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| Modern Genetic Analysis Chapter 11. Applications of Recombinant DNA Technology Expressing Eukaryotic Genes in Bacteria Figure 11-5. Two-step expression vector system based on bacteriophage T7 RNA polymerase and T7 late promoter. The chromosome of a specially engineered E. coli cell contains a copy of the T7 RNA polymerase gene under the transcrip-tional control of the lac promoter. When transcription from the lac promoter is induced by addition of IPTG, the T7 RNA polymerase gene is transcribed, and the mRNA is translated into the enzyme. The T7 RNA polymerase molecules produced then initiate transcription at a very high rate from the T7 late promoter on the expression vector. Multiple copies of the expression vector are present in such cells, although only one copy is diagrammed here. The large quantity of mRNA tran-scribed from the cDNA cloned next to the T7 late promoter is translated into abundant protein product. (Modified from H. Lodish, D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell, Molecular Cell Biology, 3d ed. Copyright © 1995 by Scientific American Books, Inc.)

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