Modulation of petite induction by low concentrations of ethidium bromide.

When starved, exponential phase cells of Saccharomyces cerevisiae are exposed to lo-25 UM ethidium bromide in buffer, the proportion of respiration deficient mutants in the population exhibits a rapid increase, followed by a pronounced decrease. This "self-rescue" by the mutagen can be dissociated from and studied independently of its mutagenic action and is shown to exhibit different requirements. These and related observations have been used to formulate a consistent model for mutagenesis by ethidium bromide and its modulation under a variety of conditions.