Methylated and unmethylated DNA compartments in the sea urchin genome

Sea urchin (Echinus esculentus) DNA has been separated into high and low molecular weight fractions by digestion with the mCpG-sensitive restriction endonucleases Hpa II, Hha I and Ava I. The separation was due to differences in methylation at the recognition sequences for these enzymes because an mCpG-insensitive isoschizomer of Hpa II (Msp I) digested Hpa II-resistant DNA to low molecular weight, showing that many Hpa II sites were in fact present in this fraction; and because 3H-methyl methionine administered to embryos was incorporated into the high molecular weight Hpa II-, Hha I- and Ava I-resistant fraction, but not significantly into the low molecular weight fraction. The fraction resistant to Hpa II, Hha I and Ava I amounted to about 40% of the total DNA. It consisted of long sequence tracts between 15 and well over 50 kg in length, in which many sites for each of these enzymes were methylated consecutively. The remaining 60% of the genome, (m-), was not significantly methylated. Methylated and unmethylated fractions were considered to be subfractions of the genome because enriched unique sequences from one fraction cross-reassociated poorly with the other fraction and specific sequences were found in either (m+) or (m-) but not in both (see below). Similar (m+) and (m-) compartments were found in embryos, germ cells and adult somatic tissues. Furthermor, we found no evidence for changes in the sequence composition of (m+) or (m-) between sperm, embryo or intestine DNAs, although low levels of exchange would not have been detected. Using cloned Echinus histone DNA, heterologous 5S DNA and ribosomal DNA probes, we have found that each of these gene families belongs to the unmethylated DNA compartment in all the tissues examined. In particular, there was no detectable methylation of histone DNA either in early embryos, which are thought to be actively transcribing the bulk of histone genes, or in sperm and gastrulae, in which most histone genes are not being transcribed. In contrast to these gene families, sequences complementary to an internally repetitious Echinus DNA clone were found primarily in the methylated DNA compartment.

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