Chi and the RecBC D enzyme of Escherichia coli

The products of genes recB and recC are responsible for conjugal recombination and for the repair of chromosomal double chain breaks in Escherichia coli. 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. On the contrary, RecBCD enzyme is a potent exonuclease which inhibits recombination by destroying linear DNA. The RecD Ejection model supposes that RecBCD enzyme enters DNA at a double-chain end and travels destructively along the DNA until (typically) it encounters the recombination hotspot sequence chi. Chi then alters the RecBCD enzyme by weakening the affinity of the RecD subunit for the RecBC heterodimer. With the loss of the RecD subunit from the enzyme, the resulting protein, RecBC(D-), becomes deficient for exonuclease activity and proficient as a recombinagenic helicase. Thus, the RecD Ejection model proposes that chi participates in recombination by acting as a toggle to convert RecBCD (a powerful exonuclease) to RecBC(D-) (a recombinase). We review the properties of RecBCD and its cognate site chi, including recent results that support both the RecD Ejection model and the view that chi plays only an indirect role in recombination.