Flexible modes of translocation by the eukaryotic CMG helicase facilitate removal or bypass of DNA blocks

Abstract

Replicative helicases in all domains of life are hexameric rings that encircle DNA while unwinding it, and studies have sought to distinguish between two different modes of action. In the steric exclusion model the helicase encircles and tracks along one strand and excludes the other strand from the inner channel. The ploughshare model posits that the helicase encircles both strands and a separation pin divides the two strands and directs them into distinct channels. Using purified CMG on model DNA substrates, we find evidence that the CMG helicase switches between both modes. In the ploughshare mode it is capable of stripping streptavidin from biotin on either DNA strand, indicating that it encircles both strands and proceeds with substantial force. In the steric exclusion mode it can simply bypass streptavidin on the lagging strand. The helicase appears to utilize the ploughshare mode by default, facilitating removal of obstacles on both strands at the replication fork, but can switch to the steric exclusion mode depending on the severity of the block. These results point to an unexpected degree of flexibility in the mode of helicase translocation that may also extend to other replicative helicases.Support or Funding InformationThis work was supported by NIH Grant GM38839 and the Howard Hughes Medical Institute