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. Influences which sets of rRNA gene variants are expressed or silenced. L-lysyl(27)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(27)-[histone H3] + S-adenosyl-L-homocysteine Isoform 1 but not isoform 2 interacts with PCNA1 and PCNA2. Interacts (via PHD domain) with HTR1 (via N-terminus). Isoform 2 interacts with IPS1. Never found in plastids and the nucleus within the same cell. Expressed in leaves, roots, stems, flowers, siliques and developing pollen. 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. Major isoform. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily.