Unwinding of a DNA triple helix by the Werner and Bloom syndrome helicases

Bloom syndrome and Werner syndrome are genome instability disorders, which result from mutations in two different genes encoding helicases, Both enzym es are members of the RecQ family of helicases, have a 3' --> 5' polarity, and require a 3' single strand tail. In addition to their activity in unwin ding duplex substrates, recent studies show that the two enzymes are able t o unwind G2 and G4 tetraplexes, prompting speculation that failure to resol ve these structures in Bloom syndrome and Werner syndrome cells may contrib ute to genome instability. The triple helix is another alternate DNA struct ure that can be formed by sequences that are widely distributed throughout the human genome. Here we show that purified Bloom and Werner helicases can unwind a DNA triple helix. The reactions are dependent on nucleoside triph osphate hydrolysis and require a free 3' tail attached to the third strand. The two enzymes unwound triplexes without requirement for a duplex extensi on that would form a fork at the junction of the tail and the tripler. In c ontrast, a duplex formed by the third strand and a complement to the triple r region was a poor substrate for both enzymes. However, the same duplex wa s readily unwound when a noncomplementary 5' tail was added to form a forke d structure. It seems likely that structural features of the tripler mimic those of a fork and thus support efficient unwinding by the two helicases.