and alters the nuclease activities so that degradation of the 5' strand is favored, producing a 3' single-stranded overhang. This overhang serves as a substrate for loading of RecA, which promotes strand invasion and homologous pairing with a homologous DNA duplex, enabling repair.

The Chi motif, first defined in Escherichia coli as 5'-GCTGGTGG-3', varies among bacteria but serves a similar function in regulating recombination after Chi recognition. The recB gene is typically encoded within the recBCD operon, and RecB is essential for efficient end processing and recombination in many bacterial systems. Mutants lacking RecB exhibit impaired DNA end processing and reduced homologous recombination.

RecB is a large, multi-domain enzyme with helicase and nuclease activities, and it interacts with RecC and RecD as part of the RecBCD complex. It remains a model system for understanding DNA end processing and recombination-based repair mechanisms in bacteria.