Effects of Chloramphenicol and Cycloheximide on the Biosynthesis of Mitochondrial Ribosomes in Tetrahymena

Effects of two antibiotics, chloramphenicol and cycloheximide, on the synthesis of mitochondrial and cytoplasmic ribosomes in Tetrahymena were studied. The results presented here show that mitochondrial ribosome synthesis was not completely inhibited by chloramphenicol (0.5 mm), but was abolished by cycloheximide (1 mm). Acrylamide gel electrophoresis analysis revealed that mitochondrial ribosomal proteins in a range of molecular weights from 13,000 to 25,000 were found particularly susceptible to chloramphenicol inhibition. Cytoplasmic ribosomal proteins did not appear to be differentially inhibited by chloramphenicol. The inhibition of mitochondrial ribosome synthesis in the presence of cycloheximide did not result from the loss of mitochondrial protein synthesis capacity as evidenced by in vitro [14C]leucine incorporation capacity of mitochondria isolated from whole cells treated with cycloheximide. However, cycloheximide greatly reduces the capacity to incorporate [3H]uridine into mitochondrial ribosomes as RNaseresistant radioactivity. These results suggest that some mitochondrial ribosomal proteins are synthesized in mitochondria and others by cytoplasmic ribosomes. The synthesis of those proteins that are produced by the mitochondrial ribosomes appears to be under the control of cytoplasmic protein synthesis. However, mitochondrial ribosome assembly can take place in the absence of cytoplasmic protein synthesis.