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TRAP230/ARC240 and TRAP240/ARC250 Mediator subunits are functionally conserved through evolution

In Saccharomyces cerevisiae Mediator, a subgroup of proteins (Srb8, Srb9, Srb10, and Srb11) form a module, which is involved in negative regulation of transcription. Homologues of Srb10 and Srb11 are found in some mammalian Mediator preparations, whereas no clear homologues have been reported for Srb8 and Srb9. Here, we identify a TRAP240/ARC250 homologue in Schizosaccharomyces pombe and demonstrate that this protein, spTrap240, is stably associated with a larger form of Mediator, which also contains conserved homologues of Srb8, Srb10, and Srb11. We find that spTrap240 and Sch. pombe Srb8 (spSrb8) regulate the same distinct subset of genes and have indistinguishable phenotypic characteristics. Importantly, Mediator containing the spSrb8/spTrap240/spSrb10/spSrb11 subunits is isolated only in free form, devoid of RNA polymerase II. In contrast, Mediator lacking this module associates with the polymerase. Our findings provide experimental evidence for recent suggestions that TRAP230/ARC240 and TRAP240/ARC250 may indeed be the Srb8 and Srb9 homologues of mammalian Mediator. Apparently Srb8/TRAP230/ARC240, Srb9/TRAP240/ARC250, Srb10, and Srb11 constitute a conserved Mediator submodule, which is involved in negative regulation of transcription in all eukaryotes.