Histone methyltransferase. Monomethylates specifically 'Lys-9' of histone H3. H3 'Lys-9Me1' (H3K9me1) functions as an epigenetic mark of repressed chromatin. L-lysyl(9)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(9)-[histone H3] + S-adenosyl-L-homocysteine Loss of function mutant (T-DNA insertion) releases the transcriptional silencing of tandem transgenes. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. Suvar3-9 subfamily.