Abstract
1. A number of stable nystatin-resistant mutants of the yeastSaccharomyces cerevisiaehave 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 andnys-3 and thatnys-1 andnys-2 were linked.5. Genetic analysis showed thatnys-1 was affected by two modifiers,Mnys-1 andMnys-2, but that onlyMnys-2 affectednys-2 andnys-3.6. The modifiersMnys-1 andMnys-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.
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