Transcription enhancer sequences

Nordheim, A., and Rich, A., 1983, Negatively supercoiled simian virus 40 DNA contains Z-DNA segments within transcriptional enhancer sequences. Nature 303 674-679. 

Regulatory regions are transcriptional control sequences, which consist of promoters, response elements, enhancers and possibly silencers, located upstream of the start site of transcription. The overall effect on gene transcription is a sum of the contributions of these elements and the activities of proteins recruited to these sites. Promoters are located immediately upstream of the start site and initiate transcription. They often contain tissue- or cell-specific elements if the gene is not ubiquitously expressed. Enhancers are positive regulatory elements which function independently of orientation and distance from the genes they regulate. 

Carotenoids have been found to exert numerous other effects of potential importance for the RPE. Carotenoids can activate transcription pathways (Ben-Dor et al., 2005 Kalariya et al., 2008 Palozza et al., 2006 Sharoni et al., 2004) for example, by activation of the antioxidant response element (ARE) (Ben-Dor et al., 2005 Sharoni et al., 2004). The ARE is an enhancer sequence responsible for the expression of many phase-II detoxification and antioxidant genes. Thus carotenoids may upregulate cellular antioxidant defenses. 

Positive regulators (enhancers) Turn on transcription when a specific effector protein binds to a specific enhancer sequence in the DNA. 

Enhancer-binding transcription factors bound to enhancer sequences at sites far from the gene... 

NtrC-P dimerizes and binds to the enhancer sequence, where it appears to catalyze an ATP-depen-dent isomerization of the closed to open forms of the transcription initiation complex (Eq. 28-1).153/154 The isomerization may depend upon looping.152 Other operons that utilize the oN subunit of RNAP often also have upstream or downstream enhancers.155156... 

The presence of an enhancer sequence may cause as much as 100- to 1000-fold increase in the rate of transcription as compared with the same transcriptional emit from which the enhancer has been deleted. A surprising fact is that enhancers as far as 1 -2 kbp upstream or even far downstream of the promoter and in either of the two possible orientations are effective. range conformational alterations in DNA. Alternatively, they might contain points of entry for RNA polymerase or for an initiation factor that could move along the DNA to the promoter region. However, the synthetic DNA molecule shown in Fig. 28-14 contains two copies of an enhancer in opposite orientations in one strand but none in the other strand.354 The... 

Figure 28-14 A "tailed circle" consisting of an enhancer linked to, but topologically separated from, a gene. One of the DNA strands of this plasmid bears two copies of an SV40 enhancer sequence, one copy inverted with respect to the other. This extra region protrudes from the circle and self-pairs to form a functional enhancer. The main body of the circle contains the p-globin gene, transcription of which is increased by the enhancer. Twisting of the enhancer has no effect on the winding of the strands on the main body of the circle nevertheless, the enhancer efficiently increases P-globin transcription. From Ptashne.355...

How can a DNA enhancer sequence located as many as several thousand base pairs from a gene transcription start site influence transcription even if its orientation is reversed ... 

In eukaryotic organisms, transcription regulation is a complex process that demands coordinated interaction of several genetic elements. The efficiency of this process mainly depends on the promoter/enhancer sequences, the copy number of the gene, and the structure and elements present at the insertion site in the host s chromatin. On the other hand, the co-transcriptional modifications (capping, splicing, polyadenylation, and transport to cytoplasm) on the primary transcript determine the stability, turnover rate, and translational capacity of the future mRNA. 

---