RecBCD

RecBCD, also known as Exonuclease V, is a protein of the E. coli bacterium that initiates recombinational repair from DNA double strand breaks which are a common result of ionizing radiation, replication errors, endonucleases, oxidative damage and a host of other factors. It is both a helicase that unwinds, or separates the strands of, DNA and a nuclease that makes single-stranded nicks in DNA.  

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RecBCD is composed of three different subunits, encoded by the recB, recC, and recD genes. Both the RecB and RecD subunits are helicases, i.e. energy-dependent molecular motors that unwind DNA or RNA.

RecBCD is unusual amongst helicases in that it recognizes a specific sequence in DNA, 5'-GCTGGTGG-3', that is known as Chi. After it initiates unwinding, RecBCD makes nicks on the strand that contains the unwound 3' end. When RecBCD encounters a Chi site on this strand as it is unwinding DNA, it makes a final nick and pauses. It has been proposed that this pause is a consequence of a conformational rearrangement in the protein that changes its properties. When RecBCD resumes unwinding, it now nicks the opposite strand (i.e. that containing the 5' unwound end). As a consequence, the 3' strand remains intact downstream of Chi. This is important because the strand exchange protein, RecA, that is responsible for the next step of recombinational repair needs a single-strand molecule with a 3' end.

RecBCD is also a model enzyme for the use of single molecule fluorescence as an experimental technique used to better understand the function of protein-DNA interactions.

References

• Singleton MR, Dillingham MS, Gaudier M, Kowalczykowski SC, Wigley DB, "Crystal structure of RecBCD enzyme reveals a machine for processing DNA breaks", Nature. (2004) Nov 11; 432 (7014): 187-93.