Suppression of a yeast mitochondrial RNA processing defect by nuclear mutations

The Saccharomyces cerevisiae strain h56 is a temperature-sensitive mit(-) mutant containing a single nucleotide substitution in the region 5' to the reading frame of the mitochondrial var1 gene. The mutation decreases the efficiency of processing of a precursor RNA such that little var1 mRNA is produced at the restrictive temperature, 36 degrees C. This communication reports the isolation and characterization of several strains carrying nuclear mutations which suppress the temperature-sensitivity of h56. Both dominant and recessive suppressor mutations were isolated. One dominant suppressor strain (h56-S4) was characterized biochemically, and the mechanism of suppression shown to involve a restoration of precursor RNA processing at the restrictive temperature, with a concomitant increase in the synthesis of the var1 protein. It appears likely that the suppressing allele encodes a component of an RNA processing endoribonuclease active on var1 transcripts. A genomic library was constructed from the h56-S4 strain, and several plasmids showing suppressed activity were isolated. A preliminary analysis of these plasmids is presented.