Regulation of mitochondrial ribosomal RNA synthesis in yeast

Summary

The regulation of mitochondrial rRNA synthesis in yeast was examined by analyzing the effects of different modes of inhibition of cytoplasmic protein synthesis on incorporation of [³H]-uridine into mitochondrial rRNA species. The effects of tyrosine limitation and cycloheximide treatment, which result in a coordinate stringent response of cytoplasmic and mitochondrial rRNA synthesis in wild-type strain A364A, were compared to the responses elicited by three different temperature-sensitive mutants, all derived from A364A. These mutants are defective at the nonpermissive temperature at steps in cytoplasmic protein synthesis involving: 1) isoleucine-tRNA charging (ts-341) 2) polypeptide chain elongation (ts-275) and 3) initiation of protein synthesis (ts-187). Whole cell kinetic studies showed that uridine uptake, reflecting essentially cytoplasmic RNA synthesis, is affected to about the same extent in each of the mutants at the nonpermissive temperature. Using well documented published values for the fraction of cytoplasmic ribosomes present in polysomes following tyrosine limitation or cycloheximide treatment in strain A364A, and in the temperature-sensitive mutants following incubation at the nonpermissive temperature, we have found a very striking correlation between the fraction of ribosomes present as polysomes and the extent to which mitochondrial rRNA synthesis is inhibited. We find that the greater the fraction of cytoplasmic ribosomes “stalled” in polysomes, the greater is the inhibition of mitochondrial rRNA synthesis. No such correlation was evident in the synthesis of cytoplasmic rRNAs. Thus under certain conditions, regulation of mitochondrial and cytoplasmic rRNA synthesis can be uncoupled. The nature of possible signals regulating mitochondrial rRNA synthesis is discussed.