Explain the RecBCD recombination system.
The central player is the RecBCD enzyme, which is made up of three different proteins. Two—RecB and RecD –are helicases. To initiate homologous recombination, one copy of the RecBCD enzyme attaches to the free ends of a chromosome at a doble-starnd break. Using its helicase activity, RecBCD then progresses along the DNA until it reaches the first copy of a consensus sequence, called the chi site. The RecBCD then produces a double-stranded molecule with a 3’ overhang. The heteroduplex is then formed, which is mediated by RecA, whch forms the protein-coated DNA filament that is able to invade the intact double helix and set up the D loop. An intermediate in formation of the D loop is the triplex structure, a three-stranded DNA helix in which the invading polynucleotide lies within the major groove of the intact helix and forms hydrogen bonds with the base pairs it encounters. Once the heteroduplex has been established, the subsequent events are common to all three recombination systems.
RuvA and RuvB catalyze the branch migration. Four copies of RuvA bind to the branch point, forming a core to which two RuvB rings, attach, one on either side. The resulting structure acts as a molecular motor, rotating the helices in the required manner. When branch migration ends, RuvAB complex detaches and is replaced by two RuvC proteins, which cleaves the Holliday structure.