Abstract
DNA in the polyploid macronucleus of the ciliated protozoan Tetrahymena thermophila contains the modified base N6-methyladenine. We identified two GATC sites which are methylated in most or all of the 45 copies of the macronuclear genome. One site is 2 kilobases 5' to the histone H4-I gene, and the other is 5 kilobases 3' to the 73-kilodalton heat shock protein gene. These sites are de novo methylated between 10 and 16 h after initiation of conjugation, during macronuclear anlage development. The methylation states of these two GATC sites and four other unmethylated GATC sites do not change in the DNA of cells cultured under conditions which change the activity of the genes, including logarithmic growth, starvation, and heat shock.