Adenine Methylation in Eukaryotic DNA

N⁶-Methyladenine (m⁶A) has been found in DNAs of various eukaryotes (algae, fungi, protozoa, and higher plants). Like bacterial DNA, DNAs of these organisms are subject to enzymatic modification (methylation) not only at cytosine, but also at adenine bases. There is indirect evidence that adenine methylation of the genome occurs in animals as well. In plants, m⁶A was detected in total, mitochondrial, and nuclear DNAs. It was observed that both adenines and cytosines can be methylated in one gene (DRM2). Open reading frames coding for homologs of bacterial adenine DNA methyltransferases were revealed in protozoan, yeast, higher plant, insect, nematode, and vertebrate genomes, suggesting the presence of adenine DNA methyltransferases in evolutionarily distant eukaryotes. The first higher-eukaryotic adenine DNA N⁶-methyltransferase (wad-mtase) was isolated from vacuolar vesicles of wheat coleoptiles. The enzyme depends on Mg²⁺ or Ca²⁺ and, in the presence of S-adenosyl-L-methionine, methylates de novo the first adenine of the sequence TGATCA in single- and double-stranded DNAs, preferring the former. Adenine methylation of eukaryotic DNA is probably involved in regulating gene expression and replication, including that of mitochondrial DNA; plays a role in controlling the persistence of foreign DNA in the cell; and acts as a component of a plant restriction— modification system. Thus, the eukaryotic cell has at least two different systems for enzymatic methylation of DNA (at adenines and at cytosines) and a special mechanism regulating the functions of genes via a combinatorial hierarchy of these interdependent modifications of the genome.__________Translated from Molekulyarnaya Biologiya, Vol. 39, No. 4, 2005, pp. 557–566.Original Russian Text Copyright © 2005 by Vanyushin.To the memory of my teacher, Academician Andrei Nikolaevich Belozersky