A Stare of Elemental Naturalist at the DNA World (Base Composition, Sequences, Methylation)
The pioneering data on base composition and pyrimidine sequences in DNA of pro- and eukaryotes are considered and their significance for the origin of genosystematics is discussed. The modern views on specificity and functional role of enzymatic DNA methylation in eukaryotes are described. DNA methylation controls all genetic functions and is a mechanism of cellular differentiation and gene silencing. A model of regulation of DNA replication by methyla-tion is suggested. Adenine DNA methylation in higher eukaryotes (higher plants) was first observed, and it was estab-lished that one and the same gene can be methylated at both cytosine and adenine moieties. Thus, there are at least two different and seemingly interdependent DNA methylation systems present in eukaryotic cells. The first eukaryot-ic adenine DNA-methyltransferase is isolated from wheat seedlings and described: the enzyme methylates DNA with formation of N<sup>6</sup>-methyladenine in the sequence TGATCA > TGm<sup>6</sup>ATCA. It is found that higher plants have endonu-cleases that are dependent on S-adenosylmethionine (SAM) and sensitive to DNA methylation status. Therefore, as in bacteria, plants seem to have a restriction-modification (R-M) system. A system of conjugated up- and down-reg-ulation of SAM-dependent endonucleases by SAM modulations is found in plants. Revelation of an essential role of DNA methylation in regulation of genetic processes is a fundament of materialization of epigenetics and epigenomics.