The isoleucine and valine biosynthetic enzyme acetolactate synthase (Ilv2p) is an attractive antifungal drug target since the isoleucine and valine biosynthetic pathway is not present in mammals, Saccharomyces cerevisiae ilv2 mutants do not survive in vivo, Cryptococcus neoformans ilv2 mutants are avirulent, and both S. cerevisiae and C. neoformans ilv2 mutants die upon isoleucine and valine starvation. To further explore the potential of Ilv2p as an antifungal drug target, we disrupted Candida albicans ILV2, and demonstrated that C. albicans ilv2 mutants were significantly attenuated in virulence, and were also profoundly starvation-cidal, with a greater than 100-fold reduction in viability after only four hours of isoleucine and valine starvation. Since fungicidal starvation would be advantageous for drug design, we explored the basis of the starvation-cidal phenotype in both S. cerevisiae and C. albicans ilv2 mutants. Since the mutation of ILV1, required for the first step of isoleucine biosynthesis, did not suppress the ilv2 starvation-cidal defects in either species, the cidal phenotype was not due to alpha-ketobutyrate accumulation. We found that starvation for isoleucine alone was more deleterious in C. albicans than S. cerevisiae, and starvation for valine was more deleterious than for isoleucine in both species. Interestingly, while the TOR inhibitor rapamycin further reduced S. cerevisiae ilv2 starvation viability, it increased C. albicans ilv1 and ilv2 viability. Furthermore, the recovery from starvation was dependent on the carbon source present during recovery for S. cerevisiae ilv2 mutants, reminiscent of isoleucine and valine starvation inducing a viable but nonculturable-like state in this species, while C. albicans ilv1 and ilv2 viability was influenced by the carbon source present during starvation, supporting a role for glucose wasting in the C. albicans cidal phenotype.
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