SMC protein RecN drives RecA filament translocation for in vivo homology search.

While the molecular repertoire of the homologous recombination pathways is well studied, the search mechanism that enables recombination between distant homologous regions is poorly understood. Earlier work suggests that the recombinase RecA, an essential component for homology search, forms an elongated filament, nucleating at the break site. How this RecA structure carries out long-distance search remains unclear. Here, we follow the dynamics of RecA after induction of a single double-strand break on the Caulobacter chromosome. We find that the RecA-nucleoprotein filament, once formed, rapidly translocates in a directional manner in the cell, undergoing several pole-to-pole traversals, until homology search is complete. Concomitant with translocation, we observe dynamic variation in the length of the filament. Importantly in vivo, the RecA filament alone is incapable of such long-distance movement; both translocation and associated length variations are contingent on action of structural maintenance of chromosome (SMC)-like protein RecN, via its ATPase cycle. In summary, we have uncovered the three key elements of homology search driven by RecN: mobility of a finite segment of RecA, changes in filament length, and ability to conduct multiple pole-to-pole traversals, which together point to an optimal search strategy.