Coactivation in vitro of the sigma(54)-dependent promoter Pu of the TOL plasmid of Pseudomonas putida by HU and the mammalian HMG-1 protein

The mechanism by which the prokaryotic histone-like protein HU replaces the integration host factor (IHF) in the coactivation of the sigma(54)-dependent promoter Pu of Pseudomonas putida has been investigated. By using a preactivated form of the cognate activator protein XylR, we show that the functional replacement of IHF with HU previously suggested in vivo can be faithfully reproduced in vitro with purified components. Furthermore, the coactivation effect of IHF on Pu could be mimicked not only by HU but also by the mammalian nonhistone chromatin protein HMG-1 and could be bypassed by intrinsically curved DNA. These results suggest that either of two different mechanisms (generation of a site-specific static DNA bend or a general flexibilization of the promoter region) gives rise to the same structural effect of stimulating transcription from Pu through changes in promoter architecture.