Drug-dependent mutants in yeast Saccharomyces cerevisiae.

Mutations in sup1 and sup2 genes may cause cycloheximide-dependent growth in yeast Saccharomyces cerevisiae. Two classes of such mutants are described in the paper: 1) high temperature sensitive mutants, which do not express their sensitivity to nonpermissive temperature in the presence of cycloheximide (conditionally dependent) and 2) mutants unable to grow in the absence of the drug (true dependent). Some of the mutants of both classes express dependence toward another antibiotic - trichodermine. The binding of H(3)-labelled cycloheximide studied by equilibrium dialysis has demonstrated that both 80S ribosomes and 60S subunits isolated from conditionally dependent mutant showed a higher affinity for the drug compared to that of a parent strain. The number of binding sites per ribosome or per 60S subunit in the cycloheximide dependent mutant remains unchanged.Circular dichroism spectra of a mutant ribosomes in the presence as well as in the absence of antibiotic revealed that sup1 and sup2 mutations alter conformation of the yeast cytoplasmic ribosomes. The binding of cycloheximide to mutant ribosomes induces a conformational shift, which presumably compensates for their functional defect.