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Mediator is an essential transcriptional cofactor of RNA polymerase II (Pol II) in eukaryotes. This cofactor is a large complex containing up to 30 subunits and consisting of four modules: head, middle, tail, and CDK/Cyclin. Generally, Mediator connects transcriptional regulators, cofactors, chromatin regulators, and chromatin remodellers, with the pre-initiation complex to provide a platform for the assembly of these factors. Many previous studies have revealed that CDK8, a subunit of the CDK/Cyclin module, is one of the key subunits mediating the pivotal roles of Mediator in transcriptional regulation. In addition to CDK8, CDK11 is conserved among vertebrates as a Mediator subunit and closely resembles CDK8. While the role of CDK8 has been studied extensively, little is known of the role of CDK11 in Mediator. We purified human CDK11 (hCDK11)-containing protein complexes from an epitope-tagged hCDK11-expressing HeLa cell line and found that hCDK11 could independently form Mediator complexes devoid of human CDK8 (hCDK8). To investigate the in vivo transcriptional activity of the complex, we employed a luciferase assay. Although hCDK11 has nearly 80% amino acid sequence identity to hCDK8, siRNA-knockdown study revealed that hCDK8 and hCDK11 possess opposing functions in viral activator VP16-dependent transcriptional regulation.

To identify novel factors involved in nuclear mRNA export in Schizosaccharomyces pombe, we isolated and characterized the ptr8(+) gene, mutation of which causes nuclear accumulation of poly (A)(+) RNA. The ptr8(+) gene encodes an S. pombe homologue of human XPB, a component of TFIIH involved in nucleotide excision repair (NER) and transcription. A temperature-sensitive mutant of ptr8(+) (ptr8-1) was highly sensitive to UV irradiation, as are human XPB cells. Northern blot analysis demonstrated that the amount of total poly (A)(+) mRNAs does not decrease significantly at the nonpermissive temperature in ptr8-1 cells, whereas a pulse-labeling assay using (35)S-methionine showed that protein synthesis decreases rapidly after incubation of cells at the nonpermissive temperature, suggesting that ptr8-1 cells have a defect in nuclear mRNA export. In Saccharomyces cerevisiae, a mutation in the SSL2 gene encoding a homologue of Ptr8p also causes a block of mRNA export at the nonpermissive temperature. In addition, expression of human XPB in ptr8-1 cells rescued the ts phenotype and the mRNA export defects, suggesting that human XPB may also play a role in mRNA export. Furthermore, we revealed a functional interaction between Ptr8p and Tho2p, a component of the TREX complex involved in mRNA export. These results suggest that XPB/Ptr8p plays roles not only in NER and transcription, but also plays a conserved role in mRNA export.