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Abstract Proper cell-type identity relies on highly coordinated regulation of gene expression. Regulatory elements such as enhancers can produce cell type-specific expression patterns, but the mechanisms underlying specificity are not well understood. We previously identified an enhancer region capable of driving specific expression in giant cells, which are large, highly endoreduplicated cells in the Arabidopsis thaliana sepal epidermis. In this study, we use the giant cell enhancer as a model to understand the regulatory logic that promotes cell type-specific expression. Our dissection of the enhancer revealed that giant cell specificity is mediated primarily through the combination of two activators and one repressor. HD-ZIP and TCP transcription factors are involved in the activation of expression throughout the epidermis. High expression of HD-ZIP transcription factor genes in giant cells promoted higher expression driven by the enhancer in giant cells. Dof transcription factors repressed the activity of the enhancer such that only giant cells maintained enhancer activity. Thus, our data are consistent with a conceptual model whereby cell type-specific expression emerges from the combined activities of three transcription factor families activating and repressing expression in epidermal cells.more » « less
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Abstract ARGONAUTES are the central effector proteins of
RNA silencing which bind target transcripts in a smallRNA ‐guided manner.Arabidopsis thaliana has 10 (ARGONAUTE ) genes, with specialized roles inAGO RNA ‐directedDNA methylation, post‐transcriptional gene silencing, and antiviral defense. To better understand specialization among genes at the level of transcriptional regulation we tested a library of 1497 transcription factors for binding to the promoters ofAGO ,AGO 1 , andAGO 10 using yeast 1‐hybrid assays. A ranked list of candidateAGO 7DNA ‐bindingTF s revealed binding of the promoter by a number of proteins in two families: the miR156‐regulatedAGO 7SPL family and the miR319‐regulatedTCP family, both of which have roles in developmental timing and leaf morphology. Possible functions forSPL andTCP binding are unclear: we showed that these binding sites are not required for the polar expression pattern of , nor for the function ofAGO 7 in leaf shape. NormalAGO 7 transcription levels and function appear to depend instead on an adjacent 124‐bp region. Progress in understanding the structure of this promoter may aid efforts to understand how the conservedAGO 7AGO 7‐triggered pathway functions in timing and polarity.TAS 3