Transcription, Splicing and mRNA Export Are Coupled Events

The machineries involved in gene expression are highly conserved from yeast to metazoans. However, a fundamental difference between these organisms is that most yeast genes lack introns whereas the converse is true in higher organisms. Recent studies of the TREX (Transcription Export) complex, which functions in mRNA export, unexpectedly revealed that this complex is recruited by the transcription machinery in yeast whereas the TREX complex appears to be recruited by the splicing machinery in mammals. Studies during the past year also revealed a possible conserved role for SR protein dephosphorylation in regulating the interaction between SR proteins and the mRNA export receptor TAP (Mex67 in yeast). There is also an interesting possibility that an SR protein-TREX complex interaction is a conserved part of the mRNA export machinery

Previous studies have led to the view that mRNAs are transported from the nucleus to the cytoplasm by machinery that is conserved from yeast to humans. Moreover, this machinery is coupled both physically and functionally to the pre-mRNA splicing machinery. During the past year, important new insights into the mechanisms behind this coupling have been made. In addition, recent advances have revealed mechanisms for the cotranscriptional loading of the export machinery on to mRNAs. Finally, a newly identified link between the nuclear exosome and the machineries required for transcription, 3’ end formation and mRNA export suggests that proper mRNP formation is co-transcriptionally monitored.