Selective bypass of a lagging strand roadblock by the eukaryotic replicative DNA helicase.
The eukaryotic replicative DNA helicase, CMG, unwinds DNA by an unknown mechanism. In some models, CMG encircles and translocates along one strand of DNA while excluding the other strand. In others, CMG encircles and translocates along duplex DNA. To distinguish between these models, replisomes were confronted with strand-specific DNA roadblocks in Xenopus egg extracts. An ssDNA translocase should stall at an obstruction on the translocation strand but not the excluded strand, whereas a dsDNA translocase should stall at obstructions on either strand. We found that replisomes bypass large roadblocks on the lagging strand template much more readily than on the leading strand template. Our results indicate that CMG is a 3'' to 5'' ssDNA translocase, consistent with unwinding via "steric exclusion." Given that MCM2-7 encircles dsDNA in G1, the data imply that formation of CMG in S phase involves remodeling of MCM2-7 from a dsDNA to a ssDNA binding mode.
PubMed ID: 21925316
PMC ID: PMC3209622
Article link: Cell.
Grant support: CA092584 NCI NIH HHS , GM034559 NIGMS NIH HHS , GM08454 NIGMS NIH HHS , GM62267 NIGMS NIH HHS , R01 GM062267 NIGMS NIH HHS , R01 GM062267-09 NIGMS NIH HHS , R01 GM062267-11 NIGMS NIH HHS
Genes referenced: mcm2