Biogenesis of mitochondria 27

Summary The isolation and characterization of five new mutants affecting mitochondrial protein synthesis in S. cerevisiae is reported. Each mutation confers in vivo resistance to the macrolide antibiotic spiramycin which acts by inhibiting mitochondrial protein synthesis in sensitive yeast. The mutants are distinguishable on the basis of their in vivo cross resistance to other antibiotics, their biochemical properties and genetic behaviour. Genetic analysis indicates the mode of inheritance to be nuclear for one mutation and cytoplasmic for the other four. Recombination analysis performed on crosses between different cytoplasmic determinants, together with data from monofactorial crosses of each determinant with sensitive strains, demonstrates at least two and possibly three cytoplasmic genetic loci conferring spiramycin resistance.The protein synthesizing activities of mitochondria isolated from the mutant strains range in response to spiramycin and other antibiotics from strong resistance through partial resistance to complete sensitivity. Based on this data the mutants showing strong antibiotic resistance in vitro might be simply classified as mitochondrial ribosome mutants and mutants sensitive in vitro as mitochondrial membrane mutants; however mutants showing partial resistances are not so readily accommodated in either class. The diverse biochemical properties cannot be correlated with the different genetic loci described; indeed three mutations, each resulting in different biochemical behaviour appear to occur at the same locus. The results are interpreted as providing further evidence for an earlier proposal of mitochondrial membrane-ribosome interactions.