Histone methyltransferase that specifically monomethylates 'Lys-27' of histone H3 (H3K27me1). Has much higher activity on nucleosomes containing H3.1 than H3.3. Involved in the formation of constitutive heterochromatin and the silencing of heterochromatic elements. L-lysyl(27)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(27)-[histone H3] + S-adenosyl-L-homocysteine Homodimer. Belongs to the class V-like SAM-binding methyltransferase superfamily.