One of two general pathways of mRNA decay in the yeast
Saccharomyces cerevisiaeoccurs by deadenylation followed by 3′-to-5′ degradation of the mRNA body. Previous results have shown that this degradation requires components of the exosome and the Ski2p, Ski3p, and Ski8p proteins, which were originally identified due to their superkiller phenotype. In this work, we demonstrate that deletion of the SKI7gene, which encodes a putative GTPase, also causes a defect in 3′-to-5′ degradation of mRNA. Deletion of SKI7, like deletion of SKI2, SKI3, or SKI8, does not affect various RNA-processing reactions of the exosome. In addition, we show that a mutation in the SKI4gene also causes a defect in 3′-to-5′ mRNA degradation. We show that the SKI4gene is identical to the CSL4gene, which encodes a core component of the exosome. Interestingly, the ski4-1allele contains a point mutation resulting in a mutation in the putative RNA binding domain of the Csl4p protein. This point mutation strongly affects mRNA degradation without affecting exosome function in rRNA or snRNA processing, 5′ externally transcribed spacer (ETS) degradation, or viability. In contrast, the csl4-1allele of the same gene affects rRNA processing but not 3′-to-5′ mRNA degradation. We identify csl4-1as resulting from a partial-loss-of-function mutation in the promoter of the CSL4gene. These data indicate that the distinct functions of the exosome can be separated genetically and suggest that the RNA binding domain of Csl4p may have a specific function in mRNA degradation.