A general suppressor of RNA polymerase I, II and III mutations in Saccharomyces cerevisiae.

A multicopy genomic library of Saccharomyces cerevisiae (strain FL100) was screened for its ability to suppress conditionally defective mutations altering the 31 kDa subunit (rpc31-236) or the 53 kDa subunit (rpc53-254/424) of RNA polymerase III. In addition to allele-specific suppressors, we identified seven suppressor clones that acted on both mutations and also suppressed several other conditional mutations defective in RNA polymerases I or II. All these clones harbored a complete copy of the SSD1 gene. The same pleiotropic suppression pattern was found with the dominant SSD1-v allele present in some laboratory strains of S. cerevisiae. SSD1-v was previously shown to suppress mutations defective in the SIT4 gene product (a predicted protein phosphatase subunit) or in the regulatory subunit of the cyclic AMP-dependent protein kinase. We propose that the SSD1 gene product modulates the activity (or the level) of the three nuclear RNA polymerases, possibly by altering their degree of phosphorylation.