STRUCTURES OF HHAL METHYLTRANSFERASE COMPLEXED WITH SUBSTRATES CONTAINING MISMATCHES AT THE TARGET BASE

Three structures have been determined for complexes between HhaI methy ltransferase (M.HhaI) and oligonucleotides containing a G:A, G:U or G: AP (AP = abasic or apurinic/apyrimidinic) mismatch at the target base pair. The mismatched adenine, uracil and abasic site are all flipped o ut of the DNA helix and located in the enzyme's active-site pocket, ad opting the same conformation as in the nipped-out normal substrate. Th ese results, particularly the flipped-out abasic deoxyribose sugar, pr ovide insight into the mechanism of base flipping. If the process invo lves the protein pushing the base out of the helix, then the push must take place not on the base, but rather on the sugar-phosphate backbon e. Thus rotation of the DNA backbone is probably the key to base flipp ing.