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Volume 271, Number 34, Issue of August 23, 1996 pp. 20258-20264
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Effects of Integration Host Factor and DNA Supercoiling on Transcription from the ilvP_G Promoter of Escherichia coli

(Received for publication, May 6, 1996, and in revised form, May 31, 1996)

Department of Microbiology and Molecular Genetics, College of Medicine, University of California, Irvine, California 92697

Integration host factor (IHF) activates transcription from the ilvP_G promoter by severely distorting the DNA helix in an upstream region of a supercoiled DNA template in a way that alters the structure of the DNA in the downstream promoter region and facilitates open complex formation. In this report, the in vivo and in vitro influence of DNA supercoiling on transcription from this promoter is examined. In the absence of IHF, promoter activity increases with increased DNA supercoiling. In the presence of IHF, the same increases in superhelical DNA densities result in larger increases in promoter activity until a maximal activation of 5-fold is obtained. However, the relative transcriptional activities of the promoter in the presence and absence of IHF at any given DNA superhelical density remains the same. Thus, IHF and increased DNA supercoiling activate transcription by different mechanisms. Also, IHF binds with equal affinities to its target site on linear and supercoiled DNA templates. Therefore, IHF binding does not activate transcription simply by increasing the local negative supercoiling of the DNA helix in the downstream promoter region or by differential binding to relaxed and supercoiled DNA templates.

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