As noted previously, enhancers generally range in length from about 50 to 200 bp and include binding sites for several transcription factors. Analysis of the roughly 50-bp enhancer that regulates expression of β-interferon, an important protein in defense against viral infections in vertebrates, provides a good example of the structure of the DNA-binding domains of several transcription factors bound to the several transcription-factor-binding sites that constitute an enhancer (Figure 1). The term enhanceosome has been coined to describe such large DNA-protein complexes that assemble from transcription factors as they bind to the multiple binding sites in an enhancer.

Fig1. Model of the enhanceosome that forms on the β-interferon enhancer. Two heterodimeric factors, Jun/ATF-2 and p50/RelA (NF-κB), and two copies each of the monomeric transcription factors IRF-3 and IRF-7, bind to the six overlapping binding sites in this enhancer. See D. Penne, T. Manniatis, and S. Harrison, 2007, Cell 129:1111.

Because of the presence of flexible regions connecting the DNA-binding domains and activation or repression do mains in transcription factors (see Figure 2), and because of the ability of interacting proteins bound to distant sites to produce loops in the DNA between their binding sites, considerable leeway in the spacing between regulatory elements in transcription-control regions is permissible. This tolerance for variable spacing between binding sites for specific transcription factors, and between promoter binding sites for the general transcription factors and for Pol II, probably contributed to rapid evolution of gene control in eukaryotes. Transposition of DNA sequences and recombination between repeated sequences over evolutionary time probably created new combinations of control elements that were subjected to natural selection and retained if they proved beneficial. The latitude in spacing between regulatory elements probably allowed many more functional combinations to be subjected to this evolutionary experimentation than would be the case if constraints on the spacing between regulatory elements were strict, as for most genes in bacteria.

Fig2. Schematic diagrams illustrating the modular structure of eukaryotic transcription activators. Transcription factors may contain more than one activation domain but rarely contain more than one DNA-binding domain. Gal4 and Gcn4 are yeast transcription activators. The glucocorticoid receptor (GR) promotes transcription of target genes when certain hormones are bound to the C-terminal activation domain. SP1 binds to GC-rich promoter elements in a large number of mammalian genes.

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مواضيع ذات صلة

Formation of Heterochromatin Silences Gene Expression at Telomeres, near Centromeres, and in Other Regions

Transcription Factor Interactions Increase Gene-Control Options

Structurally Diverse Activation and Repression Domains Regulate Transcription

Electrophoresis of Nucleic Acids

Regulation of Transcription Factor Activity

Activators Are Composed of Distinct Functional Domains

DNase I Footprinting and EMSA Detect Protein-DNA Interactions

Most Eukaryotic Genes Are Regulated by Multiple Transcription-Control Elements

DNA-Binding Domains Can Be Classified into Numerous Structural Types

Promoter-Proximal Elements Help Regulate Eukaryotic Genes

Elongation Factors Regulate the Initial Stages of Transcription in the Promoter-Proximal Region

General Transcription Factors Position RNA Polymerase II at Start Sites and Assist in Initiation