Polycomb-group (PcG) proteins mediate epigenetic gene regulation by setting H3K27me3 via Polycomb Repressive Complex 2 (PRC2). In plants, it is largely unclear how PcG proteins are recruited to their target genes. Here, we identified the PWWP-DOMAIN INTERACTOR OF POLYCOMBS1 (PWO1) protein, which interacts with all three PRC2 histone methyltransferases and is required for maintaining full H3 occupancy at several Arabidopsis genes. PWO1 localizes and recruits CURLY LEAF to nuclear speckles in nuclei, suggesting a role in spatial organization of PcG regulation. belongs to a gene family with three members having overlapping activities: triple mutants are seedling lethal and show shoot and root meristem arrest, while single mutants are early flowering. Interestingly, the PWWP domain of PWO1 confers binding to histones, which is reduced by a point mutation in a highly conserved residue of this domain and blocked by phosphorylation of H3S28. PWO1 carrying this mutation is not able to fully complement the triple mutant, indicating the requirement of this domain for PWO1 in vivo activity. Thus, the PWO family may present a novel class of histone readers that are involved in recruiting PcG proteins to subnuclear domains and in promoting Arabidopsis development.

Polycomb Group (PcG) proteins mediate chromatin repression in plants and animals by catalyzing H3K27 methylation and H2AK118/119 mono-ubiquitination through the activity of the Polycomb repressive complex 2 (PRC2) and PRC1, respectively. PcG proteins were extensively studied in higher plants, but their function and target genes in unicellular branches of the green lineage remain largely unknown. To shed light on PcG function and modus operandi in a broad evolutionary context, we demonstrate phylogenetic relationship of core PRC1 and PRC2 proteins and H3K27me3 biochemical presence in several unicellular algae of different phylogenetic subclades. We focus then on one of the species, the model red alga Cyanidioschizon merolae, and show that H3K27me3 occupies both, genes and repetitive elements, and mediates the strength of repression depending on the differential occupancy over gene bodies. Furthermore, we report that H3K27me3 in C. merolae is enriched in telomeric and subtelomeric regions of the chromosomes and has unique preferential binding toward intein-containing genes involved in protein splicing. Thus, our study gives important insight for Polycombmediated repression in lower eukaryotes, uncovering a previously unknown link between H3K27me3 targets and protein splicing.

BackgroundWater and solute transport across epithelia can occur via the transcellular or paracellular pathways. Tight junctions play a key role in mediating paracellular ion reabsorption in the kidney. In the renal collecting duct, which is a typical absorptive tight epithelium, coordination between transcellular sodium reabsorption and paracellular permeability may prevent the backflow of reabsorbed sodium to the tubular lumen along a steep electrochemical gradient.MethodsTo investigate whether transcellular sodium transport controls tight-junction composition and paracellular permeability via modulating expression of the transmembrane protein claudin-8, we used cultured mouse cortical collecting duct cells to see how overexpression or silencing of epithelial sodium channel (ENaC) subunits and claudin-8 affect paracellular permeability. We also used conditional kidney tubule–specific knockout mice lacking ENaC subunits to assess the ENaC’s effect on claudin-8 expression.ResultsOverexpression or silencing of the ENaC γ-subunit was associated with parallel and specific changes in claudin-8 abundance. Increased claudin-8 abundance was associated with a reduction in paracellular permeability to sodium, whereas decreased claudin-8 abundance was associated with the opposite effect. Claudin-8 overexpression and silencing reproduced these functional effects on paracellular ion permeability. Conditional kidney tubule–specific ENaC γ-subunit knockout mice displayed decreased claudin-8 expression, confirming the cell culture experiments' findings. Importantly, ENaC β-subunit or α-subunit silencing or kidney tubule–specific β-ENaC or α-ENaC knockout mice did not alter claudin-8 abundance.ConclusionsOur data reveal the specific coupling between ENaC γ-subunit and claudin-8 expression. This coupling may play an important role in preventing the backflow of reabsorbed solutes and water to the tubular lumen, as well as in coupling paracellular and transcellular sodium permeability.