Selective Bypass of a Lagging Strand Roadblock by the Eukaryotic REplicative DNA Helicase
Yu Fu at al.
Cell 146, 931-941
The eukaryotic replicative DNA helicase,CMG,un-winds 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-specifi DNA road blocks in Xenopus egg extracts. An ssDNA tran-slocase should stall at obstruction on the translocation strand but not the excluded strand, where as a dsDNA translocase should stall at obstructions on either strand. We found that replisomes by pass large roadblocks on the lagging strand template much more readily than on the leading strand template. Our results indicate that CMG is a 30 to 50 ssDNA translocase, consistent with unwinding via "steric exclusion. "Given that MCM2-7 encircles dsDNA in G1,the data imply that formation of CMG in Sphase involves remodeling of MCM2-7 froma dsDNA to a ssDNA binding mode.