Effect of trypsin on DNA methylation in isolated nuclei from developing sea urchin embryos

Nuclei isolated from the developing sea urchin embryo $\text{Paracentrotus lividus}$ and incubated in the presence of ³H-methyl] S-adenosylmethionine methylate their own DNA. Addition of small amounts of trypsin produces a 20-fold increase in DNA methylation. The time kinetics and the specificity of the trypsin activation of DNA methylation are described. The only products of the reaction are 5-methylcytosine and thymine. DNA adenine, guanine and cytosine are not labeled. The distribution of the counts between 5-methyl-cytosine and thymine is variable. While 5-methylcytosine originates by enzymatic methylation of DNA cytosines, the origin of the labeled thymine cannot be inferred with certainty.