Enhancers/enhancer elements

The DNA part of each control module can be divided into three main regions, the core or basal promoter elements, the promoter proximal elements and the distal enhancer elements (Figure 9.1). The best characterized core promoter element is the TATA box, a DNA sequence that is rich in A-T base pairs and located 25 base pairs upstream of the transcription start site. The TATA box is recognized by one of the basal transcription factors, the TATA box-binding protein, TBP, which is part of a multisubunit complex called TFIID. This complex in combination with RNA polymerase 11 and other basal transcription factors such as TFIIA and TFIIB form a preinitiation complex for transcription. 

Figure 9.1 The transcriptional elements of a eucaryotic structural gene extend over a large region of DNA. The regulatory sequences can be divided into three main regions (1) the basal promoter elements such as the TATA box, (2) the promoter proximal elements close to the initiation site, and (3) distal enhancer elements far from the initiation site.

Figure 9.2 Schematic model for transcriptional activation. The TATA box-binding protein, which bends the DNA upon binding to the TATA box, binds to RNA polymerase and a number of associated proteins to form the preinitiation complex. This complex interacts with different specific transcription factors that bind to promoter proximal elements and enhancer elements.

The zinc cluster regions ofGAL4 bind at the two ends of the enhancer element... 

P. B. Eamswoith, M. Wu, M. Tacquai d and M. L. Lee, Background correction device for enhanced element-selective gas cltromatograpltic detection by atomic emission spec-ti oscopy , Appl. Spectr. 48 742-746 (1994). 

HBV-based vectors efficiently target quiescent hepatocytes and HBV-specific promoter and enhancer elements allow hepatocyte specific gene expression (Protzer et al. 1999). In addition, a very favorable ratio of infectious to defective particles renders HBV-based vectors good candidates for liver-directed gene transfer. Improved HBV vectors, in which HBV gene expression was abolished (Untergasser and Protzer 2004), were used in chimpanzees to treat chronic HCV infection and did not show any toxicity (Shin et al. 2005). 

Clearly the genes, yet to be identified, which control ABA synthesis will be of interest and should offer another class of stress-inducible promoter and enhancer elements. Some intense biochemistry and protein chemistry lie ahead for those who undertake this gene cloning exercise via the cDNA route. 

Hormone response elements resemble enhancer elements in that they are not strictly dependent on position and location. They generally are found within a few hundred nucleotides upstream (50 of the transcrip-... 

Libermann, T. A., Lenardo, M., and Baltimore, D. (1990). Involvement of a second lymphoid-specific enhancer element in the regulation of immunoglobulin heavy-chain gene expression. Mol. Cell. Biol. 10 3155-3162. 

Vorderstrasse, B., Cundiff, J., and Lawrence, B.P., Developmental exposure to the potent aryl hydrocarbon receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin impairs the cell-mediated immune response to infection with influenza A virus, but enhances elements of innate immunity, J. Immunotoxicol., 1, 103, 2004. 

Enhancer response elements (specific transcription factors)... 

The nomenclature for transcription factors is confusing. Depending on their mode of action, various terms are in use both for the proteins themselves and for the DNA sequences to which they bind. If a factor blocks transcription, it is referred to as a repressor otherwise, it is called an inducer. DNA sequences to which regulatory proteins bind are referred to as control elements. In prokaryotes, control elements that serve as binding sites for RNA polymerases are called promoters, whereas repressor-binding sequences are usually called operators. Control elements that bind activating factors are termed enhancers, while elements that bind inhibiting factors are known as silencers. 

In the nucleus, the hormone-receptor complex binds to nucleotide sequences known as hormone response elements (HREs). These are short palindromic DNA segments that usually promote transcription as enhancer elements (see p. 244). The illustration shows the HRE for glucocorticoids (GRE ... 

Once integrated into the host chromosome, the assembly of new viral particles necessitates the prodnction of viral RNA transcripts and proteins. Initiation of viral transcription is also an RNA independent process where host transcription promoters and enhancer elements such as NF-kB bind to the 5 -LTR. The host transcriptional complex is then recrnited and transcription commences.Once transcription has been initiated, RNA and RNA-RNA interactions play a critical role in mediating the production of viral transcripts. The multiprotein transcription complex has a recognition factor for nonhost DNA and quickly releases from viral DNA, creating short, abortive transcripts. Processing and nuclear export of these transcripts leads to the translation of the HIV Tat protein, a small early-phase viral protein (Figure 10.4) that plays a key role in the ultimate formation of fnll-length viral RNA transcripts. 

Omirulleh, S Abraham, M., Golovkin, M., Stefanov, 1., Karabaev, M.K., Mustardy, L., Morocz, S., and Dudits, D. (1993). Activity of a chimeric promoter with the doubled CaMV 35S enhancer element in protoplast-derived cells and transgenic plants in maize. Plant Mol. Biol. 21(3) 415-428. 

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