Antagonism by propidium of petite induction by ethidium and ethidium azide in Saccharomyces cerevisiae.

Propidium, a phenanthridinium dye similar to ethidium, did not induce petite mutations in non-growing yeast cells in contrast to ethidium. Combined exposure to ethidium and an excess of propidium for periods up to 2 h resulted in the expected petite induction expressed after subsequent plating on growth medium. As incubation was continued with propidium, the numbers of petites declined on subsequent plating whether the drug had been added before, during, or after the mutagenic treatment by ethidium. Propidium decreased petite induction by the monoazide analog of ethidium when applied before but not after photolytic attachment of the drug.