ATP alters the diffusion mechanics of MutS on mismatched DNA.

The mismatch repair (MMR) initiation protein MutS forms at least two types of sliding clamps on DNA: a transient mismatch searching clamp (∼1 s) and an unusually stable (∼600 s) ATP-bound clamp that recruits downstream MMR components. Remarkably, direct visualization of single MutS particles on mismatched DNA has not been reported. We have combined real-time particle tracking with fluorescence resonance energy transfer (FRET) to image MutS diffusion dynamics on DNA containing a single mismatch. We show searching MutS rotates during diffusion independent of ionic strength or flow rate, suggesting continuous contact with the DNA backbone. In contrast, ATP-bound MutS clamps that are visually and successively released from the mismatch spin freely around the DNA, and their diffusion is affected by ionic strength and flow rate. These observations show that ATP binding alters the MutS diffusion mechanics on DNA, which has a number of implications for the mechanism of MMR.