STRUCTURAL BASIS FOR MUTH ACTIVATION IN ESCHERICHIA-COLI MISMATCH REPAIR AND RELATIONSHIP OF MUTH TO RESTRICTION ENDONUCLEASES

MutS, MutL and MutH are the three essential proteins for initiation of methyl-directed DNA mismatch repair to correct mistakes made during D NA replication in Escherichia coli, MutH cleaves a newly synthesized a nd unmethylated daughter strand 5' to the sequence d(GATC) in a hemi-m ethylated duplex, Activation of MutH requires the recognition of a DNA mismatch by MutS and MutL. We have crystallized MutH in two space gro ups and solved the structures at 1.7 and 2.3 Angstrom resolution, resp ectively, The active site of MutH is located at an interface between t wo subdomains that pivot relative to one another, as revealed by compa rison of the crystal structures, and this presumably regulates the nuc lease activity, The relative motion of the two subdomains in MutH corr elates with the position of a protruding C-terminal helix, This helix appears to act. as a molecular lever through which MutS and MutL may c ommunicate the detection of a DNA mismatch and activate MutH, With seq uence homology to Sau3AI and structural similarity to PvuII endonuclea se, MutH is clearly related to these enzymes by divergent evolution, a nd this suggests that type II restriction endonucleases evolved from a common ancestor.