Enhancer elements

Defining transcriptional regulatory signals represents an essential step toward our understanding of eucaryotic gene expression. Analysis of genes transcribed by polymerase II has revealed a complex and interrelated set of “promoter elements.” Enhancers or activators are among the set of eucaryotic promoter elements that appear to increase transcriptional efficiency in a manner relatively independent of their position and orientation with respect to a nearby gene. The prototype enhancer, the 72 bp tandem repeat of SV40 DNA, was initially identified as a c&i-essential element located more than 100 nucleotides upstream from the cap site of the early viral genes (Benoist and Chambon, Nature 290, 304-310, 1981; Gruss et al., PNAS 78, 943-947, 1981). Deletion of this element reduces early gene expression of T antigen by a factor of at least 100 and concomitantly abolishes virus viability. Subsequently, it was found that the SV40 enhancer as well as analogs isolated from other animal viruses can function not only when linked to their natural genes but also in association with heterologous genes (Banerji et al., Cell 27, 299-308, 1981). This finding helped clarify the earlier observation by Capecchi (Cell 22, 479-488, 1980) that association of an SV40-origin-containing DNA fragment enhanced transformation by the herpes simplex thymidine kinase (HSV-tk) gene of tkcells. What was thought to be enhanced integration appears, in retrospect, to have been enhanced transcription mediated by the unsuspected SV40 activator element. In other experiments, several groups have demonstrated that the presence of viral enhancers augments expression of such diverse genes as rabbit @-globin, HSV-tk, conalbumin, chick lysozyme, mouse metallothionin, and the ~21 transforming protein of Harvey murine sarcoma virus. Resistance to enhancement of certain genes, such as human cr-globin, remains unexplained (Mellon et al., Cell 27,279-288, 1981; Humphries et al., Cell 30, 173-I 83, 1982). The remarkable ability of enhancer sequences to function upstream from, within, or downstream from eucaryotic genes distinguishes them from classic promoter elements, such as the polymerase binding site and the GoldbergHogness box (see figure), which are immediately 5’ to the gene. The equally surprising finding that activators can function in either orientation suggests that they may be able to exert regional transcriptional control for functions encoded on either DNA strand (Fromm and Berg, J. Mol. Appl. Gen. 7, 457-481, 1982; Moreau et al., Nucl. Acids Res. 9, 6047-6068, 1981). Preliminary results from several laboratories suggest that enhancers can act over considerable distances (perhaps >lO kb). More important than linear distance may be the sequences located between the enhancer element and the promoter adjacent to the gene being assayed. Some DNA sequences (perhaps strong promoters themselves) can interfere with the ability of activators to function across a region. Although more work is needed to support this conclusion, the implications of these findings for regional gene control and coordinate regulation are important. In addition to their role as novel genetic elements modulating eucaryotic gene expression, enhancers have proved useful for investigating genes with weak promoters. For example, detection of gene products under the control of the mouse mammary tumor virus promoter after acute transfection of most tissue culture cells is extremely difficult, even in the presence of glucocorticoids; however, addition of an enhancer sequence to this genetic unit increases RNA and protein synthesis to readily detectable levels (Lee et al., Nature 294, 228-232, 1981; Huang et al., Cell 27, 245-255, 1981). Thus enhancement permits evaluation of promoter mutations in the presence and absence of inducing agents. Activators have been found within a number of DNA viruses, including SV40, polyoma virus, BKV, bovine papilloma virus, adenoviruses, and perhaps herpes simplex virus. They are often present as short tandem repeats of 50 to 100 nucleotides. Since viruses with a single copy of the sequence remain viable, the purpose of a duplicated element is unclear. Enhancers have also been identified within the U-3 region of a number of retroviral LTRs, the portion of these genomes known to contain transcriptional regulatory sequences. Consistent with the ability of enhancers to augment the transcriptional activity of heterologous genes, LTRs have been shown to activate,adjacent cellular genes following the integration of proviral DNA 5’ or 3’ to the eucaryotic gene in question. Activation of cellular protooncogenes may lead to cell transformation or tumor production by a mechanism called promoter insertion (Temin, Cell 28, 3-5, 1982). Since this activation can stimulate endogenous genes that use their own promoters, the process might more appropriately be called “enhancer insertion.” Given the plethora of endogenous retroviruses, an abundant source of potential activators may exist within all eucaryotic cells. The presence of enhancer sequences on transposon-like retroviral elements offers a tempting explanation for a wide variety of regulatory events.