The Role of Nuclear Cap Binding Protein Cbc1p of Yeast in mRNA Termination and Degradation

ABSTRACTThe cyc1-512mutation in Saccharomyces cerevisiaecauses a 90% reduction in the level of iso-1-cytochrome cbecause of the lack of a proper 3'-end-forming signal, resulting in low levels of eight aberrantly longcyc1-512mRNAs which differ in length at their 3' termini. cyc1-512can be suppressed by deletion of either of the nonessential genes CBC1and CBC2, which encode the CBP80 and CBP20 subunits of the nuclear cap binding complex, respectively, or by deletion of the nonessential gene UPF1, which encodes a major component of the mRNA surveillance complex. The upf1-? deletion suppressed the cyc1-512defect by diminishing degradation of the longer subset ofcyc1-512mRNAs, suggesting that downstream elements or structures occurred in the extended 3' region, similar to the downstream elements exposed by transcripts bearing premature nonsense mutations. On the other hand, suppression of cyc1-512defects by cbc1-? occurred by two different mechanisms. The levels of the shorter cyc1-512transcripts were enhanced in the cbc1-? mutants by promoting 3'-end formation at otherwise-weak sites, whereas the levels of the longercyc1-512transcripts, as well as of all mRNAs, were slightly enhanced by diminishing degradation. Furthermore,cbc1-? greatly suppressed the degradation of mRNAs and other phenotypes of a rat7-1strain which is defective in mRNA export. We suggest that Cbc1p defines a novel degradation pathway that acts on mRNAs partially retained in nuclei.