Physical coupling of activation and derepression activities to maintain an active transcriptional state at FLC.

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Establishment and maintenance of gene expression states is central to development and differentiation. Transcriptional and epigenetic mechanisms interconnect in poorly understood ways to determine these states. We explore these mechanisms through dissection of the regulation of Arabidopsis thaliana FLOWERING LOCUS C (FLC). FLC can be present in a transcriptionally active state marked by H3K36me3 or a silent state marked by H3K27me3. Here, we investigate the trans factors modifying these opposing histone states and find a physical coupling in vivo between the H3K36 methyltransferase, SDG8, and the H3K27me3 demethylase, ELF6. Previous modeling has predicted this coupling would exist as it facilitates bistability of opposing histone states. We also find association of SDG8 with the transcription machinery, namely RNA polymerase II and the PAF1 complex. Delivery of the active histone modifications is therefore likely to be through transcription at the locus. SDG8 and ELF6 were found to influence the localization of each other on FLC chromatin, showing the functional importance of the interaction. In addition, both influenced accumulation of the associated H3K27me3 and H3K36me3 histone modifications at FLC We propose the physical coupling of activation and derepression activities coordinates transcriptional activity and prevents ectopic silencing.