Partial independence of synthesis and membrane attachment of mitochondrial and cytoplasmic precursors of electron transfer complexes III and IV in adapting Bakers' yeast

Abstract

When anaerobically grown Saccharomyces cerevisiae are aerated under conditions which may deplete them of cytoplasmically translated, mitochondrial inner membrane enzyme precursors, they show no immediate decrease in in vivo mitochondrial translational activity compared with cells which have not been so depleted. Similarly, cells depleted of mitochondrially translanted precursors show no immediate decrease in their cytoplasmic translation of mitochondrial inner membrane proteins. These experiments suggest that the synthesis and nonspecific membrane attachment of mitochondrially and cytoplasmically translated inner membrane proteins are not stringently delimited by a prior depletion of inner membrane precursors elaborated by the “other” genetic system. It is thus possible to demonstrate a degree of uncoupling of the activities of the two genetic systems. The oxygen inductions of reduced CoQ cytochrome c reductase (complex III) and of cytochrome c oxidase (complex IV) activities in cells which have been sequentially exposed first to cycloheximide and then to chloramphenicol, or first to chloramphenicol and then to cycloheximide reflect the levels to which specifically integrated, mitochondrial and cytoplasmic precursors of these complexes can accumulate in the absence of concomitant translational activity by the “other” genetic system. These data again suggest the degree to which the translational activities of the two genetic systems can be uncoupled. A detailed study of the inductions of these two complexes in cells exposed first to chloramphenicol shows that the modes of induction of the two complexes are different. Complex III develops approximately 50% of its activity as an expression of a precursor (presumably mitochondrially translated) which is already present in the anaerobic cells, but which requires oxygen-induced cytoplasmic translation for its expression. The remainder of the induced complex III activity appears to require oxygen-induced mitochondrial translation for its expression. There was no analogous anaerobically present component evident during complex IV induction.