Adenine methylation in eukaryotes: Apprehending the complex evolutionary history and functional potential of an epigenetic modification

While N-6-methyladenosine (m(6)A) is a well-known epigenetic modification inbacterial DNA, it remained largely unstudied in eukaryotes. Recent studies have brought to fore its potential epigenetic role across diverse eukaryotes with biological consequences, which are distinct and possibly even opposite to the well-studied 5-methylcytosine mark. Adenine methyltransferases appear to have been independently acquired by eukaryotes on at least 13 occasions from prokaryotic restriction-modification and counter-restriction systems. On at least four to five instances, these methyltransferases were recruited as RNA methylases. Thus, m(6)A marks in eukaryotic DNA and RNA might be more wide spread and diversified than previously believed. Several m(6)A-binding protein domains from prokaryotes were also acquired by eukaryotes, facilitating prediction of potential readers for these marks. Further, multiple lineages of the AlkB family of dioxygenases have been recruited as m(6)A demethylases. Although members of the TET/JBP family of dioxygenases have also been suggested to be m(6)A demethylases, this proposal needs more careful evaluation.