EFFECTS OF INTEGRATION HOST FACTOR AND DNA SUPERCOILING ON TRANSCRIPTION FROM THE ILVP(G) PROMOTER OF ESCHERICHIA-COLI

Integration host factor (IHF) activates transcription from the ilvP(G) promoter by severely distorting the DNA helix in an upstream region o f a supercoiled DNA template in a way that alters the structure of the DNA in the downstream promoter region and facilitates open complex fo rmation. In this report, the in vivo and in vitro influence of DNA sup ercoiling on transcription from this promoter is examined. In the abse nce of IHF, promoter activity increases with increased DNA supercoilin g. In the presence of IHF, the same increases in superhelical DNA dens ities result in larger increases in promoter activity until a maximal activation of 5-fold is obtained. However, the relative transcriptiona l activities of the promoter in the presence and absence of IHF at any given DNA superhelical density remains the same. Thus, IHF and increa sed DNA supercoiling activate transcription by different mechanisms. A lso, IHF binds with equal affinities to its target site on linear and supercoiled DNA templates. Therefore, IHF binding does not activate tr anscription simply by increasing the local negative supercoiling of th e DNA helix in the downstream promoter region or by differential bindi ng to relaxed and supercoiled DNA templates.