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Adenylate kinase (Adk1p) is a pivotal enzyme in both energetic and adenylic nucleotide metabolisms. In this paper, using a transcriptomic analysis, we show that the lack of Adk1p strongly induced expression of the PHO and ADE genes involved in phosphate utilization and AMP de novo biosynthesis respectively. Isolation and characterization of adk1 point mutants affecting PHO5 expression revealed that all these mutations also severely affected Adk1p catalytic activity, as well as PHO84 and ADE1 transcription. Furthermore, overexpression of distantly related enzymes such as human adenylate kinase or yeast UMP kinase was sufficient to restore regulation. These results demonstrate that adenylate kinase catalytic activity is critical for proper regulation of the PHO and ADE pathways. We also establish that adk1 deletion and purine limitation have similar effects on both adenylic nucleotide pool and PHO84 or ADE17 expression. Finally, we show that, in the adk1 mutant, upregulation of ADE1 depends on synthesis of the previously described effector(s) (S)AICAR ((N-succinyl)-5-aminoimidazol-4-carboxamide ribotide), while upregulation of PHO84 necessitates the Spl2p positive regulator. This work reveals that adenylic nucleotide availability is a key signal used by yeast to co-ordinate phosphate utilization and purine synthesis.

Guanylic nucleotide biosynthesis is a conserved and highly regulated process. Drugs reducing GMP synthesis affect the immunological response and mutations enabling guanylic-derivative recycling lead to severe mental retardation. While the effects of decreased GMP synthesis have been well documented, the consequences of GMP overproduction in eukaryotes are poorly understood. In this work, we selected and characterized several mutations making yeast hypoxanthine-guanine phosphoribosyltransferase insensitive to feedback inhibition by GMP. In these mutants, accumulation of guanylic nucleotides can be triggered by addition of extracellular guanine. We show that such an accumulation is highly toxic for yeast cells and results in arrest of proliferation and massive cell death. This growth defect could be partially suppressed by overexpression of Rfx1p, a transcriptional repressor of the DNA damage response pathway. Importantly, neither guanylic nucleotide toxicity nor its suppression by Rfx1p was associated with an alteration of forward mutation frequency.