Histone methyltransferase that specifically methylates histone H3 to form H3K79me. This methylation is required for telomere silencing, correct growth and development, and for resistance to DNA damage induced by UV LIGHT. 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 Expressed during the whole life cycle. Individuals exhibit 4-5 hours delay in development. Development does progress with fewer fruiting bodies and smaller spore heads. No apparent changes in cell cycle regulation. 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.