The role of nuclear cap binding protein Cbc1p of yeast in mRNA terminationand degradation

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-for ming signal, resulting in low levels of eight aberrantly long cyc1-512 mRNA s 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 t he CBP80 and CBP20 subunits of the nuclear cap binding complex, respectivel y, or by deletion of the nonessential gene UPF1, which encodes a major comp onent of the mRNA surveillance complex. The upf1-Delta deletion suppressed the cyc1-512 defect by diminishing degradation of the longer subset of cyc1 -512 mRNAs, suggesting that downstream elements or structures occurred in t he extended 3' region, similar to the downstream elements exposed by transc ripts bearing premature nonsense mutations. On the other hand, suppression of cyc1-512 defects by cbc1-Delta occurred by two different mechanisms, The levels of the shorter cyc1-512 transcripts were enhanced in the cbc1-Delta mutants by promoting 3'-end formation at otherwise-weak sites, whereas the levels of the longer cyc1-512 transcripts, as well as of all mRNAs, were s lightly enhanced by diminishing degradation. Furthermore, cbc1-Delta greatl y suppressed the degradation of mRNAs and other phenotypes of a rat7-1 stra in which is defective in mRNA export. We suggest that Cbc1p defines a novel degradation pathway that acts on mRNAs partially retained in nuclei.