Analysis of putative RNase sensitivity and protease insensitivity of demethylation activity in extracts from rat...

The mechanism for demethylation of DNA in rat myoblasts has recently been studied using a new in vitro system that monitors demethylation in whole cell extracts. Previous investigations using this system had indicated that demethylation is resistant to conditions that are normally assumed to denature or digest proteins. Remarkably, it was reported that the activity appeared to be sensitive to the action of ribonuclease, suggesting a role for RNA in the demethylation of DNA. This manuscript reports that, upon further purification of the extract, demethylation activity has properties that are different. When subjected to more rigorous procedures for digestion of proteins, the demethylase activity disappears. Furthermore, RNase sensitivity of the extract disappears when a quantity of unmethylated competitor DNA is added to the reaction mix or when extracts treated with RNase are subsequently treated with protease. Although a role for RNA cannot be completely discounted, it is unlikely that this demethyl-ation reaction involves RNA cofactors or ribozyme components. These results have important implications for the mechanism of DNA demethylation and they exemplify the potential pitfalls of experiments in which new biological roles for RNA are evaluated using RNase sensitivity experiments.