Histone methyltransferase that specifically monomethylates 'Lys-27' of histone H3 (H3K27me1). Has higher activity on nucleosomes containing H3.1 than H3.3. Involved in the formation of constitutive heterochromatin and the silencing of heterochromatic elements. May act as a positive regulator of the G1-S transition. Influences which sets of rRNA gene variants are expressed or silenced. Up-regulated by E2FB. L-lysyl(27)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(27)-[histone H3] + S-adenosyl-L-homocysteine Interacts with PCNA1 and PCNA2. Interacts (via PHD domain) with HTR1 (via N-terminus). Interacts with IPS1. Expressed in leaves, roots, stems, flowers and siliques. Up-regulated in tissues where cell division is active. No visible phenotype. Atxr5 and atxr6 double mutant is smaller than wild-type plants, shows partial decondensation of the chromocenter, decreased H3K27 monomethylation and increased DNA re-replication. The binding to histone H3.2 is unaffected by mono-, di, or trimethylation at H3K9, but is strongly reduced by increasing levels of H3K4 methylation. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily.