Histone methyltransferase. Methylates 'Lys-79' of histone H3. Required for Polycomb Group (PcG) and trithorax Group (trxG) maintenance of expression. Also involved in telomeric silencing but do not in centric heterochromatin. Probably participates in pairing sensitivity. 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 in embryos, larvae and adults. 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. Was named 'grappa' because the eyes of mutant flies are of a color similar to that of the Italian spirit. Belongs to the class I-like SAM-binding methyltransferase superfamily. DOT1 family.