The effect of flanking sequences on the de novo methylation of C-G pairs by the human DNA methylase.

The HeLa DNA methylase can methylate selected cytosine residues in oligodeoxynucleotides as small as 12-16 nucleotides in length in vitro. The maximum methylation rate seems to require oligomers having more than one C-G in the molecule even when only one of the C-G pairs is methylated. Compounds which contain a high G+C content also seem to be favored substrates. The use of defined synthetic oligodeoxynucleotides permits one to demonstrate that flanking DNA sequences can be critical in determining whether a C-G site can be methylated.