A kinetic analysis of spontaneous rho- mutations in yeast.

Spontaneous mutation to the petite state at the level of the individual cell was studied in a haploid strain of yeast by the technique of pedigree analysis. Results indicated that (1) the mutability of rho+ cells within a population in log phase is variable; (2) rho+ mitotic buds are, on the average, about 50% more mutable than the rho+ cells from which they arose; (3) the mutability of a rho+ cell tends to decrease as it produces consecutive buds: (4) the probability that a mother cell will become rho- at or immediately subsequent to cell division is, on the average, one third the probability that its bud will be rho-; (5) most, if not all spontaneous rho- mutant cells contain mitochondrial DNA as judged from suppressiveness measurements. The data indicate that the spontaneous production of a mutant cell is a multi-step process. Neither a replicative advantage of defective mitochondrial DNA nor the existence of a "master" mitochondrial genome provides a satisfactory explanation of the process. Either selective dispensation of defective mitochondria to the bud at cytokinesis or normal retention by the mother cell of factors influencing the amplification or rate of induction of defective mitochondrial DNA could be involved.