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

Abstract

The cyc1-512 mutation in Saccharomyces cerevisiae causes a 90% reduction in the level of iso-1-cytochrome c because of the lack of a proper 3′-end-forming signal, resulting in low levels of eight aberrantly longcyc1-512 mRNAs which differ in length at their 3′ termini. cyc1-512 can be suppressed by deletion of either of the nonessential genes CBC1 and 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-Δ deletion suppressed the cyc1-512defect by diminishing degradation of the longer subset of cyc1-512 mRNAs, 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-Δ occurred by two different mechanisms. The levels of the shorter cyc1-512 transcripts were enhanced in the cbc1-Δ mutants by promoting 3′-end formation at otherwise-weak sites, whereas the levels of the longercyc1-512 transcripts, as well as of all mRNAs, were slightly enhanced by diminishing degradation. Furthermore,cbc1-Δ greatly suppressed the degradation of mRNAs and other phenotypes of a rat7-1 strain which is defective in mRNA export. We suggest that Cbc1p defines a novel degradation pathway that acts on mRNAs partially retained in nuclei.

ACKNOWLEDGMENTS

We thank Michael R. Culbertson (University of Wisconsin) for the UPF1 disrupter and other UPF1 plasmids; Charles N. Cole (Dartmouth Medical School) for the rat7-1 andRAT7⁺ strains; V. Raju (Department of Cardiology, University of Rochester) for advice on Northern blot analysis; Edward Pagani (Pfizer Inc., Groton, Conn.) for a generous gift of thiolutin; Jay R. Greenberg (Department of Biochemistry and Biophysics, University of Rochester), Ding-Fang Yun (Cadus Pharmaceutical Corp., Tarrytown, N.Y.), Alan Sachs (University of California, Berkeley), Roy Parker (University of Arizona), Scott Butler (University of Rochester), Stuart Peltz (University of Medicine and Dentistry of New Jersey), and Allan Jacobson (University of Massachusetts Medical School) for helpful discussions; and Satarupa Das for assistance in the preparation of the cbc2-Δ disruptant.

This investigation was supported by NIH research grant GM12702 (to F.S.) and by an FCAR Canadian Postdoctoral Fellowship (to P.C.).