A key to many aspects of genetic engineering is the ability to obtain a sufficient level of expression of a gene of interest. The use of genetic engineering to produce proteins of commercial importance, such as erythropoietin, tissue plasminogen activator, and many others, is well established. However, the cost of producing such products could be decreased by the ability to express a gene that encodes the protein at a higher level. Gene therapy, which involves the introduction of a nucleic acid into cells of a patient to express the nucleic acid for some therapeutic purpose, also depends upon obtaining a sufficient level of expression to achieve the desired result. In other applications, delivery of genes encoding a toxin (e.g., diphtheria toxin, ricin, tk) can be used to kill cancer cells, and other genes can be specifically tailored to kill infectious organisms. Again, obtaining an optimized or sufficient level of expression is a key to success. Genetic vaccines, which express proteins that can induce and/or modulate an immune response, also require adequate levels of gene expression.
Therefore, a need exists for promoters and enhancers that can provide appropriate levels of gene expression (e.g., great, intermediate, or low gene expression levels), as needed for the particular application or purpose, in target cells of interest. The present invention fulfills this and other needs.