RecBCD

RecBCD, also known as Exonuclease V, is an enzyme of the "E. coli" bacterium that initiates recombinational repair from potentially lethal double strand breaks in DNA which may result from ionizing radiation, replication errors, endonucleases, oxidative damage and a host of other factors.cite book | author = Spies M, Kowalczykowski SC | authorlink = | editor = Higgins P | others = | title = Bacterial Chromosomes | edition = | language = | publisher = ASM Press | location = Washington, D.C | year = 2003 | origyear = | pages = pages 389–403 | chapter = Homologous recombination by RecBCD and RecF pathways | quote = | isbn = 1-55581-232-5 | oclc = | doi = | url = | accessdate = ] The RecBCD enzyme is both a helicase that unwinds, or separates the strands of, DNA and a nuclease that makes single-stranded nicks in DNA.

tructure

The enzyme complex is composed of three different subunits called RecB, RecC, and RecD and hence the complex is named RecBCD. Each subunit is encoded by a separate gene:

Function

Both the alpha (RecD) and beta (RecB) subunits are helicases, "i.e." energy-dependent molecular motors that unwind DNA or RNA. The beta subunit in addition has a nuclease function. Finally the gamma subunit (RecC) recognizes a specific sequence in DNA, 5'-GCTGGTGG-3', that is known as χ (the Greek letter chi).

RecBCD is unusual amongst helicases because of its ability to recognize the Chi DNA sequence. After it initiates unwinding, RecBCD cleaves both DNA strands endolytically, although the 5' tail is cleaved less often. When RecBCD encounters a χ site on this strand as it is unwinding DNA, it pauses and the digestion of the 3' tail ceases.cite journal | author = Spies M, Amitani I, Baskin RJ, Kowalczykowski SC | title = RecBCD enzyme switches lead motor subunits in response to chi recognition | journal = Cell | volume = 131 | issue = 4 | pages = 694–705 | year = 2007 | month = November | pmid = 18022364 | pmc = 2151923 | doi = 10.1016/j.cell.2007.09.023 | url = | issn = ] When RecBCD resumes unwinding, it now cleaves the opposite strand ("i.e.", the 5' tail). As a consequence, the 3' strand remains intact downstream of χ. The RecA protein is then loaded onto the 3' tail and RecBCD dissociates from the DNA. In a final step, RecA initiates homologous recombination to complete the repair of the double stranded DNA break.

Application

RecBCD is a model enzyme for the use of single molecule fluorescence as an experimental technique used to better understand the function of protein-DNA interactions.cite journal | author = Bianco PR, Brewer LR, Corzett M, Balhorn R, Yeh Y, Kowalczykowski SC, Baskin RJ | title = Processive translocation and DNA unwinding by individual RecBCD enzyme molecules | journal = Nature | volume = 409 | issue = 6818 | pages = 374–8 | year = 2001 | month = January | pmid = 11201750 | doi = 10.1038/35053131 | url = | issn = ]

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