Position effect takes precedence over target sequence in determination of adenine methylation patterns in the nuclear genome of a eukaryote, Tetrahymena thermophila.

Approximately 0.8% of the adenine residues in the macronuclear DNA of the ciliated protozoan Tetrahymena thermophila are modified to N 6-methyladenine. DNA methylation is site specific and the pattern of methylation is constant between clonal cell lines. In vivo, modification of adenine residues appears to occur exclusively in the sequence 5'-NAT-3', but no consensus sequence for modified sites has been found. In this study, DNA fragments containing a site that is uniformly methylated on the 50 copies of the macronuclear chromosome were cloned into the extrachromosomal rDNA. In the novel location on the rDNA minichromosome, the site was unmethylated. The result was the same whether the sequences were introduced in a methylated or unmethylated state and regardless of the orientation of the sequence with respect to the origin of DNA replication. The data show that sequence is insufficient to account for site-specific methylation in Tetrahymena and argue that other factors determine the pattern of DNA methylation.