Rec-D

I.Pic of

 Recombination

•  RecD is a helicase that moves down ssDNA 5' to 3' that also acts as  an exonuclease, cutting the DNA as it moves down the strand.  Sister Cromatids first pair to make chromosomes, then two homologous chromosomes pair at the Metaphase plate. In order to facilitate recombination a double stranded break must occur in one of the homologous pairs.  RedBCD recognizes the double stranded break site and acts as an endonuclease to "chew away" the strands. RecBCD continues to degrade the stands until RecC recognizes a Chi site which terminates degradation providing a free 3' end on the bottom strand.  RecBCD binds to the free blunt ends of double stranded DNA, and moves along the dsDNA, unwinding it.  It acts as an endonuclease, degrading both single strands, until it encounters the chi site, where it's endonuclease activity changes.  It then creates a 3' whisker, where RecA aligns the ssDNA with it's homologous target region on the duplex DNA.

   

Rec D Function  

•  The product of the RecD gene, which combines with the RecB and RecC proteins to comprise the RecBCD enzyme, is not required for either recombination or repair. The RecBCD enzyme is a powerful exonuclease that inhibits recombination by destroying linear DNA. The RecD Ejection model proposes that the RecBCD enzyme enters DNA at the double-chain end and travels destructively along the DNA until it encounters the recombination hotspot sequence chi. Chi then alters the RecBCD enzyme by weakening the affinity of the RecD portion of the complex to the RecBC heterodimer. After losing the RecD subunit from the enzyme, the resulting protein becomes unable to perform exonuclease activity.  It instead functions as a recombinagenic helicase.

IV. related disease