Histone methyltransferase. Methylates 'Lys-79' of histone H3. Nucleosomes are preferred as substrate compared to free histones (PubMed:12123582). Binds to DNA (PubMed:12628190). L-lysyl(79)-[histone H3] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(79)-[histone H3] + 3 S-adenosyl-L-homocysteine Interacts with MLLT10. In contrast to other lysine histone methyltransferases, it does not contain a SET domain, suggesting the existence of another mechanism for methylation of lysine residues of histones. Belongs to the class I-like SAM-binding methyltransferase superfamily. DOT1 family.