The Bloom’s syndrome helicase, BLM, is a member of\ud the highly conserved RecQ family, and possesses\ud both DNA unwinding and DNA strand annealing\ud activities. BLM also promotes branch migration of\ud Holliday junctions. One role for BLM is to act in conjunction\ud with topoisomerase IIIa to process homologous\ud recombination (HR) intermediates containing a\ud double Holliday junction by a process termed dissolution.\ud However, several lines of evidence suggest\ud that BLM may also act early in one or more of the\ud recombination pathways to eliminate illegitimate or\ud aberrantly paired DNA joint molecules. We have\ud investigated whether BLM can disrupt DNA displacement\ud loops (D-loops), which represent the\ud initial strand invasion step of HR. We show that\ud mobile D-loops created by the RecA recombinase\ud are a highly preferred substrate for BLM with the\ud invading strand being displaced from the duplex.\ud We have identified structural features of the D-loop\ud that determine the efficiency with which BLM promotes\ud D-loop dissociation. We discuss these results\ud in the context of models for the role of BLM as an\ud ‘anti-recombinase’
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