Characterization of the ptr6(+) gene in fission yeast: A possible involvement of a transcriptional coactivator TAF in nucleocytoplasmic transport of mRNA

Transport of mRNA from the nucleus to the cytoplasm is one of the important steps in gene expression in eukaryotic cells. To elucidate a mechanism of mRNA export, we identified a novel ptr [poly (A)(+) RNA transport] mutation , ptr6, which causes accumulation of mRNA in the nucleus and inhibition of growth at the nonpermissive temperature. The ptr6(+) gene was found to enco de an essential protein of 393 amino acids, which shares significant homolo gy in amino acid sequence with yTAFII67 of budding yeast Saccharomyces cere visiae and human hTAFII55, a subunit of the general transcription factor co mplex TFIID. A Ptr6p-GFP fusion protein is localized in the nucleus, sugges ting that Ptr6p functions there. Northern blot analysis using probes for 10 distinct mRNAs showed that the amount of tbp(+) mRNA encoding the TATA-bin ding protein is increased five- to sixfold, whereas amounts of others are r apidly; decreased at the nonpermissive temperature in Ptr6-1. ptr6 has no d efects in nuclear import of an NLS-GFP fusion protein. These results sugges t that Ptr6p required for mRNA transport is a Schizosaccharomyces pombe hom ologue of yTAFII67 and hTAFII55. This is the first report suggesting that a TAF is involved in the nucleocytoplasmic transport of mRNA in addition to the transcription of the protein-coding genes.