SUPPRESSION OF A YEAST MITOCHONDRIAL RNA PROCESSING DEFECT BY NUCLEARMUTATIONS

The S. cerevisiae strain h56 is a temperature-sensitive mit(-) mutant containing a single nucleotide substitution in the region 5' to the re ading frame of the mitochondrial var1 gene. The mutation decreases the efficiency of processing of a precursor RNA such that little var1 mRN A is produced at the restrictive temperature, 36 degrees C. This commu nication 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 isolate d. One dominant suppressor strain (h56-S4) was characterized biochemic ally, and the mechanism of suppression shown to involve a restoration of precursor RNA processing at the restrictive temperature, with a con comitant increase in the synthesis of the var1 protein. It appears lik ely that the suppressing allele encodes a component of an RNA processi ng endoribonuclease active on var1 transcripts. A genomic library was constructed from the h56-S4 strain, and several plasmids showing suppr essed activity were isolated. A preliminary analysis of these plasmids is presented.