It is becoming increasingly clear that the intrinsic and protein-induced topological properties of the DNA helix influence transcriptional efficiency. In this report we describe the properties of two upstream activating regions that influence transcription from the non-overlapping tandem promoters of the ilvGMEDA operon of Escherichia coli. One 20 base-pair region between the promoter sites contains an intrinsic DNA bend that activates transcription from the downstream promoter. The other region contains an integration host factor (IHF) binding site that overlaps the upstream promoter site. IHF binding at this site represses transcription from the upstream promoter and enhances transcription from the downstream promoter. IHF also induces a severe bend in the DNA at its target binding site in the upstream promoter region. The activating property of the 20 base-pair DNA sequence located between the promoters is dependent upon the helical phasing of the sequence-directed DNA bend that it encodes. However, the IHF-mediated activation of transcription is not dependent upon the helical phasing (spatial orientation) of the upstream IHF and downstream promoter sites. The IHF-mediated activation of transcription is also uninfluenced by the presence or absence of the intrinsic DNA bend between its binding site and the downstream promoter site. These results suggest the interesting possibility that IHF activates transcription from the nearby downstream promoter simply by bending the DNA helix in the absence of specific IHF-RNA polymerase or upstream DNA-RNA polymerase interactions.