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Modern Genetic Analysis Chapter 11. Applications of Recombinant DNA Technology
Expressing Eukaryotic Genes in Bacteria

We learned in Chapter 10 that genes for any eukaryotic protein can be cloned in E. coli expression vectors. Organisms containing introduced foreign DNA are referred to as being transgenic. The introduced foreign gene is called a transgene. Hence bacteria containing eukaryotic genes are transgenic bacteria. Such transgenic bacterial cultures can be used as "factories" for the synthesis of valuable eukaryotic proteins that are otherwise difficult to obtain. Broadly speaking, the term biotechnology is the name given to this area of researchthe application of recombinant DNA techniques in commercial enterprises. Patented "designer organisms," both prokaryotic and eukaryotic, are already finding widespread use, and biotechnology is expected to become a dominant sector of the economy early in the twenty-first century.

When bacteria are used to produce a eukaryotic protein, it is desirable to design the system so as to produce as large an amount of the protein as possible. There are several such systems for overproducing foreign proteins in E. coli. One system uses phage T7 RNA polymerase operating on a T7 promoter. Late in infection, phage T7 synthesizes enormous amounts of gene products from several sites termed late promoters. Because host chromosomal genes are not synthesized at this point, these products are the only proteins synthesized. Figure 11-5 depicts the two components of this system. In the first component, the T7 RNA polymerase gene is arranged so that it is transcribed from the lac promoter, from which transcription is normally shut off by the lac repressor, a negative regulatory protein (see Chapter 14). However, on the addition of the lactose analog IPTG (isopropyl b -d-thiogalactoside), the repressor is inactivated and large amounts of T7 RNA polymerase are synthesized. In the second component of the system, the gene of choice [in this example, a human blood-clotting factor, factor VIII (defective in hemophiliacs)] is inserted (as a cDNA) adjacent to a late T7 promoter. The T7 RNA polymerase then transcribes this gene at high levels, resulting in high production of the factor VIII protein in the bacteria.

Now, many proteins such as human insulin (Figure 11-6), human growth hormone (Figure 11-7), and a wide range of pharmaceuticals are mass-produced from genetically engineered bacteria and fungi. Figure 11-8 shows an example in which the genetic engineering resulted in a drug that happened to give a strikingly different color to the bacterial colony when streaked onto a petri plate.

© 1999 by W. H. Freeman and Company. All rights reserved.