Nucleation of Polycomb silencing at FLC

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The Dean group studies the control of flowering time in the reference plant, thalecress (Arabidopsis thaliana). Several flowering pathways converge to regulate a gene called FLC and variation in expression of this gene contributes enormously to natural variation in flowering time of Arabidopsis types collected from around the world. Winter conditions switch off expression of the FLC gene, removing the brake to flowering, and thus accelerating the flowering process. This induction of a plant's flowering process by exposure to the prolonged cold of winter is termed "vernalization." Genetic, molecular and computational analysis of vernalization in thalecress has shown it involves a cellular memory mechanism, one that also occurs during development of our own bodies. In the plant, cold switches off expression of FLC and this repressed state is maintained by Polycomb proteins that add methylation groups to proteins associated with FLC DNA. These modifications are passed from the mother to the daughter DNA strand to maintain the repressed state of the gene through the development as epigenetic regulation and similar mechanisms keep many thousands of genes switched off (silenced) during vertebrate development. A key question in the epigenetic field is what determines whether a specific gene becomes silenced or not. Recent work in the Dean lab has made progress in this question through characterization of a change in the sequence of the FLC gene that blocks the silencing by cold. This then enabled the factor that binds to that specific region to be identified. Use of a tagged version of the factor then enabled isolation of a range of proteins that were in many cases unexpected and which suggested what might be involved in the silencing switch. This proposal aims to follow up these findings to fully describe the multiple factors required to specify a gene for epigenetic silencing. Understanding from a study such as this can provide concepts important to epigenetic regulation across many genomes.