Repression of Flowering inArabidopsisRequires Activation ofFLOWERING LOCUS CExpression by the Histone Variant H2A.Z

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Abstract
The histone variant H2A.Z has been implicated in numerous chromatin-mediated processes, including transcriptional activation, euchromatin maintenance, and heterochromatin formation. In yeast and humans, H2A.Z is deposited into chromatin by a conserved protein complex known as SWR1 and SRCAP, respectively. Here, we show that mutations in the Arabidopsis thaliana homologs of two components of this complex, ACTIN-RELATED PROTEIN6 (ARP6) and PHOTOPERIOD-INDEPENDENT EARLY FLOWERING1 (PIE1), produce similar developmental phenotypes and result in the misregulation of a common set of genes. Using H2A.Z-specific antibodies, we demonstrate that ARP6 and PIE1 are required for the deposition of H2A.Z at multiple loci, including the FLOWERING LOCUS C (FLC) gene, a central repressor of the transition to flowering. Loss of H2A.Z from chromatin in arp6 and pie1 mutants results in reduced FLC expression and premature flowering, indicating that this histone variant is required for high-level expression of FLC. In addition to defining a novel mechanism for the regulation of FLC expression, these results support the existence of a SWR1-like complex in Arabidopsis and show that H2A.Z can potentiate transcriptional activation in plants. The finding that H2A.Z remains associated with chromatin throughout mitosis suggests that it may serve an epigenetic memory function by marking active genes and poising silenced genes for reactivation.