A GENERAL SUPPRESSOR OF RNA POLYMERASE-I, POLYMERASE-II AND POLYMERASE-III MUTATIONS IN SACCHAROMYCES-CEREVISIAE

A multicopy genomic library of Saccharomyces cerevisiae (strain FL100) was screened for its ability to suppress conditionally defective muta tions altering the 31 kDa subunit (rpc31-236) or the 53 kDa subunit (r pc53-254/424) of RNA polymerase III. In addition to allele-specific su ppressors, we identified seven suppressor clones that acted on both mu tations and also suppressed several other conditional mutations defect ive in RNA polymerases I or II. All these clones harbored a complete c opy of the SSD1 gene. The same pleiotropic suppression pattern was fou nd with the dominant SSD1-v allele present in some laboratory strains of S. cerevisiae. SSD1-v was previously shown to suppress mutations de fective in the SIT4 gene product (a predicted protein phosphatase subu nit) 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 alteri ng their degree of phosphorylation.