Ethidium-bromide-induced loss and retention of cytoplasmic drug resistance factors in yeast

By using a strain of Saccharomyces cerevisiae harboring three cytoplasmic resistance factors to oligomycin, erythromycin and chloramphenicol, the effect of ethidium bromide (EB) on the loss and retention of the resistance factors was examined. Comparison was also made of the petite mutation with the loss of each resistance factor.Although resistance to each drug was specific and separately determined, EB induced the segregation of the erythromycin-resistance factor from the chloramphenicol-resistance factor with a very low frequency, and segregation of both from the oligomycin-resistance factor at a higher rate. There was a close correlation between the rate of the petite mutation and that of the loss of any resistance factro in the whole cell population treated with EB.We interpret these findings as indicating that the drug-resistance factors studied are linked together in the mitochondrial genome.