Restriction endonuclease MutH and similar endonucleases
MutH is a 28kD endonuclease involved in methyl-directed DNA mismatch repair in gram negative bacteria. MutH is both sequence-specific and methylation-specific, introducing a nick in the unmethylated strand of a hemi-methylated d(GATC) DNA duplex. MutH is homologous to the type II restriction endonuclease Sau3AI which also recognizes the d(GATC) sequence however, Sau3AI cleaves both strands regardless of their methylation state. The active form of MutH is monomeric while that of Sau3AI is homodimeric. In addition to MutH, MutS, involved in mismatch recognition, and MutL, involved in mediating the interactions between MutH and MutS, are essential in initiating mismatch repair in Escherichia coli.