Enhancer element

Enhancers are classically defined as DNA sequences which stimulate transcription of a linked gene from its authentic start site. This stimulation is observed with the enhancer placed upstream or downstream of the gene, at a distance of several thousand base pairs and in an orientation-independent manner (for a review see [80]). Enhancers were originally described in viruses but have since been identified in a number of cellular genes. The first cellular enhancer to be found was in the major intron of the mouse heavy chain locus [81-83] and subsequently in the mouse k [84,85] and human IgH loci [86]. 

The IgH enhancer is spread over some 400 base pairs and is located between the JH cluster and the n switch region such that canonical VH-D-JH joining or heavy 

Immunoglobulin enhancers differ from previously identified viral enhancers in their spectrum of activity, enhancers from SV40 or polyoma virus being active in a variety of cell types [87,115,161,162]. Transfection experiments have established that both the IgH [81,82], and the k enhancers [84] are preferentially active in plasmacytoma cells. Although weak activity has been described in fibroblasts, this weak activity manifests a strong dependence on distance [88,89], The fact that some deletions of the IgH enhancer increase its activity in non-lymphoid transfectants has been used to infer the existence of repressor molecules in non-lymphoid cells 

which bind to the El and E3 elements respectively. No factors have yet been described that bind in vitro to E2 or E4. Thus, despite the fact that elements El through E4 show homology to one another, NF-juEl and NF-pi,E3 show distinct binding specificity as shown by competition experiments [105,106], Analysis of the dimethyl sulphate interference patterns obtained for NF-/J.E1 shows that the binding site extends beyond the E consensus. Both NF-/J.E1 and NF-juE3 are found to have a widespread distribution amongst mammalian cell types. 

No enhancer has yet been found in the mouse A locus, although transfection experiments indicate that expression of a transfected, rearranged A, light chain gene in certain plasmacytoma lines manifests an enhancer requirement (unpublished observations). However, the mouse A locus possesses only two VA genes and, therefore, there may be no requirement to place an enhancer downstream of the 

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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. 

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. 

Fig. 1.30. Structure of a typical eucaryotic transcription start site. Enhancer elements and UAS elements (UAS upstream activating sequences) are binding sites for positive and negative regulatory DNA-binding proteins. The TATA box is the binding site for the TATA box binding protein (TBP) and serves to position the RNA polymerase holoenzyme on the promoter. For promoters that do not possess a TATA box, this function is fulfilled by an initiator region.

Transcription factor proto-oncogenes that are silent or expressed at low levels in the progenitor cell are activated when placed imder control of potent enhancer elements of genes, which are normally highly expressed. 

The intron/exon, 5 UTR, 3 UTR, and the poly-A tail all affect gene expression by modulating the turnover and intracellular trafficking of the transcript. The promoter, usually accompanied by an enhancer element, controls the number of transcripts that are synthesized. The enhancer, acting through the promoter, can dramatically increase gene expression. 

The enhancer element contains binding sites for activator proteins that promote transcription. These can be located adjacent... 

Comparison of cellular promoters to viral promoters shows that the cellular promoters are weaker than the viral promoters, as reflected in the level of gene expression. However, addition of an enhancer element can increase promoter strength as noted above with the combination of the CMV enhancer and the cellular chick beta-actin promoter. 

DNA bending can cause an enhancer element that is far from the promoter in the linear DNA molecule, to interact with the transcription-initiation complex. 

Enhancer elements and transcription factors 5 -ATGA(C/G)TCAT AP-1, cjun, GCN4 (yeast)... 

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