Catalysis of the reaction: L-lysyl9- + S-adenosyl-L-methionine = H+ + N6-methyl-L-lysyl9- + S-adenosyl-L-homocysteine. This reaction is the addition of a methyl group to the unmethylated lysine residue at position 9 of histone H3, producing histone H3K9me.
histone lysine N-monomethyltransferase activity (H3-K9 specific)