Yta10p (Afg3p) and Yta12p (Rcal1p), members of the conserved AAA family of ATPases, are subunits of the mitochondrial m-AAA protease, an inner membrane ATP-dependent metallopeptidase. Deletion of YTA10 or YTA12 impairs degradation of non-assembled inner membrane proteins and assembly of respiratory chain complexes. Mutations of the proteolytic sites in either YTA10 or YTA12 have been shown to inhibit proteolysis of membrane-integrated polypeptides but not the respiratory competence of the cells, suggesting additional activities of Yta10p and Yta12p. Here we demonstrate essential proteolytic functions of the m-AAA protease in the biogenesis of the respiratory chain. Cells harbouring proteolytically inactive forms of both Yta10p and Yta12p are respiratory deficient and exhibit a pleiotropic phenotype similar to Deltayta10 and Deltayta12 cells. They show deficiencies in expression of the intron-containing mitochondrial genes COX1 and COB. Splicing of COX1 and COB transcripts is impaired in mitochondria lacking m-AAA protease, whilst transcription and translation can proceed in the absence of Yta10p or Yta12p. The function of the m-AAA protease appears to be confined to introns encoding mRNA maturases. Our results reveal an overlapping substrate specificity of the subunits of the m-AAA protease and explain the impaired assembly of respiratory chain complexes by defects in expression of intron-containing genes in mitochondria lacking m-AAA protease.