COREGULATION BY BENT DNA - FUNCTIONAL SUBSTITUTIONS OF THE INTEGRATION HOST FACTOR SITE AT SIGMA(54)-DEPENDENT PROMOTER PU OF THE UPPER-TOLOPERON BY INTRINSICALLY CURVED SEQUENCES

The role of integration host factor (IHF) in the regulation of the sig ma(54)-dependent promoter Pu of the TOL plasmid of Pseudomonas putida has been examined. We have selected in vivo insertions of intrinsicall y curved DNA that restore the responsiveness of an IHF-binding site de letion variant of Pu to the cognate activator of the system, XylR. We found five Pu derivatives which had inserted a core sequence with 6 ph ased [A](6) tracts, flanked by different lengths of DNA at the locatio n of the former IHF site. They displayed 40-100% of the activity of Pu , were independent of IHF, and maintained the overall geometry of the wild-type promoter. The induction patterns of Pu, compared to those of hybrid promoters, were virtually indistinguishable. This supports the notion that, in native conditions, IHF co-regulates the system by pro viding a structural aid for promoter architecture and not by interacti ng directly with the RNA polymerase, as it has been suggested with oth er known IHF-dependent promoters.