Bloom syndrome and
Werner syndrome are
genome instability disorders, which result from mutations in two different genes encoding helicases. Both
enzymes 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 unwinding duplex substrates, recent studies show that the two
enzymes are able to unwind G2 and G4 tetraplexes, prompting speculation that failure to resolve these structures in
Bloom syndrome and
Werner syndrome cells may contribute to
genome instability. The triple helix is another alternate
DNA structure 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 triphosphate hydrolysis and require a free 3' tail attached to the third strand. The two
enzymes unwound triplexes without requirement for a duplex extension that would form a fork at the junction of the tail and the triplex. In contrast, a duplex formed by the third strand and a
complement to the triplex region was a poor substrate for both
enzymes. However, the same duplex was readily unwound when a noncomplementary 5' tail was added to form a forked structure. It seems likely that structural features of the triplex mimic those of a fork and thus support efficient unwinding by the two helicases.