Splicing is required for rapid and efficient mRNA export in metazoans

Pre-mRNA splicing is among the last known nuclear events before export of mature mRNA to the cytoplasm, At present; it is not known whether splicing and mRNA export are biochemically coupled processes. In this study, we have injected pre-mRNAs containing a single intron or the same mRNAs lacking an intron (Delta i-mRNAs) into Xenopus oocyte nuclei. We find that the spliced mRNAs are exported much more rapidly and efficiently than the identical Delta i-mRNAs. Moreover, competition studies using excess Delta i-mRNA indicate that different factor(s) are involved in the inefficient export df Delta i-mRNA vs. the efficient export of spliced mRNA. Consistent with this conclusion, spliced mRNA and Delta i-mRNA, though identical in sequence, are assembled into different messenger ribonucleoprotein particles (mRNP) in vitro. Strikingly, the mRNA in the spliced mRNP, but not in the Delta i-mRNP, is exported rapidly and efficiently. We conclude that splicing generates a specific nucleoprotein complex that targets mRNA for export. Our results, revealing a link between splicing and efficient mRNA export, may explain the reports that an intron is required for efficient expression of many protein-coding genes in metazoans.