Figure 1. A promoter fusion construct generated in vitro. The expression of the lacZ reporter gene in this fusion construct is controlled by the enhancer sequence located upstream. Promoter (Prom.) and polyadenylation (PolyA) sequences are required for proper expression of genes in eukaryotic cells. The line under the fusion gene construct denotes the product of transcription, messenger RNA (mRNA), which is then translated into the protein -galactosidase.

Figure 2. A cotransfection assay. Two different fusion constructs, an effector (top) and a lacZ reporter (bottom), are introduced into the same cell. The effector gene encodes an activator that can stimulate the expression of the reporter gene by binding to DNA sequences within the enhancer. This assay can be used not only to delineate the specific sequences in the enhancer, but also to further study the functions of the activator protein.

Figure 3. Enhancer trap. A reporter gene construct without an enhancer is used to integrate randomly into the genome of an organism. This gene will not be expressed unless it is inserted near an enhancer, thus ‘trapping’ that enhancer.

Figure 4. Promoter trap. In this design of promoter trap, the lacZ reporter gene contains neither an enhancer nor a promoter but has a splice acceptor site (SA) placed upstream. This reporter gene will not be expressed unless it is inserted within an intron (sequence between the exons) of an endogenous gene in the same transcription direction. Splicing is an RNA processing event that joins exon RNA sequences together by removing the intron sequences. The splice acceptor located upstream of lacZ ensures that its RNA appears in the spliced product (mRNA) for translation into protein. Enh., enhancer; Prom., promoter.