Genetic analysis of nystatin-resistant mutants ofSaccharomyces cerevisiae

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Abstract
Primary genes, designated as NYSA, NYSD, NYSE, NYSF (dominant) and nysB, nysC (recessive) are responsible for resistance to nystatin ranging from 30 to 80 u/ml. A dominant modifier gene increases the resistance conferred by NYSF from 80 to 140 u/ml and a recessive modifier gene enhances resistance due to NYSA by 20 u/ml (from 40 to 60 u/ml). One nystatin-resistant mutant is apparently cytoplasmic and this ‘mutation’ suppresses specifically the action of modifying factors which increase the level of resistance conferred by the NYSA gene. Interaction resulting in additive levels of resistance is shown by gene NYSD in combination with each of the genes nysB, nysC and NYSE. A model of step-wise increases in resistance due to polygenic primary genes and nuclear and cytoplasmic modifying factors is proposed.