METHYLATION OF SLIPPED DUPLEXES, SNAPBACKS AND CRUCIFORMS BY HUMAN DNA(CYTOSINE-5)METHYLTRANSFERASE

When human DNA(cytosine-5)methyltransferase erase was used to methylat e a series of snapback oligodeoxynucleotides of differing stem lengths , each containing a centrally located CG dinucleotide recognition site , the enzyme required a minimum of 22 base pairs in the stem for maxim um activity, Extrahelical cytosines in slipped duplexes that were 30 b ase pairs in length acted as effective methyl accepters and were more rapidly methylated than cytosines that were Watson-Crick paired, Duple xes containing hairpins of CCG repeats in cruciform structures in whic h the enzyme recognition sequence was disrupted by a C . C mispair wer e also more rapidly methylated than control Watson-Crick-paired duplex es, Since enzymes have higher affinities for their transition states t han for their substrates, the results with extrahelical and mispaired cytosines suggest that these structures can be viewed as analogs of th e transition state intermediates produced during catalysis by methyltr ansferases.