Structural Maintenance of Chromosome (SMC) proteins play central roles in chromosome dynamics across domains of life. While their function in chromosome organization and segregation in bacteria is well-characterized, their contribution to pathways of genome integrity maintenance is less understood. Here, I will present evidence for requirement of the highly conserved SMC protein, RecN, in mediating homology search and repair in vivo during double-strand break (DSB) repair. Using quantitative live cell imaging, we follow temporal and spatial dynamics of the recombinase, RecA, after induction of a single DSB on the chromosome of Caulobacter crescentus. We find that the RecA-nucleoprotein filament is mobile and moves in a directional manner across the length of the cell, undergoing several such cycles until homology search is complete. These dynamics are independent of the presence of a repair template. Instead, such large-scale translocations of the filament as well as remodeling of its architecture is driven by RecN. We show that filament dynamics is lost in the absence of RecN. Rates of RecA loading or integrity of the RecA-nucleoprotein filament are unaffected in the absence of RecN. Together, our data suggest that RecN arrives after RecA has loaded at the break, following which it triggers RecA filament mobility via its ATPase cycle. Our findings are consistent with a model where symmetry breaking by RecN likely acts as the driver for directional RecA filament translocation. Such symmetry breaking highlights a conserved feature of SMC proteins across domains of life, irrespective of their specific functions in modulating chromosome dynamics.

Keywords: DNA damage, double-strand break repair, homologous recombination, SMC proteins, live-cell imaging, bacteria