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. 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. Required for small-RNA-induced H3K27 trimethylation. Involved in the negative regulation of lifespan in a germline-independent fashion.