Nucleic Acids Research, 1989, Vol. 17, No. 22 9219-9229
© 1989
MOLECULAR BIOLOGY |
In vitro methylation of CpG-rich islands
1Departimenti di Scienze Biochimiche e Rome, Italy 2Biopatologia Umana, Università di Roma ‘La Sapienza’ Rome, Italy 3Centri di Genetica Evoluzionistica e Rome, Italy 4Biologia Molecolare del CNR Rome, Italy
*To whom correspondence should be addressed at Dipartimento di Scienze Biochimiche, Università ‘La Sapienza’, Piazzale A.Moro 5, 00185 Rome, Italy
Received July 24, 1989. Revised October 12, 1989. Accepted October 12, 1989.
CpG islands are distinguishable from the bulk of vertebrate DNA for being unmethylated and CpG-rich. Since CpG doublets are the specific target of eukaryotic DNA methyltransferases, CpG-rich sequences might be expected to be good methyl-accepting substrates in vitro, despite their unmethylated in vivo condition. This was tested using a partially purified DNA-methyltransferase from human placenta and several cloned CpG rich or CpG-depleted sequences. The efficiency of methylation was found to be proportional to the CpG content for CpG-depleted regions, which are representative of the bulk genome. However, methylation was much less efficient for CpG frequencies higher than 1 in 12 nucleotides, reaching only 60% of the expected level. That suggests that the close CpG spacing typical of CpG-islands somehow inhibits mammalian DNA methyltransferase. The implications of these findings on the in vivo pattern of DNA methylation are discussed.
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