LACK OF CORRELATION BETWEEN BINDING OF ECORII METHYLTRANSFERASE TO DNA DUPLEXES CONTAINING MISMATCHES AND THE PROMOTION OF C-MUTATIONS TO T-MUTATIONS

The cytosine methyltransferases (MTases) M. HhaIand M. HpaII bind subs trates in which the target cytosine is replaced by uracil or thymine, i.e. DNA containing a U:G or a T:G mismatch. We have extended this obs ervation to the EcoRII MTase (M. EcoRII) and determined the apparent K d for binding. Using a genetic assay we have also tested the possibili ty that MTase binding to U:G mismatches may interfere with repair of t he mismatches and promote C:G to T:A mutations. We have compared two m utants of M. EcoRII that are defective for catalysis by the wild-type enzyme for their ability to bind DNA containing U:G or T:G mismatches and for their ability to promote C to T mutations. We find that althou gh all three proteins are able to bind DNAs with mismatches. only the wild-type enzyme promotes C:G to T:A mutations in vivo. Therefore, the ability of M. EcoRII to bind U:G mismatched duplexes is not sufficien t for their mutagenic action in cells.