Rmd9p controls the processing/stability of mitochondrial mRNAs and its overexpression compensates for a partial deficiency of Oxa1p in Saccharomyces cerevisiae

Oxa1p is a key component of the general membrane insertion machinery of eukaryotic respiratory complex subunits encoded by the mitochondrial genome. In this study, we have generated a respiratory-deficient mutant, oxa1-E65G-F229S, that contains two substitutions in the predicted intermembrane space domain of Oxa1p. The respiratory deficiency due to this mutation is compensated for by overexpressing RMD9. We show that Rmd9p is an extrinsic membrane protein facing the matrix side of the mitochondrial inner membrane. Its deletion leads to a pleiotropic effect on respiratory complex biogenesis. The steady state level of all the mitochondrial mRNAs encoding respiratory complex subunits is strongly reduced in the Delta rmd9 mutant, and there is a slight decrease in the accumulation of two RNAs encoding components of the small subunit of the mitochondrial ribosome. Overexpressing RMD9 leads to an increase in the steady state level of mitochondrial RNAs, and we discuss how this increase could suppress the oxa1 mutations and compensate for the membrane insertion defect of the subunits encoded by these mRNAs.