Co-mutagenic effects of propidium on petite induction by ethidium in Saccharomyces cerevisiae

Propidium, whose structure is closely related to ethidium bromide, induced a low level of petites in yeast, but only at high concentrations with long incubation time, and only in growth medium. When added to growing cells, propidium also caused a large increase in petite induction by ethidium even at submutagenic concentrations of ethidium. Incorporation of adenine into DNA was inhibited by propidium in mitochondria but not in nuclei. Propidium by itself had no effects on fragmentation of pre-existing DNA, but enhanced mitochondrial DNA degradation provoked by ethidium. The proportion of suppressive clones occurring among the petites from ethidium treatment was reduced by the presence of propidium. All of these results indicated that propidium treatment led to degradation of the mitochondrial DNA in petites induced by ethidium but not in native (intact) mitochondrial DNA, nor in spontaneous petite colonies. The results are discussed in terms of possible mechanisms of modulation of petite induction.