Enzymatic DNA modifications in isolated nuclei from developing sea urchin embryos

Nuclei were prepared from developing sea urchin embryos. The isolated nuclei incubated in vitro in the presence of 14 C-methyl S -adenosylmethionine methylate their own DNA. The DNA methylase activity is lost by heating the nuclei at 65°C for 10 min. Some kinetic properties of the reaction are described. 14 C-formate and 3 H-deoxythymidine are not incorporated into DNA of the isolated nuclei. 5-Methylcytosine and thymine are the only products of the DNA methylation by the isolated nuclei. DNA adenine and guanine are not labelled under the same conditions. A mechanism of synthesis at the polymer level of the two DNA ‘minor’ bases by the action of specific DNA cytosine methylases and DNA-5 methylcytosine aminohydrolases is suggested. This mechanism is discussed as the possible chemical basis of the synchron model of cell differentiation.