PRC1-mediated H2A.Zub promotes gene expression by preventing H3.1K27me1 incorporation in Arabidopsis

The PcG complexes guarantee the correct temporal and spatial expression of genes. As such, they are crucial in orchestrating plant developmental phase transitions. While PRC2-mediated H3K27me3 is primarily associated with gene repression, PRC1 has a more versatile role that is not fully understood. Moreover, although PRC1 can monoubiquitinate canonical H2A or its variant H2A.Z, the circumstances under which this occurs remain unknown. Here, we show that in Arabidopsis H2A.Z is the variant that undergoes monoubiquitination during the transition from seed-to-seedling. H2A.Zub incorporation recruits PRC2 to repress seed-specific genes; nevertheless, it also activates gene expression by preventing the incorporation of H3K27me1. Interestingly, the removal of two methyl groups from H3K27me3-marked genes is a prerequisite for H2A.Zub incorporation, placing REF6 activity at the onset of gene reprogramming. Our data also indicate that H2A.Zub plays a role in promoting histone exchange to maintain gene expression patterns. These findings extend our understanding of PRC1-mediated gene regulation. Overall design: Chromatin immunoprecipitation DNA-sequencing (ChIP-seq) for histone modification H3K27me3 on Arabidopsis seeds 48 hours after imbibition (48 HAI) as well as ChIP-seq for histon variant H2A.Z and histone modifications H3K27me3 and H3K27me1 in Arabidopsis seedlings 10 days after germination (10 DAG).