STARVATION FOR A SPECIFIC AMINO-ACID INDUCES HIGH-FREQUENCIES OF RHO(-) MUTANTS IN SACCHAROMYCES-CEREVISIAE

Auxotrophic yeast cells were starved on solid media for their respecti ve essential amino acid in the course of ''adaptive mutation'' experim ents. Thereby, high proportions of mitochondrially respiratory deficie nt (rho(-)) mutants accumulated among the cells stressed on selective plates. Using a strain with a plus-four frameshift mutation in a chrom osomal gene involved in lysine biosynthesis, we observed that many of the revertant colonies which arose late under the selective pressure w ere composed of mixtures of rho(+) and rho(-) cells, indicating that t hey originated from founder cells containing intact as well as defecti ve mitochondrial genomes. We show that in spite of the slower growth o f rho(-) cells the late-appearing colonies cannot be interpreted as de scending From rho(-) revertants present before selective plating.