Petitalot, A. | bioRxiv Science

bioRxiv2026

ULTRAPETALA1 remodels PRC2 recruitment to nucleosomes

Polycomb Repressive Complex 2 (PRC2) establishes transcriptional repression through trimethylation of histone H3 lysine 27 (H3K27me3), a modification essential for developmental patterning. Here, we describe a novel plant-specific PRC2 variant (PRC2.3) that employs a distinct nucleosome-targeting mechanism mediated by accessory factor ULTRAPETALA1 (ULT1), which promotes H3K27me3 deposition at over 1,300 developmental genes. The cryo-EM structure of the PRC2SWN-ULT1-nucleosome complex reveals that ULT1 antagonizes the canonical PRC2 binding mode and instead bridges PRC2 to the nucleosome using its own interaction surfaces. ULT1 binds consecutive purines in the nucleosomal DNA and the histone H2A/H2B acidic patch, while enabling PRC2 to accommodate H3K36 modifications. We further show that in planta ULT1 enhances H3K27me3 at purine-rich loci and at genes associated with H3K36 marks, and identify the ULT1-H2A/H2B interface as required for reproductive transition and flower organogenesis. Our findings demonstrate that PRC2 can deploy mechanistically distinct recruitment strategies to control key developmental switches.

Molecular Biology↗

bioRxiv2024

The dual trxG/PcG protein ULTRAPETALA1 modulates H3K27me3 and directly enhances POLYCOMB REPRESSIVE COMPLEX 2 activity for fine-tuned reproductive transitions

AbstractThe antagonistic POLYCOMB (PcG) REPRESSIVE COMPLEX 2 (PRC2) and trithorax (trxG) chromatin machineries play a major role in orchestrating gene expression during the development of multicellular eukaryotes. These complexes are well known for depositing and maintaining the repressive H3K27me3 and activating H3K4me3 marks, respectively. However, the mechanisms that govern the switch between these functions remains 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 that antagonizes the PRC2 enzymatic subunit CURLY LEAF (CLF), also exhibits a repressive function, increasing H3K27me3 levels at over a thousand genes. We discovered a physical interaction between ULT1 and PRC2 components, particularly the SWINGER (SWN) enzymatic subunit. We further show that in vitro ULT1 significantly enhances the enzymatic activity of PRC2SWN, and to a lesser extent also that of PRC2CLF, corroborating our epigenomic and developmental genetic data that reveal different ULT1 activity depending on the catalytic subunit of the PRC2 complex. This study provides new insights into the relative activities of CLF and SWN and introduces a novel mechanistic framework for a chromatin switch mediated by a bivalent trxG/PcG factor. Key messageULTRAPETALA1 counteracts or promotes PRC2 activity at hundreds of developmental genes in Arabidopsis thaliana, and activates the deposition of the repressive H3K27me3 chromatin mark via direct interaction with PRC2. This is the first instance of a bivalent factor which functions as a cofactor of PRC2 HMTs.

genetics↗