In vitro methylation of CpG-rich islands.
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.
Carotti
D
D
Departimenti di Scienze Biochimiche, Università di Roma La Sapienza, Italy.
Palitti
F
F
Lavia
P
P
Strom
R
R
D001483:Base Sequence; D003001:Cloning, Molecular; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000302:isolation & purification / Q000378:metabolism; D015226:Dinucleoside Phosphates; D005260:Female; D006801:Humans; D008745:Methylation; D010920:Placenta / Q000201:enzymology; D011247:Pregnancy; D012436:S-Adenosylmethionine / Q000378:metabolism
D004248:DNA (Cytosine-5-)-Methyltransferases; D015226:Dinucleoside Phosphates; D012436:S-Adenosylmethionine; C015772:CpG dinucleotide