The Bloom's syndrome helicase (BLM) interacts physically and functionally with p12, the smallest subunit of human DNA polymerase delta

Bloom's syndrome (BS) is a cancer predisposition disorder caused by mutation of the BLM gene, encoding a member of the RecQ helicase family. Although the phenotype of BS cells is suggestive of a role for BLM in repair of stalled or damaged replication forks, thus far there has been no direct evidence that BLM associates with any of the three human replicative DNA polymerases. Here, we show that BLM interacts specifically in vitro and in vivo with p12, the smallest subunit of human POL {delta} (hPOL {delta}). The hPOL {delta} enzyme, as well as the isolated p12 subunit, stimulates the DNA helicase activity of BLM. Conversely, BLM stimulates hPOL {delta} strand displacement activity. Our results provide the first functional link between BLM and the replicative machinery in human cells, and suggest that BLM might be recruited to sites of disrupted replication through an interaction with hPOL {delta}. Finally, our data also define a novel role for the poorly characterized p12 subunit of hPOL {delta}.