A genetic analysis of resistance to nystatin in<i>Saccharomyces cerevisiae</i>

Abstract

1. A number of stable nystatin-resistant mutants of the yeast Saccharomyces cerevisiae have been isolated from platings of a sensitive wild-type strain on low concentrations of the antibiotic. 2. These mutants were found to be resistant to 10, 15 or 60 units of drug/ml. 3. Analysis of meiotic segregants from crosses of these mutants to wild-type indicate that resistance is determined by two types of genes; resistance genes and modifiers. 4. Functional analysis of the mutants demonstrated the existence of three recessive resistance genes, nys -l, nys -2 and nys -3 and that nys -1 and nys -2 were linked. 5. Genetic analysis showed that nys -1 was affected by two modifiers, M nys -1 and M nys -2, but that only M nys -2 affected nys -2 and nys -3. 6. The modifiers M nys -1 and M nys -2 are dominant. 7. An investigation of the effects of temperature and medium on resistance demonstrated marked interactions between genotype and environment for both the resistance genes and the modifiers. 8. Second-step mutants have been isolated by plating first-step mutants on higher concentrations of the drug. Some of these are resistant to 800 units/ml. 9. Some possible mechanisms of nystatin resistance are discussed.