RNA polymerase III defects suppress a conditional-lethal poly(A) polymerase mutation in Saccharomyces cerevisiae

We isolated spontaneous extragenic suppressors of a temperature-sensitive, lethal poly(A) polymerase mutation (pap1-1) in Saccharomyces cerevisiae that restore growth at the restrictive temperature of 30 degrees. Three of five suppressors represent alleles of the PDS2 complementation group. The recessive pds2-1 mutation exerts dominant allele-specific suppression over pap1-1, suggesting a direct functional interaction. The suppressor restores to near normal the steady-state concentrations of various mRNAs and total poly(A) reduced by pap1-1 at 30 degrees. Transcriptional chase experiments detect no reduction in the decay rates of mRNAs in the suppressor strain, suggesting that the restoration of steady-state message levels results from increased stable mRNA synthesis. Molecular cloning shows PDS2 to be allelic to RET1, which encodes the second-largest subunit of RNA polymerase III. We observe alterations in both the length and the steady-state amounts of RNA polymerase III transcripts in pds2-1 strains. Previously identified ret1 alleles do not suppress pap1-1, indicating that the pds2 alleles we isolated represent a specific class of RET1 mutations that suppress pap1-1. Suppression of pap1-1 by mutations in an RNA polymerase III subunit suggests a number of potentially novel interactions between these enzymes.