The nucleolus is involved in mRNA export from the nucleus in fission yeast

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Abstract
To elucidate the mechanism of mRNA export from the nucleus, we isolated five novel temperature-sensitive mutants (ptr7 to ptr11) that accumulate poly(A)+ RNA in the nuclei at the nonpermissive temperature in Schizosaccharomyces pombe. Of those, the ptr11 mutation was found in the top2+ gene encoding DNA topoisomerase II. In addition to the nuclear accumulation of poly(A)+ RNA, ptr11 exhibited the cut (cell untimely torn) phenotype at the nonpermissive temperature, like the previously isolated mutant, ptr4. In these two mutants, cytokinesis occurred without prior nuclear division, resulting in cleavage of the undivided nuclei by the septum. To investigate the relationship between mRNA export defects and the cut phenotype observed in ptr4 and ptr11, we analyzed 11 other mutants displaying the cut phenotype and found that all these tested mutants accumulate poly(A)+ mRNA in the aberrantly cleaved nuclei. Interestingly, nuclear accumulation of poly(A)+ mRNA was observed only in the anucleolate nuclei produced by aberrant cytokinesis. In addition, nuc1, the S. pombe mutant exhibiting a collapsed nucleolus, trapped poly(A)+ mRNA in the nucleolar region at the nonpermissive temperature. In ptr11 and nuc1, mRNA transcribed from the intron-containing TBP gene showed nuclear accumulation, but not transcripts from the intron-less TBP cDNA, suggesting that the export pathway differs between the spliced and unspliced TBP mRNAs. These findings support the notion that a subset of mRNAs in yeast is exported from the nucleus through transient association with the nucleolus.