The role of respiration in canavanine tolerance – reviving the Goldilocks legend
The yeast Saccharomyces cerevisiae is a petite-positive species, i.e., it can survive without respiration. This unique feature makes S. cerevisiae an excellent model for testing the role of respiration in stress tolerance. We exposed the respiration-proficient wild type (WT) and respiration-deficient (petite) strains to heat shock, the toxic amino acid thialysine, and the antifungal fluconazole, yet we did not observe any difference in colony-forming ability between WT and petites. In contrast, colony formation was severely inhibited in petites when grown on plates containing canavanine, the toxic analog of arginine. Interestingly, the significant differences observed on plates between respiration-proficient and respiration-deficient strains were not observed in liquid media containing canavanine. Under these conditions, there was no differential effect on glucose consumption. We then hypothesized that, due to a low diffusion rate on agar media, ethanol accumulation in the colony synergizes with canavanine to cause toxicity, yet there was no synthetic delay when combining canavanine, glucose, and ethanol in plates. Alternatively, the slow diffusion rate into the colony may limit the availability of nutritional factors. While increasing glucose concentration did not improve colony formation of petite mutants in the presence of canavanine, increasing arginine did. Next, we found that petite mutants produce less arginine than WT cells, indicating that when exposed to sublethal doses of canavanine, the low diffusion rate of arginine building blocks combined with petites` inherent deficiency to produce arginine results in the inhibition of colony formation. We then examined colony formation of WT cells when grown on a non-fermentable carbon source. Surprisingly, no colonies were formed, and canavanine was fungistatic in liquid media. We conclude that both lack of respiration and complete dependence on respiration are synthetic with canavanine in colony-formation inhibition, but through different mechanisms.