H2A monoubiquitination in Arabidopsis is generally independent of LHP1 and PRC2 activity

Gene silencing by chromatin marks occurs in plants and animals, but there are often some differences in the details. Polycomb repressive complex 1 (PRC1) and PRC2 were first characterized in animals and shown to repress gene expression in part through histone modification; PRC1 has histone H2A E3 ubiquitin ligase activity, and PRC2 has histone H3 lysine 27 (H3K27) tri-methyltransferase activity. In animals, the classic hierarchical model for Polycomb group (PcG) complex recruitment proposes that the H3K27me3 produced by PRC2 recruits PRC1, leading to H2A ubiquitination. Zhou and Romero-Campero et al. investigated histone modifications genome-wide in wild-type and mutant plants to determine whether this activity sequence holds true in plants, and found that it does not; PRC2 activity is dispensable to establish H2AK121ub marks at most genes. Genome Biol. 10.1186/s13059-017-1197-z