Expression of mitochondrial release factor in relation to respiratory competence in yeast.

Mitochondria have a single release factor that recognizes all stop codons in mRNAs. The yeast mitochondrial release factor, mRF1, is a protein of 43 kDa that emerges from its precursor by cleavage of a mitochondrial targeting sequence. mRF1 is localized exclusively in mitochondria, even when it is overproduced. A several-fold increase in mRF1 levels slightly inhibits the growth of wild-type cells on media containing a non-fermentable carbon source. A direct antisuppressor effect of overproduced mRF1 is observed, since the MRF1 gene on a multicopy plasmid causes Gly(-) phenotypes of the leaky mit(-) point mutations in mtDNA. We also examine steady-state mRF1 levels in a respiratory-deficient mrf1-780 mutant with inhibited mitochondrial translation. We show that both the mRF1 protein and the MRF1 transcript are elevated in mrf1-780 cells. A similar increase in mRF1 expression is observed in the rho(0) strain with no mitochondrial translation. This is indicative of retrograde signalling in the regulation of MRF1 expression. According to our hypothesis, inhibition of translation in the mrf1-780 strain is due to mitoribosome stalling at the stop codon and the observed elevated level of release factor is a secondary effect of respiratory deficiency.