Polycomb repression works without Siesta [TRIP]

Polycomb group proteins are essential for epigenetic repression of developmental genes. They act as multi-subunit complexes whose biochemical functions are yet to be fully characterised. One of the complexes, canonical Polycomb Repressive Complex 1 (PRC1), acts as an E3 ligase, depositing a single ubiquitin molecule on histone H2A. It can also bind to histone H3 tri-methylated at lysine 27 (H3K27me3), which is critical to propagate the repressed state of regulated genes epigenetically. The RING1 subunit of PRC1, responsible for the ubiquitin ligase activity, forms other complexes. These complexes can ubiquitylate H2A but cannot bind H3K27me3. It was proposed that H2A ubiquitylation is an essential part of the repressive mechanism and that variant RING1 complexes evolved from ancestral canonical PRC1 to diversify the Polycomb system and enable the evolution of vertebrate-specific traits. However, systematic tracing of genes encoding subunits of distinct variant RING1 complexes argues that these complexes appeared early in animal evolution and likely had functions unrelated to epigenetic repression. To address this problem, we leveraged the power of Drosophila genetics to discover that canonical PRC1 and variant RING1 complexes monoubiquitylate H2A across distinct genomic regions. We found that the sole Drosophila PCGF protein specific for variant RING1 complexes, which we named Siesta, is not required for epigenetic repression of homeotic genes, but controls larval locomotion independently of H2A ubiquitylation. Exploiting the division of labour between PRC1 and Siesta-RING1 complexes, we employed thousands of reporters integrated in parallel to conclude that H2A ubiquitylation has no major repressive effect on transcription. We propose that variant RING1 complexes are not part of the Polycomb regulatory system and that the current PRC1 nomenclature needs revision. TRIP experiments in cultured Drosophila cells. Two experiments started from independently grown cell cultures were performed for each genetic background and condition.