Nucleic Acids Research, Vol 26, Issue 20 4566-4573, Copyright © 1998 by Oxford University Press
KM Karrer and TA VanNuland
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.
ARTICLES
Position effect takes precedence over target sequence in determination of adenine methylation patterns in the nuclear genome of a eukaryote, Tetrahymena thermophila
Department of Biology, Marquette University, Milwaukee, WI 53201-1881, USA. karrerk@mu.edu
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