Summary: This study describes the isolation and characterization of a mutant (strain GP122) of with a partial deficiency of phosphoribosylpyrophosphate (PRPP) synthetase activity. This strain was isolated in a purine auxotroph by a procedure designed to select guanosine-utilizing mutants. Strain GP122 had roughly 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, including: (1) poor growth on purine bases; (2) decreased accumulation of 5-aminoimidazole ribonucleotide (the substrate of the blocked reaction) under conditions of purine starvation; (3) excretion of anthranilic acid when grown in medium lacking tryptophan; (4) increased resistance to inhibition by 5-fluorouracil; (5) derepressed levels of aspartate transcarbamylase and orotate phosphoribosyltransferase, enzymes involved in the pyrimidine biosynthetic pathway;(6) growth stimulation by PRPP-sparing compounds(e.g.guanosine,histidine); (7)poor growth in low phosphate medium; and (8) increased heat lability of the defective enzyme. This mutant strain also had increased levels of guanosine 5'-monophosphate reductase. This genetic lesion, designated , was mapped by conjugation and phage P22-mediated transduction at 35 units on the Salmonella linkage map.


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