The <scp>RING1</scp> subunit of Polycomb Repressive Complex 1 monoubiquitinates histone <scp>H2A</scp> and mediates repression independently of Polycomb Repressive Complex 2 in the model diatom <i>Phaeodactylum tricornutum</i>

Polycomb Repressive Complex 1 (PRC1) and its associated mark, H2A monoubiquitination (H2Aub), cooperate with PRC2 for transcriptional silencing in multicellular organisms. However, PRC1 is absent from many unicellular lineages, leaving its functional conservation and relationship with PRC2 unclear. In the model diatom Phaeodactylum tricornutum, the role of the PRC1 subunit RING1 was investigated to assess whether PRC1-mediated repression is evolutionary conserved and how it interacts with PRC2. RING1 knockout mutants were generated to investigate H2Aub deposition. Genome-wide H2Aub profiles were integrated with existing H3K27me3 maps, and transcriptome analyses were performed to compare RING1 and the PRC2 catalytic subunit Enhancer of Zeste mutants, thereby evaluating their respective contributions to gene and transposable elements (TEs) repression. Loss of RING1 abolished H2Aub and revealed that H2Aub predominantly marks repressed genes lacking H3K27me3, while both marks colocalize on TEs. Transcriptome data indicate that H2Aub primarily mediates repression of genes with high H2Aub enrichment, whereas TEs marked by both H2Aub and H3K27me3 are mainly regulated by H3K27me3. These results demonstrate that PRC1 and PRC2, which originated before multicellularity, act independently to establish transcriptional silencing in P. tricornutum. PRC1 is functionally conserved in this diatom, with H2Aub and H3K27me3 fulfilling distinct yet complementary regulatory roles.