The phage shock protein operon (pspABCE) of Es-cherichla coll is strongly induced by adverse environmental conditions. Expression is controlled principally at the transcriptlonal level, and transcription is directed by the sigma factor σ⁵⁴. PspB and PspC are required for high-level psp expression during osmotic shock, ethanol treatment and f1 infection, but heat-Induced expression is Independent of these proteins. We report here that the promoter region contains an upstream activation sequence (UAS) that is required for psp induction and has the enhancer-like ability to activate at a distance. A DNA-bindlng activity is detected in crude protein extracts that Is dependent on the UAS and Induced by heat shock. We further show that Integration host factor (IHF) binds in vitro to a site between the UAS and σ⁵⁴ recognition sequence. In bacteria lacking IHF, psp expression is substantially reduced In response to high temperature and ethanol. During osmotic shock In contrast, psp expression Is only weakly stimulated by IHF, and IHF mutants can strongly induce the operon. The dependence of psp expression on IHF varies with the inducing condition, but does not correlate with dependence on PspB and PspC, indicating distinct, agent-specific activation mechanisms.