Histone methyltransferase that methylates 'Lys-4' of histone H3 (H3K4me). H3K4me represents a specific tag for epigenetic transcriptional activation. Functions in segment determination through interaction with genes of bithorax (BX-C) and antennapedia (ANT-C) complexes. Acts as an activator of BX-C. Involved in the very early regulation of homeotic genes expressed only in the posterior region of the embryo. L-lysyl(9)-[histone H3] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(9)-[histone H3] + 3 S-adenosyl-L-homocysteine Interacts with ash1 via its SET domain. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily.