In vivo homology search via symmetry-breaking action of SMC protein RecN on the RecA filament

While the molecular repertoire of the homologous recombination pathway is well studied, the mechanism of homology search that enables recombination between distant homologous regions is poorly understood. To address this, wefollow the dynamics of the recombinase RecA, an essential component of homology search, after induction of a double-strand break on the Caulobacter chromosome. We find that the RecA-nucleoprotein filament translocates in a directional manner along the DNA, undergoing several pole-to-pole traversals, until homology search is complete. This translocation and remodeling ofthe filament is driven by the SMC protein, RecN. While RecA loading rates or filament integrity are unaffected in the absence of RecN, the translocation dynamics of RecA after association with the break site is contingent on RecN action via its ATPase cycle. Using a stochastic model, we show that RecN induced symmetry breaking drives translocation of the RecA filament, pointing to a novel optimal search strategy.