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The interaction of AGL15 with members of the SIN3/HDAC1 complex suggests a mechanism that could explain its function as a transcriptional repressor in planta
Histone deacetylase (HDAC) enzymes remove acetyl groups from histones, and hypoacetylation results in a decrease in the space between the nucleosome and the DNA that is wrapped around it. Tighter wrapping of the DNA diminishes accessibility for transcription factors, leading to transcriptional repression (for a review, see de Ruijter et al., 2003). SIN3-associated proteins (SAP18 and SAP30) have been hypothesized to stabilize the SIN3–HDAC interaction (discussed by Silverstein and Ekwall, 2005). SAP18, like other core members of the SIN3/HDAC1 complex, is not believed to directly associate with the chromatin (reviewed by Silverstein and Ekwall, 2005), but does interact with sequence-specific DNA binding proteins (e.g., Espinas et al., 2000; Song and Galbraith, 2006; Zhu and Hanes, 2000; this study), implicating it as a bridge protein connecting the core SIN3/HDAC1 complex targeted genes.
When directed to regulatory regions of genes, SAP18 or other components of the HDAC complex lead to repression of gene expression (Song and Galbraith, 2006; Song et al., 2005; Wu et al., 2000; Yang et al., 1996; Zhang et al., 1997; Zhu et al., 2001). We demonstrated that SAP18 is able to associate with DNA-bound AGL15 in vitro, but is not able to directly interact with a strong AGL15 binding site (Figure 7). Likewise, SAP18 also interacts with DNA-bound ERF3 in vitro, but not with the ERF3 binding site (Song and Galbraith, 2006).
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