Nutrition of “Adenineless” Auxotrophs of Yeasts

Abstract

Nutritional studies on 29 "adenineless" yeast strains were carried out. In no case could guanine, xanthine, or their ribonucleosides or ribonucleotides be used to satisfy the purine requirement. The cultures fell into three nutritional groups: group A, absolute purine requirement satisfied by adenine or hypoxanthine; group B, stimulatory purine requirement (leaky) satisfied by adenine or hypoxanthine; group C, absolute purine requirement satisfied only by adenine. Although adenosine, adenylic acid (AMP), inosine, and inosinic acid (IMP) could not be used by the absolute mutants, the leaky mutants were stimulated by these compounds. The rapid growth on adenine observed with members of all three nutritional groups was delayed by guanine. No inhibition was observed with guanosine or guanylic acid (GMP). Guanine and its derivatives did not inhibit growth of wild-type Saccharomyces cerevisiae. The slow growth of the leaky mutants in unsupplemented medium was inhibited by guanine, guanosine, and GMP. The results were interpreted as follows. (i) Yeasts lack GMP-reductase. (ii) Group A and B yeasts are blocked before IMP; group C yeasts lack one of the enzymes leading from IMP to AMP. (iii) The absolute mutants cannot take ribonucleotides or ribonucleosides into the cell and cannot break down these compounds extracellularly to the free base; the leaky mutants can slowly use extracellular nucleosides and nucleotides. (iv) Guanine competes with adenine for entry into the cell. (v) Guanine inhibits de novo synthesis of AMP. The inhibition is probably a weak feedback effect which can only be demonstrated with leaky strains which synthesize purines at suboptimal rates.

Mutant of Yeast Sensitive to 2,6-Diaminopurine

“…This is not in accord with the results of Harris and MacWilliam (12), who found that 8-azaadenine and 8-azaguanine were metabolized by S. cerevisiae but had no effect on growth. Prototrophic strains of S. cerevisiae have not previously been shown to be inhibited by guanine, but Demain (9) found that guanine did inhibit the growth of leaky "purineless" mutants. He interpreted the failure of guanine to satisfy the purine requirement of "purineless" mutants as an absence of guanosine monophosphate reductase (GMP reductase; EC 1.6.6.a.).…”