Yeast episome: oligomycin resistance associated with a small covalently closed non-mitochondrial circular DNA

We have isolated a single step spontaneous mutant of S. cerevisiae resistant simultaneously to oligomycin, venturicidin, chloramphenicol, cycloheximide and triethyltin. This multiple drug resistance results from the interaction of two genetic factors showing both chromosomal location and episomal characteristics. One factor (π) confers oligomycin resistance, the other (τ) confers the other resistances. π can be lost spontaneously while τ can be completely eliminated with ethidium bromide. All π⁺ strains, whether grande or petite, τ⁺ or τ^?, carry a covalently closed circular DNA while π^? strains are devoid of it. We hypothesise that this circular DNA may play an informational role in the biogenesis and/or function of membranes.