The dual trxG/PcG protein ULTRAPETALA1 modulates H3K27me3 and directly enhances POLYCOMB REPRESSIVE COMPLEX 2...

The antagonistic POLYCOMB (PcG) REPRESSIVE COMPLEX 2 (PRC2) and trithorax (trxG) chromatin machineries orchestrate gene expression during the development of multicellular eukaryotes. These complexes deposit and maintain the repressive trimethyl mark on H3K27 and the activating trimethyl mark on H3K4, respectively. However, the mechanisms governing the switch between these functions remain elusive, especially in plants, whose lifelong, flexible development relies heavily on this process. Here we demonstrate that the plant-specific ULTRAPETALA1 (ULT1) protein, previously reported as a trxG factor antagonizing the PRC2 enzymatic subunit CURLY LEAF (CLF), also exhibits a pro-PRC2 function, increasing H3K27 trimethylation levels at over 1,000 genes. We discovered that ULT1 physically interacts with PRC2 components, particularly the SWINGER (SWN) enzymatic subunit, and that in vitro it significantly enhances the enzymatic activity of PRC2^SWN, and to a lesser extent also that of PRC2^CLF. This corroborates our epigenomic and developmental genetic data that reveal different ULT1 activity depending on the PRC2 catalytic subunit. This study provides new insights into the relative activities of CLF and SWN and introduces a mechanistic framework for a chromatin switch mediated by a bivalent trxG/PcG factor.