Physiological consequences of mitochondrial antibiotic-resistant mutations in Paramecium

Summary

A set of mitochondrial antibiotic-resistant mutants of Paramecium have been analyzed with respect to their growth-rates, cytochromic content and respiratory properties. The mutants could be arranged in a continuous series ranging from strains equivalent to wild-type to severely affected ones; affected strains display longer generation times, reduced amount of cytochrome oxidase and very high levels of cyanideinsensitive respiration. Perfect phenocopies of the mutants were obtained by treating wild-type cells with low concentrations of erythromycin suggesting that the mutations exert their pleiotrophic effect by perturbating mitochondrial protein synthesis in agreement with the idea that these mutations affect the mitochondrial ribosomes. In the mitochondria of some of the mutants, electrons can be channelled with equal efficiency into the “classical” cyanide-sensitive pathway and the alternate cyanide insensitive (and SHAM_sensitive) one, providing direct demonstration of the branching of these two respiratory pathways. In the absence of any added inhibitor, however, electrons tend to be channelled in the cyanide-sensitive pathway.

All the physiological data fit perfectly the genetic data concerning the “stability” of the various mutations in “mixed mitochondrial populations”, i.e., markers that were known to be strongly counter-selected with respect to wild-type in such populations correspond to severely affected strains, while markers that were known to be “stable” correspond to “healthy” strains. A more quantitative analysis of the data shows that that there is little or no “complementation” between wild-type and mutated mitochondria in mixed cells indicating a high extent of functional autonomy of mitochondria in Paramecium.