THE NUCLEAR KLUYVEROMYCES-LACTIS MRF1 GENE ENCODES A MITOCHONDRIAL CLASS-I PEPTIDE-CHAIN RELEASE FACTOR THAT IS IMPORTANT FOR CELL VIABILITY

We report the isolation and characterization of the Kluyveromyces lact is MRF1 gene encoding mitochondrial peptide chain release factor mRF-1 . Over-expression of the KlMRF1 gene has a strong antisuppressive effe ct in a Saccharomyces cerevisiae mitochondrial nonsense suppressor str ain. Inactivation of KlMRF1 results in a dual phenotype: most cells di e after about 10-13 generations, while a small number of cells exceed this limit. We propose that the lethality is related to a loss of mito chondrial genome integrity. Surviving Klmrf1 cells are able to grow sl owly on the non-fermentable substrate glycerol, indicating the existen ce of a second mitochondrial release factor activity. Our previous com parative analysis of class I release factors is refined by the incorpo ration of KlmRF-1 and ten recently identified prokaryotic release fact or sequences.