The Nrd1p-Nab3p-Sen1p complex constitutes a vital component of the nuclear mRNA surveillance in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the NNS (Nrd1p-Nab3p-Sen1p) complex along with the nuclear exosome, TRAMP, and CTEXT rapidly degrades aberrant mRNAs. Consistently, nrd1-1/nrd1-2 mutant cells displayed an impairment of the decay of all kinds of aberrant mRNAs, just like the yeast mutants deficient in Rrp41p, Rrp6p, and Rrp4p. Interestingly, however, nrd1{Delta}CID mutant strain (in which Nrd1p failed to interact with RNAPII) exhibits a selective abrogation of the degradation of aberrant messages generated during transcription elongation, splicing and 3'-end maturation, without affecting the decay rate of the export-defective mRNAs. Mutation in the 3'-end processing factor, Pcf11p, in contrast, displayed a selective abolition of the decay of the export-defective mRNAs without influencing the faulty messages spawned in the early phase of mRNP biogenesis. Thus, mRNA decay by Nrd1p is accompanied by its co-transcriptional recruitment onto the faulty messages, which initially relies on RNAPII during transcription elongation and on Pcf11p post-transcription. Furthermore, Nrd1p binding to the export-defective messages leads to the recruitment of Rrp6p to promote their decay.