Genetic analysis of nystatin-resistant mutants ofSaccharomyces cerevisiae

Abstract

SUMMARYPrimary genes, designated asNYSA, NYSD, NYSE, NYSF(dominant) andnysB,nysC(recessive) are responsible for resistance to nystatin ranging from 30 to 80 u/ml. A dominant modifier gene increases the resistance conferred byNYSFfrom 80 to 140 u/ml and a recessive modifier gene enhances resistance due toNYSAby 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 theNYSAgene. Interaction resulting in additive levels of resistance is shown by geneNYSDin combination with each of the genesnysB,nysCandNYSE. A model of step-wise increases in resistance due to polygenic primary genes and nuclear and cytoplasmic modifying factors is proposed.