Like the age-old “chicken-or-the-egg” question, epigeneticists have wondered which comes first, nucleosome positioning or DNA methylation. One school of thought is that DNA must be unwrapped from nucleosomes to allow methyltransferase access before methylation can occur, but a new study in Nature suggests that this theory isn’t all that it’s cracked up to be.
A great group of scientists from the Jacobsen lab at UCLA deep sequenced Arabidopsis mononucleosomes and compared the data with DNA methylation profiles to tease out how the two interact. The research team showed that:
- DNA methylation has a 10-base periodicity on nucleosomal DNA, indicating that the DNA can be methylated when still bound to nucleosomes
- DNA methylation occurs preferentially on nucleosomal DNA
- Nucleosomes are more abundant in exons than in introns, and so is DNA methylation
- These relationships are highly conserved in humans and other species.
This work is especially interesting in light of a recent paper showing that DNA methylation leads to more compact nucleosomes. So while the question of which comes first hasn’t been completely settled, the recent evidence makes nucleosome position the odds-on favorite.
Don’t be a chicken; go read the full paper at Nature, May 2010.