PARTICIPATION OF THE PURINE REPRESSOR IN CONTROL OF THE CARBAMOYLPHOSPHATE SYNTHETASE OPERON IN SALMONELLA-TYPHIMURIUM

Previous work has shown that the carAB operon of Salmonella typhimuriu m is transcribed from tandem promoters, P1 and P2, that are negatively controlled by pyrimidines and arginine, respectively. The results rep orted here show that purines also negatively control expression of car AB and that this effect is absent in a purR::Tn10 derivative. Primer-e xtension experiments established that the purine effect is exerted at P1, thus redefining this promoter as sensitive to both purines and pyr imidines. The results of gel-retardation experiments as well as DNase I and premethylation footprintings indicate that the purine repressor interacts with a PUR box 85 bp upstream of P1. Modification of this PU R box by site-directed mutagenesis abolishes the repression by purines in a carA::lacZ fusion, confirming that this box functions in vivo in purine control of carAB expression.