PcrA Helicase Dismantles RecA Filaments by Reeling in DNA in Uniform Steps

SummaryTranslocation of helicase-like proteins on nucleic acids underlies key cellular functions. However, it is still unclear how translocation can drive removal of DNA-bound proteins, and basic properties like the elementary step size remain controversial. Using single-molecule fluorescence analysis on a prototypical superfamily 1 helicase, Bacillus stearothermophilus PcrA, we discovered that PcrA preferentially translocates on the DNA lagging strand instead of unwinding the template duplex. PcrA anchors itself to the template duplex using the 2B subdomain and reels in the lagging strand, extruding a single-stranded loop. Static disorder limited previous ensemble studies of a PcrA stepping mechanism. Here, highly repetitive looping revealed that PcrA translocates in uniform steps of 1 nt. This reeling-in activity requires the open conformation of PcrA and can rapidly dismantle a preformed RecA filament even at low PcrA concentrations, suggesting a mode of action for eliminating potentially deleterious recombination intermediates.

Graphical AbstractFigure optionsDownload high-quality image (141 K)Download as PowerPoint slideHighlights
► PcrA helicase reels in lagging strands rather than unwinding replicating duplex DNA
► During this reeling-in process, PcrA strips RecA filaments off the DNA efficiently
► PcrA activity requires anchoring to the duplex through the 2B subdomain
► PcrA translocates on DNA in uniform steps