Molecular analysis of N super(6)-methyladenine patterns in Tetrahymena thermophila) nuclear DNA

We have cloned two DNA fragments containing 5'-GATC-3' sites at which the adenine is methylated in the macronucleus of the ciliate Tetrahymena thermophila . Using these cloned fragments as molecular probes, we analyzed the maintenance of methylation patterns at two partially and two uniformly methylated sites. Our results suggest that a semiconservative copying model for maintenance of methylation is not sufficient to account for the methylation patterns we found during somatic growth of Tetrahymena . Although we detected hemimethylated molecules in macronuclear DNA, they were present in both replicating and nonreplicating DNA.