Polycomb group (PcG) protein. Catalytic subunit of the Esc/E(z) complex, which methylates 'Lys-9' and 'Lys-27' of histone H3, leading to transcriptional repression of the affected target gene. While PcG proteins are generally required to maintain the transcriptionally repressive state of homeotic genes throughout development, this protein is specifically required during the first 6 hours of embryogenesis to establish the repressed state. The Esc/E(z) complex is necessary but not sufficient for the repression of homeotic target genes, suggesting that the recruitment of the distinct PRC1 complex is also required. L-lysyl(27)-[histone H3] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(27)-[histone H3] + 3 S-adenosyl-L-homocysteine Component of the Esc/E(z) complex, composed of Caf1-55, esc, E(z), Su(z)12, and possibly pho. The Esc/E(z) complex may also associate with Pcl and HDAC1/Rpd3 during early embryogenesis. This complex is distinct from the PRC1 complex, which contains many other PcG proteins like Pc, Ph, Psc, Su(z)2. The two complexes however cooperate and interact together during the first 3 hours of development to establish PcG silencing. Interacts with corto in vitro. Interacts with Nup93-1 (PubMed:31784359). Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. EZ subfamily.