ATP binding to the sigma 54-dependent activator XylR triggers a protein multimerization cycle catalyzed by UAS DNA.

The events that take place at the prokaryotic enhancer of the Pu promoter of Pseudomonas putida prior to the engagement of the sigma 54-RNA polymerase (sigma 54-RNAP) have been studied in vitro. ATP hydrolysis by XylR, the cognate regulator of the system, is preceded by the multimerization of XylR at the enhancer, which is itself triggered by the sole allosteric effect of ATP binding to the protein. Since ADP is unable 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 either the nonmultimerized or the multimerized state, respectively. These results support a cyclic mechanism of ATP-dependent association/dissociation of XylR at the promoter UAS that precedes any involvement of the polymerase in transcription initiation.