Crystallographic and functional studies of very short patch repair endonuclease

Vsr endonuclease plays a crucial role in the repair of TG mismatched base p airs, which are generated by the spontaneous degradation of methylated cyti dines; Vsr recognizes the mismatched base pair and cleaves the phosphate ba ckbone 5' to the thymidine. We have determined the crystal structure of a t runcated form of this endonuclease at 1.8 Angstrom resolution. The protein contains one structural zinc-binding module. Unexpectedly, its overall topo logy resembles members of the type II restriction endonuclease family. Subs equent mutational and biochemical analyses showed that certain elements in the catalytic site are also conserved. However, the identification of a cri tical histidine and evidence of an active site metal-binding coordination t hat is novel to endonucleases indicate a distinct catalytic mechanism.