A Regulatory Gene ( use ) Affecting the Expression of pyrA and Certain Other Pyrimidine Genes

Theuse-1 mutation inSalmonella typhimurium confers a complex and pleiotrophic phenotype which is primarily characterized as a temperature-dependent sensitivity to uracil. This sensitivity can be reversed by arginine or citrulline, but not by ornithine, suggesting that theuse-1 mutation affects the synthesis or the activity (or both) of carbamoylphosphate synthetase or ornithine carbamoyltransferase (or both). Activity measurements showed thatuse-1 caused superrepression of both of these enzymes, especially when uracil was present in the medium. Dihydro-orotase and dihydro-orotate oxidase were also superrepressed, but aspartate carbamoyltransferase and orotate phosphoribosyltransferase were not. Lowered nucleotide triphosphate and guanosine tetra- and pentaphosphate pools inuse-1 strains indicated that the mutation affected synthesis or breakdown of all of these phosphorylated compounds, but the UTP pool increased by a larger relative factor inuse-1 strains in the presence of uracil. The uracil-sensitive phenotype of theuse-1 mutation is a complex response to several environmental factors: temperature, aerobiosis, carbon sources, and uracil concentration. Uracil sensitivity was eliminated by alteration of one or more of these factors. Uracil sensitivity was suppressed by several genetic alterations. These include introduction intouse-1 strains of a multi-copy ColE1 derivative which carries the structural gene(s) for carbamoylphosphate synthetase, episomes that carryuse , mutations includingargR andpyrH , and various unclassified intergenic suppressor mutations. These genetic changes increased significantly the expression of carbamoylphosphate synthetase or ornithine carbamoyltransferase (or both). The activity ofuse-1 is not known, but the facts that it altered expression of at least four unlinked genes (pyrA, pyrC, pyrD , andargI ) and that theEscherichia coli F′133 complemented it establish it as atrans -acting regulatory factor.