Altered Purine Metabolism in Regulatory Mutants of Saccharomyces cerevisiae

Armitt & Woods (1970) and Lomax & Woods (1973) reported the isolation of a series of mutants of Saccharomyces cerevisiae that did not require purines for growth, but excreted substantial amounts of purines into the growth medium. Genetic studies have assigned these yeast strains to six unlinked genes (purl, pur2, pur3, pur4, pur.5, pzv6), of which one, pur6, had both recessive and dominant (PUR6) alleles. Purine excretion was prevented by another gene, su-pur, except in an allele of purl called purI(ps), in which only partial suppression took place. Mapping revealed that pur6 was allelic to ade4, which has been identified (Gross & Woods, 197 1) as the structural gene for amidophosphoribosyltransferase EC 2.4.2.14), the first enzyme of the pathway of purine biosynthesis de novo. Purl was allelic to adel2 (Lomax & Woods, 1971), the gene for adenylosuccinate synthetase (EC 6.3.4.4); this locus is in fact bifunctional, and in addition to the catalysis of the adenylosuccinate synthetase reaction it also regulates the rate of the de novo purine biosynthetic pathway (Dorfman, 1965, 1971; Lomax & Woods, 1971). There is also evidence that the pur5 locus is involved in the regulation of the synthesis or function of inosinate dehydrogenase (EC 1.2.1.14) (W. J. E. Gardner & R. W. Woods, unpublished observations). Because the pur mutants excrete purines into the medium at considerable rates but can depend entirely on purine biosynthesis de novo to supply purines for growth, it has been assumed that they have accelerated rates of purine biosynthesis de novo and hence that they are in some way altered in the regulation of this pathway. The excretion of purines, mostly hypoxanthine and inosine (Lomax & Woods, 1973), into the medium has therefore been thought to be a direct consequence of the accelerated purine biosynthesis de novo, and hence a way of disposing of unneeded inosinate. In order to test these hypotheses, and to elucidate other aspects of purine metabolism in these yeast strains, we have made quantitative studies of the metabolism of radioactive glycine, adenine, guanine and hypoxanthine.