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Nucleic Acids Research, 2000, Vol. 28, No. 18 3594-3599
© 2000 Oxford University Press

Examination of the DNA substrate selectivity of DNA cytosine methyltransferases using mass tagging

Valaiporn Rusmintratip1, Arthur D. Riggs2 and Lawrence C. Sowers1,3,*

1Division of Molecular Medicine, 2Division of Biology and 3Division of Pediatrics, City of Hope National Medical Center, 1500 East Duarte Road, Duarte, CA 91010, USA

The biological significance of cytosine methylation is as yet incompletely understood, but substantial and growing evidence strongly suggests that perturbation of methylation patterns, resulting from the infidelity of DNA cytosine methyltransferase, is an important component of the development of human cancer. We have developed a novel in vitro assay that allows us to quantitatively determine the DNA substrate preferences of cytosine methylases. This approach, which we call mass tagging, involves the labeling of target cytosine residues in synthetic DNA duplexes with stable isotopes, such as 15N. Methylation is then measured by the formation of 5-methylcytosine (5mC) by gas chromatography/mass spectrometry. The DNA substrate selectivity is determined from the mass spectrum of the product 5mC. With the non-symmetrical duplex DNA substrate examined in this study we find that the bacterial methyltransferase HpaII (duplex DNA recognition sequence CCGG) methylates the one methylatable cytosine of each strand similarly. Introduction of an A-C mispair at the methylation site shifts methylation exclusively to the mispaired cytosine residue. In direct competition assays with HpaII methylase we observe that the mispaired substrate is methylated more extensively than the fully complementary, normal substrate, although both have one HpaII methylation site. Through the use of this approach we will be able to learn more about the mechanisms by which methylation patterns can become altered.

* To whom correspondence should be addressed. Tel: +1 626 359 8111; Fax: +1 626 301 8862; Email: lsowers@coh.org


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