Analysis of Schizosaccharomyces pombe Mediator reveals a set of essential subunits conserved between yeast and metazoan cells

`With the identification of eight new polypeptides, we here complete the su bunit characterization of the Schizosaccharomyces pombe RNA polymerase II h oloenzyme. The complex contains homologs to all 10 essential gene products present in the Saccharomyces cerevisiae Mediator, but lacks clear homologs to any of the 10 S. cerevisiae components encoded by nonessential genes. S. pombe Mediator instead contains three unique components (Pmc2, -3, and -6) , which lack homologs in other cell types. Presently, pmc2(+) and pmc3(+) h ave been shown to be nonessential genes. The data suggest that S. pombe and S. cerevisiae share an essential protein module, which associates with non essential species-specific subunits. in support of this view, sequence anal ysis of the conserved yeast Mediator components Med4 and Med8 reveals seque nce homology to the metazoan Mediator components Trap36 and Arc32. Therefor e, 8 of 10 essential genes conserved between S. pombe and S. cerevisiae als o have a metazoan homolog, indicating that an evolutionary conserved Mediat or core is present in all eukaryotic cells. Our data suggest a closer funct ional relationship between yeast and metazoan Mediator than previously anti cipated.