Characterization of a Salmonella typhimurium Mutant Defective in Phosphoribosylpyrophosphate Synthetase

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Abstract
This study describes the isolation and characterization of a mutant (strain GP122) of S. typhimurium with a partial deficiency of phosphoribosylpyrophosphate (PRPP) synthetase activity. This strain was isolated in a purE deoD gpt purine auxotroph by a procedure designed to select guanosine-utilizing mutants. Strain GP122 had .apprx. 15% of the PRPP synthetase activity and 25% of the PRPP pool of its parent strain. The mutant exhibited many of the predicted consequences of a decreased PRPP pool and a defective PRPP synthetase enzyme; those included poor growth on purine bases, decreased accumulation of 5-aminoimidazole ribonucleotide (the substrate of the blocked purE reaction) under conditions of purine starvation, excretion of anthranilic acid when grown in medium lacking tryptophan, increased resistance to inhibition by 5-fluorouracil, derepressed levels of aspartate transcarbamylase and orotate phosphoribosyltransferase (enzymes involved in the pyrimidine de novo biosynthetic pathway), growth stimulation by PRPP-sparing compounds (e.g., guanosine, histidine), poor growth in low phosphate medium, and increased heat lability of the defective enzyme. This mutant strain also had increased levels of GMP reductase. This genetic lesion, designated prs, was mapped by conjugation and phage P22-mediated transduction at 35 U on the Salmonella linkage map.

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