Direct observation of subunit rotation during DNA strand exchange by serine recombinases

Serine recombinases are proposed to catalyse site-specific recombination by a unique mechanism called subunit rotation. Cutting and rejoining DNA occurs within an intermediate synaptic complex comprising a recombinase tetramer bound to two DNA sites. After double-strand cleavage at both sites, one half of the complex rotates 180° relative to the other, before re-ligation of the DNA ends. We used single-molecule FRET (smFRET) methods to provide compelling direct physical evidence for subunit rotation by recombinases Tn3 resolvase and Sin. Synaptic complexes containing fluorescently labelled DNA show FRET fluctuations consistent with the subunit rotation model. FRET changes were associated with the rotation steps, on a timescale of 0.4–1.1 \({{\mbox{s}}}^{-1}\) ${\text{s}}^{-1}$ , as well as opening and closing of the gap between the scissile phosphates during cleavage and ligation. Multiple rounds of recombination were observed within the ~25 s observation period, including frequent consecutive rotation events in the cleaved-DNA state without evidence of intermediate ligation.