E0075

DNA-protein Interactions in Mismatch Repair. Wei Yang¹, Murray Junop¹, Hon Ling, Changill Ban, Galina Obmolova² and Peggy Hsieh², ¹Laboratory of Molecular Biology and ²Genetics and Biochemistry Branch, NIDDK, NIH, Bethesda, MD 20892

Initiation of the methyl-dependent mismatch repair in E. coli requires recognition of two DNA sites, a mismatch site by MutS in a non-sequence specific mode and a DNA cleavage site at a specific sequence by MutH. MutS possesses an ATPase activity and initiates repair by recognizing DNA containing mispaired or unpaired nucleotides. MutH is a sequence- and methylation-specific endonuclease, which recognizes a hemimethylated d(GATC) sequence and cleaves the newly synthesized strand 5’ to a transiently unmodified d(GATC) sequence. MutH is a latent endonuclease and activated by the MutS associated with a mismatch and MutL, a mediator between MutS and MutH, in the presence of ATP. The two DNA recognition sites can be separated by as much as 1000 bps and either 5’ or 3’ to one another. We have determined the crystal structures of each of these proteins, both alone and complexed with ligands, and pursued structure-based studies of the mechanism of mismatch repair. The structural and biochemical data we have obtained have revealed the relationship between MutH and type II restriction endonuclease as well as elucidating a novel induced-fit between MutS and a mismatch-containing DNA.