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1997), and DNA access can be further occluded by juxtaposition to nearby nucleosomes in chromatin fibers ( Schalch et al.
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In the process, substantial surface area of DNA faces the histone octamer ( Luger et al. 2004 Sekiya and Zaret 2007), and folding of nucleosomes into higher-order structures ( Schwarz and Hansen 1994). 1998), binding of corepressor complexes ( Fan et al. Arrays of nucleosome cores can be condensed by binding of linker histone near the nucleosome dyad axis ( Zhou et al. The primary means of condensation is by the wrapping of DNA around an octamer of the four core histones to create the nucleosome core particle ( Kornberg 1977). Genes in eukaryotic cells are packaged into chromatin, allowing the condensation of 2 m of DNA into a nucleus of microns in diameter. Herein we review the field and describe how pioneer factors may enable cellular reprogramming. Passive and active roles for the pioneer factor FoxA occur in embryonic development, steroid hormone induction, and human cancers. In addition, pioneer factor binding can actively open up the local chromatin and directly make it competent for other factors to bind. Such initial binding can passively enhance transcription by reducing the number of additional factors that are needed to bind the DNA, culminating in activation.
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#Pioneer enable dot by dot series#
However, in cases in which the binding of a series of factors is sequential in time and thus not initially cooperative, special “pioneer transcription factors” can be the first to engage target sites in chromatin. Binding cooperativity can be sufficient for many kinds of factors to simultaneously engage a target site in chromatin and activate gene expression. This raises the question of how target sites at silent genes become bound de novo by transcription factors, thereby initiating regulatory events in chromatin. Indeed, genome-wide location analyses have revealed that, for all transcription factors tested, the vast majority of potential DNA-binding sites are unoccupied, demonstrating the inaccessibility of most of the nuclear DNA. Yet much of the DNA in the eukaryotic cell is in nucleosomes and thereby occluded by histones, and can be further occluded by higher-order chromatin structures and repressor complexes. Transcription factors are adaptor molecules that detect regulatory sequences in the DNA and target the assembly of protein complexes that control gene expression.