The production of mature export-competent transcripts is under the surveillance of quality control steps where aberrant mRNP molecules resulting from inappropriate or inefficient processing and packaging reactions are subject to exosome-mediated degradation. Previously, we have shown that the heterologous expression of bacterial Rho factor in yeast interferes in normal mRNP biogenesis leading to the production of full-length yet aberrant transcripts that are degraded by the nuclear exosome with ensuing growth defect. Here, we took advantage of this new tool to investigate the molecular mechanisms by which an integrated system recognizes aberrancies at each step of mRNP biogenesis and targets the defective molecules for destruction. We show that the targeting and degradation of Rho-induced aberrant transcripts is associated with a large increase of Nrd1 recruitment to the transcription complex via its CID and RRM domains and a concomitant enrichment of exosome component Rrp6 association. The targeting and degradation of the aberrant transcripts is suppressed by the overproduction of Pcf11 or its isolated CID domain, through a competition with Nrd1 for recruitment by the transcription complex. Altogether, our results support a model in which a stimulation of Nrd1 co-transcriptional recruitment coordinates the recognition and removal of aberrant transcripts by promoting the attachment of the nuclear mRNA degradation machinery.

In eukaryotic cells, the nascent pre-mRNA molecule is coated sequentially with a large set of processing and binding proteins that mediate its transformation into an export-competent ribonucleoprotein particle (mRNP) that is ready for translation in the cytoplasm. We have implemented an original assay that monitors the dynamic interplay between transcription and mRNP biogenesis and that allows the screening for new factors linking mRNA synthesis to translation in Saccharomyces cerevisiae. The assay is based on the perturbation of gene expression induced by the bacterial Rho factor, an RNA-dependent helicase/translocase that acts as a competitor at one or several steps of mRNP biogenesis in yeast. We show that the expression of Rho in yeast leads to a dose-dependent growth defect that stems from its action on RNA polymerase II-mediated transcription. Rho expression induces the production of aberrant transcripts that are degraded by the nuclear exosome. A screen for dosage suppressors of the Rho-induced growth defect identified several genes that are involved in the different steps of mRNP biogenesis and export, as well as other genes with both known functions in transcription regulation and unknown functions. Our results provide evidence for an extensive cross talk between transcription, mRNP biogenesis, and export. They also uncover new factors that potentially are involved in these interconnected events.The expression of protein-encoding genes in eukaryotic cells is a multistep process in which the genetic message is transcribed from DNA into a pre-mRNA molecule that undergoes 5Ј-end capping, splicing, 3Ј-end cleavage, and polyadenylation before being assembled into a ribonucleoprotein particle (mRNP) and exported to the cytoplasm for translation. Genetic and biochemical studies, using the yeast Saccharomyces cerevisiae as a model organism in particular, suggest that the various mRNA processing and packaging events known as mRNP biogenesis are coupled physically and functionally to transcript elongation. The current view is that the nascent transcript emerging from the transcription elongation complex is coated sequentially with a variety of proteins that ensure its integrity and mediate its maturation and transport from the site of transcription to the nuclear pore for export (2,29,68). The cotranscriptional assembly of export-competent mRNPs seems to be facilitated by the C-terminal domain of the largest subunit of RNA polymerase II (RNAP II), which serves as a platform for the sequential recruitment of various factors. Among the general factors that are important for the production of export-competent mRNPs in yeast, the four-subunit THO complex (Hpr1p, Mft1p, Tho2p, and Thp1p) associates with RNAP II at early steps of transcription elongation and facilitates the loading onto the nascent transcript of two mRNP export factors, Yra1p and Sub2p (1,7,71). The hexameric complex formed by THO, Yra1p, and Sub2p, termed the transcription and export complex (TREX) (64), recruits the heterodimeric export receptor Mex6...