RING1 and BMI1 catalytic activities play distinct roles in plant PcG-mediated gene regulation.

PcG proteins play a crucial role in the regulation of eukaryotic development. Two major complexes, PRC1/2, are responsible for H2Aub and H3K27me3, respectively. While the core composition of PRC2 is conserved across species and its activity is required for gene repression, PRC1 components are diverse, and the role of H2Aub is not yet fully understood. In Arabidopsis (Arabidopsis thaliana), both B-cell-specific Moloney murine leukemia virus integration site 1 (BMI1A/B/C) and Really Interesting New Gene 1 (RING1A/B) display H2Aub activity, but whether any link or difference exists between RING1 and BMI1 proteins' in vivo activity in Arabidopsis remains unknown. Here, we show that the catalytic activities of RING1 and BMI1 proteins, rather than their mere presence, are required for gene regulation. We found that BMI1 affects the H2Aub on both H2Aub/H3K27me3- and only-H2Aub-marked genes, whereas genes marked only by H2Aub largely depend on RING1A/B. Our data also show specific subsets of genes in which H3K27me3 levels are partially maintained by the RING-domain proteins independently of their catalytic activities, and other subsets of genes where BMI1s assist RING1s' activities for H2Aub marking, highlighting their unique and common functions.