Dnmt2 was one of the first cytosine-5 RNA methylases identified in a multicellular organism [16••]. Although Dnmt2-mediated methylation of cytosine 38 in the

anticodon loop of tRNAAsp was conserved in plant, flies and mice, none of these organisms lacking the functional Dnmt2 protein displayed any morphological differences to their wild-type counterparts [16••]. In contrast, morpholino-mediated loss of Dnmt2 in zebrafish reduced the size of the morphants by half and specifically affected liver, retina and brain development due to a failure to conduct late differentiation [17]. Over-expression of Dnmt2 on the other hand prolonged the life span of Drosophila by more than 50% and increased the resistance to stress

[ 18]. In line with these studies, Drosophila Dnmt2 loss-of-function INK 128 nmr Nutlin3a mutants showed reduced viability under stress conditions, and Dnmt2-mediated methylation protected tRNAs from stress-induced ribonuclease cleavage ( Figure 1a) [ 9]. Cleavage of tRNAs is a conserved response to several stress stimuli in eukaryotes and the tRNA fragments are produced to repress translation by displacing translation initiation and elongation factors from mRNAs or by interfering with efficient transpeptidation [19, 20 and 21]. However, whether and how increased tRNA cleavage in Drosophila Dnmt2 mutants is directly linked to stress tolerance and protein translation is currently unknown. While tRNA cleavage is mediated by angiogenin in mammals, the only identified tRNA nuclease in Drosophila so far is Dicer [ 22]. Interestingly, also Astemizole expression of Dicer is down-regulated by oxidative stress and Dicer knockout cells can be hypersensitive towards oxidative stress whereas its over-expression confers stress resistance

[ 23]. Other functions that have been linked to Dnmt2 but may be independent of its tRNA methyltransferase activity are silencing of retro-transposons and control of RNA viruses in Drosophila as well as RNA-mediated paramutations in the mouse [ 24]. Together, these data implicate that Dnmt2 is functionally redundant for normal development of most multicellular organisms but implicated in cellular stress responses at least in adult flies [ 24]. At least two more enzymes NSun2 and NSun4 can generate 5-methylcytidine in RNA in mammals (Figure 1a and b) [25 and 26]. Both belong to the S-Adenosylmethionine (AdoMet)-dependent methyltransferase superfamily and at least five more putative m5C RNA methylases in mammals (NOP2, NSun3, and NSun5–7) are predicted to methylate RNA based on sequence conservation of key catalytic residues [12]. Although the substrate specificities are unknown, NSun1 and NSun5, in addition to NSun2 and Nsun4, have been identified as mRNA-binding proteins [27].