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.