Deciphering the role of PRC2 accessory proteins in promoting cold-induced epigenetic switching in Arabidopsis thaliana

All cells in multicellular organisms contain the same genetic material. Cell differentiation and adaptation to environmental signals require the accurate switching of gene expression states and the maintenance of those states through cell division, even in the absence of the initial signal. Polycomb complexes play an important role in these switching and epigenetic silencing mechanisms. The Arabidopsis thaliana developmental regulator FLOWERING LOCUS C (FLC) has emerged as an excellent system to dissect Polycomb Repressive Complex 2 (PRC2) regulation. FLC is expressed as plants germinate in autumn to prevent premature flowering. The prolonged cold of winter epigenetically silences FLC, aligning flowering with the following spring. The epigenetic silencing involves a cold-induced PRC2 switch and deposition of the repressive chromatin mark H3K27me3 in the FLC nucleation region. This switch requires the core PRC2 and two accessory proteins VIN3 and VRN5, but the molecular mechanism underlying the switch and the subsequent memory of cold exposure remain largely unknown. The work in this thesis aimed to understand the role of VIN3 and VRN5 in the repression of FLC. In the first Results chapter, the mechanism of VIN3 binding at FLC is studied; VIN3 was shown to contain a domain that can associate with nucleic acids. The second Results chapter dissects the function of the conserved C-terminal domain of VIN3 and VRN5; this domain mediates protein-protein interactions between VIN3 and VRN5 and adopts a novel polymerization fold. In the third Results chapter, the local chromatin conformation at FLC is studied; the expression state of FLC was found to correlate with distinct 3D conformation. The 3’ end of FLC containing a promoter for FLC antisense transcripts is involved in setting this 3D conformation. Overall, the work advances our understanding of the molecular function of PRC2 accessory proteins in epigenetic silencing