Delayed Methylation of DNA in Developing Sea Urchin Embryos

Abstract

IN eukaryotes 5-methylcytosine seems to be the only minor methylated base^1,2, and this is synthesized at the polynucleotide level using S-adenosyl methionine. Although in most circumstances the majority of this methylation occurs very shortly after the DNA has been synthesized^1,3 this is apparently not essential. Thus I have shown in mouse L929 cells⁴ that following incubation with ³H-methyl methionine for 3 h about 15% of the 5-methylcytosines produced are in DNA synthesized before the label was added, and also following cessation of DNA synthesis either at the end of S phase or on treatment of cells with hydroxyurea or aminopterin, methylation was found to continue. Methylation of DNA has also been shown to occur in G₂ in Physarum polycephalum⁵. It has been claimed that no methylation of DNA occurs in the sea urchin Lytechinus variegatus until gastrulation⁶ although some has been shown to occur throughout development in Sphaerechinus granularis and Paracentrotus lividus^1,7. These latter experiments, however, indicated that the maximum rate of methylation occurred at about the time of gastrulation; the developing sea urchin embryo is thus an ideal system to study the delayed methylation of DNA.