DNA methylation in Drosophila--a critical evaluation

Drosophila belongs to the so-called "Dnmt2 only" organisms, and does not contain any of the canonical DNA methyltransferases (Dnmt1 and Dnmt3). Furthermore, no functional homologs of known 5-methylcytosine reader proteins are found. Nevertheless, there is strong evidence for DNA methylation in this organism. It has been suggested that DNA methylation in Drosophila is simply a byproduct of Dnmt2, which is a DNA methyltransferase (Dnmt) according to structure and type of catalysis but functions in vivo as a tRNA methyltransferase. However, concerning the very specific timing of cytosine methylation in Drosophila, their suggested functions in control of retrotransposon silencing and genome stability, and the obvious DNA methylation activity of Dnmt2 enzymes in the protozoans Dictyostelium discoideum and Entamoeba histolytica, we tend to disagree with this notation. Dnmt2 probably serves, and not only in Drosophila, as a methyltransferase of both specific DNA and tRNA targets.