INTEGRATION HOST FACTOR SUPPRESSES PROMISCUOUS ACTIVATION OF THE SIGMA(54)-DEPENDENT PROMOTER PU OF PSEUDOMONAS-PUTIDA

In the presence of m-xylene, the Pu promoter of the TOL plasmid of Pse udomonas putida is activated by the prokaryotic enhancer binding prote in XyIR. The intervening DNA segment between the upstream activating s equences (UASs) and those for RNA polymerase binding contains an integ ration host factor (IHF) attachment site that is required for full tra nscriptional activity. In the absence of IHF, the Pu promoter can be c ross-activated by other members of the sigma(54)-dependent family of r egulatory proteins. Such illegitimate activation does not require the binding of the heterologous regulators to DNA and it is suppressed by bent DNA structures, either static or protein induced, between the pro moter core elements (UAS and RNA polymerase recognition sequence). The role of IHF in some sigma(54) promoters is, therefore, not only a str uctural aid for assembling a correct promoter geometry but also that o f an active suppressor (restrictor) of promiscuous activation by heter ologous regulators for increased promoter specificity.