Polycomb group (PcG) protein. Catalytic subunit of a the mes-2/mes-3/mes-6 complex, which methylates 'Lys-27' of histone H3, leading to transcriptional repression of the affected target genes. PcG proteins act by forming multiprotein complexes, which are required to maintain the transcriptionally repressive state of homeotic genes throughout development. In association with the nfya-1-NF-Y complex, may play a role in repressing the expression of the homeobox protein egl-5 in tissues such as the head (PubMed:17574230). PcG proteins are not required to initiate repression, but to maintain it during later stages of development. The mes-2/mes-3/mes-6 complex may participate in the global inactivation of the X chromosomes in germline cells. This complex is required to exclude mes-4 from the inactivated X-chromosomes in germline cells (PubMed:12077420, PubMed:15380065). Required for small-RNA-induced H3K27 trimethylation (PubMed:26365259). Involved in the negative regulation of lifespan in a germline-independent fashion (PubMed:22212395). 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 Interacts directly with mes-6 via its N-terminal domain (PubMed:11320248). Forms a heterotrimeric complex with mes-3 and mes-6 (PubMed:11320248, PubMed:15380065). Does not interact with mes-4 (PubMed:11320248). In adults, it is predominantly expressed in the germline, and weakly expressed in intestinal cells (PubMed:9609829, PubMed:29702639). Expressed in the hypoderm (PubMed:29702639). Expressed both maternally and zygotically. Expressed in all cells of early embryos. In late embryos and L1 larva, it is weakly expressed, while it is expressed at intermediate levels in the germline of L4 larvae. RNAi-mediated knockdown results in extended lifespan in wild type worms and in a glp-1(e2141) mutant background which lacks a germline (PubMed:22212395). Also leads to reduced H3K27me3 levels on metaphase chromosomes (PubMed:26904949). Double RNAi-mediated knockdown together with mes-6 RNAi results in ectopic expression of the homeobox protein egl-5 in the head region (PubMed:17574230). This ectopic expression of egl-5 in the head region is enhanced in a nfya-1 bp4 mutant background (PubMed:17574230). In addition in this background in males, there is ectopic expression of egl-5 in the mid-body region including in seam cells and hypodermal nuclei, and there is ectopic ray formation (PubMed:17574230). Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. EZ subfamily.