Binding and melting of D-loops by the Bloom syndrome helicase

Bloom syndrome is a rare autosomal disorder characterized by predisposition to cancer and genomic instability. BLM, the structural gene mutated in ind ividuals with the disorder, encodes a DNA helicase belonging to the RecQ fa mily of helicases. These helicases have been established to serve roles in both promoting and preventing recombination. Mounting evidence has implicat ed a function for BLM during DNA replication; specifically, BLM might be in volved in rescuing stalled or collapsed replication forks by a recombinatio n-based mechanism. We have tested this idea by examining the binding and me lting activity of BLM on oligonucleotide substrates containing D-loops, DNA structures that model the presumed initial intermediate formed during homo logous recombination. We find that BLM preferentially melts those D-loops t hat are formed more favorably by the strand exchange protein Rad51, but who se polarity could be less favorable for enabling restoration of an active r eplication fork. We propose a model in which BLM selectively dissociates re combination intermediates likely to be unfavorable for recombination-promot ed replication.