Release of mRNPs from the transcription unit: a distinct step?
Because the pre-mRNA travels with Pol II significantly beyond the polyadenylation signal, there may be additional time for co-transcriptional processing of the nascent RNA before it is released from the TU. Several recent studies in yeast suggest that an mRNA processing surveillance mechanism operates at the TU prior to mRNP release. First, mutations in mRNA nuclear export factors lead to the retention of mRNAs at their sites of transcription, and these mRNAs become hyperadenylated (Jensen et al., 2001). Second, (pre)-mRNAs that are cleaved but not polyadenylated, owing to a mutation in PAP, also accumulate at TUs and can be released upon inactivation of components of the nuclear exosome (Hilleren et al., 2001), which are thought to function mainly as 3' to 5' exonucleases (Mitchell and Tollervey, 2001). Interestingly, retention of transcripts aberrantly processed at their 3' ends does not depend on Pol II, since retention also occurs when the transcript is synthesized by T7 RNA polymerase (Dower and Rosbash, 2002). These findings suggest that components of the nuclear exosome have novel functions in regulating the release mRNPs from the TU. A previous study implicated the exosome in monitoring pre-mRNA splicing (Bousquet-Antonelli et al., 2000).
How do mutations in export factors result in retention of transcripts at TUs? Several lines of evidence link mRNA transport with transcription. First, pre-mRNA splicing deposits a set of proteins called the exon-junction complex (EJC) on mRNA, and this complex promotes the nucleocytoplasmic transport of the mRNP (reviewed in Reed and Hurt, 2002). Second, even in the absence of splicing, two nuclear export factors in yeast, Yra1p and Sub2p, and their human counterparts, ALY and UAP56, are recruited to TUs through direct binding to the THO transcription elongation complex (Lei et al., 2001; Strasser et al., 2002). This evolutionarily conserved transcription/export complex (TREX) is detectable throughout the TU (Strasser et al., 2002), and Yra1p has been detected in downstream regions of the TU in a separate study (Lei et al., 2001). The co-transcriptional binding of these factors to nascent RNA raises the possibility of a feedback mechanism that is active at the TU. This link between RNA processing, mRNP release and nuclear export is reminiscent of previous studies in human cells, showing that transcripts that exhibit defective splicing or polyadenylation are retained at the TU (Custodio et al., 1999; Horowitz et al., 2002). It remains to be determined how transcripts are retained at TUs and whether mRNP retention in humans depends on components of the nuclear exosome.