EFFECT OF APHIDICOLIN ON DNA METHYLTRANSFERASE IN THE NUCLEUS

Methylation of cytosine in the genomic DNA plays an important role in mammalian embryogenesis. DNA methyltransferase activity, which contrib utes mainly to the maintenance of the methylation pattern during proli feration, is under the control of the cell cycle, its activity being h igher in the S phase than in the other phases (Adams, R.L.P., 1990, Bi ochem. J. 265, 309-320). In the present study, we examined how DNA met hyltransferase is regulated in the cells arrested at S phase by aphidi colin treatment. The activity and protein levels of DNA methyltransfer ase in the nuclei were kept constant in proliferating mouse erythroleu kemia cells, and increased about twofold after 6 h incubation in the p resence of aphidicolin. This increase of DNA methyltransferase levels by aphidicolin treatment was positively correlated with the cell popul ation at S phase. De novo synthesis of DNA methyltransferase protein w as increased by the treatment. In addition, the relative half life of pulse labeled DNA methyltransferase was prolonged by aphidicolin treat ment. Both increase in synthesis and prolongation of half life of DNA methyltransferase in S phase contributed to the increase of the activi ty and the protein levels by aphidicolin treatment. Prolongation of ha lf life was abolished by cycloheximide, suggesting that newly synthesi zed protein(s) with a short half life participated in the degradation of DNA methyltransferase.