ATP BINDING TO THE SIGMA(54)-DEPENDENT ACTIVATOR XYIR TRIGGERS A PROTEIN MULTIMERIZATION CYCLE CATALYZED BY UAS DNA

The events that take place at the prokaryotic enhancer of the Po promo ter of Pseudomonas putida prior to the engagement of the sigma(54)-RNA polymerase (sigma(54)-RNAP) have been studied in vitro. ATP hydrolysi s by XylR, the cognate regulator of the system, is preceded by the mul timerization of XylR at the enhancer, which is itself triggered by the sole allosteric effect of ATP binding to the protein. Since ADP is un able to support multimerization, ATP hydrolysis might be followed by a return to the nonmultimerized state. This notion is supported further by the properties of mutant proteins that seem to be frozen, in eithe r the nonmultimerized or the multimerized state, respectively. These r esults support a cyclic mechanism of ATP-dependent association/dissoci ation of XylR at the promoter UAS that precedes any involvement of the polymerase in transcription initiation.