In differentiating mouse myoblasts DNA methyltransferase is posttranscriptionally and posttranslationally regulated.

Upon the onset of mouse myoblast differentiation there is a rapid drop in DNA methyltransferase activity followed by a genome wide demethylation [Jost and Jost (1994) J. Biol. Chem. 269, 10040-10043]. Here we show by using specific antibodies directed against DNA methyltransferase that upon differentiation there was a rapid drop in nuclear DNA methyltransferase whilst the internal control histone H1 remained constant. The loss of nuclear methyltransferase was not due to a translocation of the enzyme from the nucleus to the cytoplasm where there was an increase in creatine phosphokinase protein. In vitro run on experiments carried out with growing and differentiating myoblast nuclei showed no difference in the rate of DNA methyltransferase mRNA synthesis. As measured by Northern blot hybridization the relative half life of DNA methyltransferase mRNA in growing and differentiating cells in the presence of Actinomycin D was 5 h and 1 h 30 min respectively, whereas in the same cells the half life of histone H4 mRNA was in both cases 80 min. As measured by a combination of pulse chase experiments with labeled leucine and immunoprecipitation, the relative half-life of DNA methyltransferase in growing and differentiating cells was approximately 18 h and 4 h 30 min respectively.