REGULATION OF DNA METHYLTRANSFERASE DURING DIFFERENTIATION OF F9 MOUSE EMBRYONAL CARCINOMA-CELLS

DNA becomes demethylated when F9 mouse embryonal carcinoma cells diffe rentiate into parietal endoderm. DNA methyltransferase (DNA-MTase) act ivity decreased by 50% during 1 week of differentiation. The level of DNA-MTase mRNA was also diminished accordingly, but the transcription rate of the DNA-MTase gene measured by run-on transcription was essent ially unchanged, indicating regulation of DNA-MTase expression at a po sttranscriptional step. The decline of DNA-MTase mRNA paralleled that of histone H3 mRNA in accord with the notion that DNA-MTase is prefere ntially expressed in the S phase of the cell cycle. Since DNA-MTase ex pression decreases in parallel with DNA synthesis, DNA demethylation d uring differentiation of F9 cells appears not to be due to limited exp ression of DNA-MTase. However, the plasmid pAFP7000CAT, alpha-fetoprot ein (AFP), which is strongly de novo methylated when transfected into F9 stem cells became only weakly methylated after transfection into th e F9 parietal endoderm derivative P1, indicating that the activity of DNA-MTase within parietal endoderm cells is more strongly diminished t han is apparent from measurements of mRNA amounts and of overall DNA-M Tase activity in vitro. The discrepancy between DNA-MTase expression a nd its actual activity within the cell indicates the existence of a no vel mechanism controlling the activity of DNA-MTase. (C) 1995 Wiley-Li ss.