INTERACTION OF OLIGONUCLEOTIDES CONTAINING 6-O-METHYLGUANINE WITH HUMAN DNA (CYTOSINE-5-)-METHYLTRANSFERASE

Thirty-base-pair synthetic oligonucleotide duplexes containing a single meG·C (meG = 6-O-methylguanine) or A·C base pair at the 16th position (i.e., 5′-CCCGTTTAAATATACXTATACCCGGGTACC-3′, where X = A or meG) were used to study de novo methylation by the purified human DNA (cytosine-5)-methyltransferase isolated from CEM cells. Both duplexes containing meG·C and A·C base pairs show enhanced methyl group acceptor properties. Subsequent introduction of hemimethylated sites at the 15th position of the top strand (the C residue next to the abnormal base pair) and the 7th, 15th (which represents the C residue in the 6meG·C and A·C base pairs), and 27th positions of the bottom strand were used to study the maintenance methylation of the hemimethylated duplexes by the methylase. This revealed striking differences in the rate, amount, and sites of methylation, which are dependent on the position of the hemimethylated site in the duplex. The possible mechanism of action of the methylase is discussed. The data show that 6-O-methylguanine residues in DNA can have other genetic effects apart from their miscoding behavior and that meG·C and A·C base pairs exert different effects in terms of methylation. © 1990, American Chemical Society. All rights reserved.