Specific protection of methylated CpGs in mammalian nuclei.
We have compared nuclear accessibility of methylated and nonmethylated sequences using restriction enzymes. MspI, which cuts CpG sites in naked DNA regardless of methylation, cut DNA in intact mouse liver or brain nuclei almost exclusively at CpG islands. Bulk chromatin was not significantly cleaved by MspI but was cleaved extensively by enzymes that do not recognize CpG. Quantitative analysis of limit digests showed that MspI and another methyl-CpG insensitive enzyme, Tth, have a strong bias against cutting methylated sites in these nuclei. Southern analysis confirmed this at three genomic loci. Our results suggest that resistance to nucleases is mediated by factors that are bound specifically to methylated CpGs. MeCP, a protein that binds to methylated DNA in vitro, may be one such factor, since nuclease resistance was significantly reduced in an MeCP-deficient cell line.
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MRC Human Genetics Unit, Western General Hospital, Edinburgh, Scotland.
Macleod
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AP
D044503:5-Methylcytosine; D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism; D002843:Chromatin / Q000648:ultrastructure*; D003596:Cytosine / Q000031:analogs & derivatives* / Q000378:metabolism; D004262:DNA Restriction Enzymes / Q000378:metabolism*; D019003:Deoxyribonuclease HpaII; D015252:Deoxyribonucleases, Type II Site-Specific / Q000378:metabolism; D004303:Dosage Compensation, Genetic; D008745:Methylation; D051379:Mice; D015183:Restriction Mapping
D002843:Chromatin; D044503:5-Methylcytosine; D003596:Cytosine; D004262:DNA Restriction Enzymes; D019003:Deoxyribonuclease HpaII; D015252:Deoxyribonucleases, Type II Site-Specific; C027046:TCGA-specific type II deoxyribonucleases