Genetic investigation of purine nucleotide imbalance in Saccharomyces cerevisiae.

Because metabolism is a complex balanced process involving multiple enzymes, understanding how organisms compensate for transient or permanent metabolic imbalance is a challenging task that can be more easily achieved in simpler unicellular organisms. The metabolic balance results not only from the combination of individual enzymatic properties, regulation of enzyme abundance, but also from the architecture of the metabolic network offering multiple interconversion alternatives. Although metabolic networks are generally highly resilient to perturbations, metabolic imbalance resulting from enzymatic defect and specific environmental conditions can be designed experimentally and studied. Starting with a double amd1 aah1 mutant that severely and conditionally affects yeast growth, we carefully characterized the metabolic shuffle associated with this defect. We established that the GTP decrease resulting in an adenylic/guanylic nucleotide imbalance was responsible for the growth defect. Identification of several gene dosage suppressors revealed that TAT1, encoding an amino acid transporter, is a robust suppressor of the amd1 aah1 growth defect. We show that TAT1 suppression occurs through replenishment of the GTP pool in a process requiring the histidine biosynthesis pathway. Importantly, we establish that a tat1 mutant exhibits synthetic sickness when combined with an amd1 mutant and that both components of this synthetic phenotype can be suppressed by specific gene dosage suppressors. Together our data point to a strong phenotypic connection between amino acid uptake and GTP synthesis, a connection that could open perspectives for future treatment of related human defects, previously reported as etiologically highly conserved.

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