RecQ family members combine strand pairing and unwinding activities to catalyze strand exchange.

RecQ helicases are critical for maintaining genomic integrity. In this study, we show that three RecQ members (WRN, deficient in the Werner syndrome; BLM, deficient in the Bloom syndrome; and Drosophila melanogaster RecQ5b (dmRecQ5b)) possess a novel strand pairing activity. Furthermore, each of these enzymes combines this strand pairing activity with its inherent DNA unwinding capability to perform coordinated strand exchange. In this regard, WRN and BLM are considerably more efficient than dmRecQ5b, apparently because dmRecQ5b lacks conserved sequences C-terminal to the helicase domain that contribute to DNA binding, strand pairing, and strand exchange. Based on our findings, we postulate that certain RecQ helicases are structurally designed to accomplish strand exchange on complex replication and recombination intermediates. This is highly consistent with proposed roles for RecQ members in DNA metabolism and the illegitimate recombination and cancer-prone phenotypes associated with RecQ defects.