Polycomb-mediated gene silencing by the BAH–EMF1 complex in plants

Polycomb proteins implement genome-wide transcriptional repression in multicellular organisms. The evolutionarily conserved Polycomb repressive complex 2 (PRC2) catalyzes histone H3 Lys27 trimethylation (H3K27me3) that is read and effected by Polycomb repressive complex 1 (PRC1) in animals, but the interpretation of this mark remains unclear in plants. Here we report that in the eudicot Arabidopsis thaliana two homologous BAH (Bromo adjacent homology) domain–containing proteins form a plant-specific complex with EMBRYONIC FLOWER 1 (EMF1), and that the BAH–EMF1 complex (BAH–EMF1c) reads and effects the H3K27me3 mark and mediates genome-wide transcriptional repression. Furthermore, in the monocot rice a homolog of the Arabidopsis BAH-domain proteins also binds methylated H3K27 and forms a complex with the rice homolog of EMF1, suggesting that BAH–EMF1c is conserved in flowering plants. Therefore, our results show that the plant-specific BAH–EMF1c fulfills PRC1-like functions in higher plants, suggesting a convergent evolution of PRC1 activity in plants and animals.