Genetic evidence for selective degradation of RNA polymerase subunits by the 20S proteasome in Saccharomyces cerevisiae.

scs32 was isolated as an extragenic suppressor of a temperature-sensitive (ts) mutation (rpo26-31) in the gene encoding Rpo26p, a subunit common to yeast nuclear RNA polymerases (RNAPs). rpo26-31 also confers inositol auxotrophy, inhibits the assembly of RNAPI and RNAPII and reduces the steady-state level of Rpo26p and the largest subunit of RNAPI (Rpo11p or A190p) and RNAPII (Rpo21p). rpo26-31p accumulated to wild-type levels in the scs32 strain; nevertheless, the amount of assembled RNAPII remained at a reduced level at high temperature. Hence, scs32 only partially suppressed the ts phenotype and was unable to suppress the Ino-phenotype of rpo26-31. SCS32 is identical to PUP3, which encodes a subunit of the yeast proteasome. scs32 was able to suppress the phenotype of other ts alleles of RPO26, all of which reduce the steady-state level of this subunit. However, scs32 was unable to suppress the ts phenotype of mutant alleles of RPO21, or result in accumulation of the unstable rpo21-4p. These observations suggest that the stability of non-functional or unassembled forms of Rpo26p and Rpo21p are regulated independently.