Mediator is a large complex of up to 33 proteins that is conserved from plants to fungi to humans - the number and representation of individual subunits varying with species {1-2]. It is arranged into four different sections, a core, a head, a tail and a kinase-activity part, and the number of subunits within each of these is what varies with species. Overall, Mediator regulates the transcriptional activity of RNA polymerase II but it would appear that each of the four different sections has a slightly different function [3]. Mediator exists in two major forms in human cells: a smaller form that interacts strongly with pol II and activates transcription, and a large form that does not interact strongly with pol II and does not directly activate transcription. Notably, the 'small' and 'large' Mediator complexes differ in their subunit composition: the Med26 subunit preferentially associates with the small, active complex, whereas cdk8, cyclin C, Med12 and Med13 associate with the large Mediator complex [4]. This family includesthe C terminal region of a number of eukaryotic hypothetical proteins which are homologous to the Saccharomyces cerevisiae protein IWS1. IWS1 is known to be an Pol II transcription elongation factor and interacts with Spt6 and Spt5 [5,6].

1: A unified nomenclature for protein subunits of mediator complexes linking transcriptional regulators to RNA polymerase II. Bourbon HM, Aguilera A, Ansari AZ, Asturias FJ, Berk AJ, Bjorklund S, Blackwell TK, Borggrefe T, Carey M, Carlson M, Conaway JW, Conaway RC, Emmons SW, Fondell JD, Freedman LP, Fukasawa T, Gustafsson CM, Han M, He X, Herman PK, Hinnebusch AG, Holmberg S, Mol Cell. 2004;14:553-557. PMID:15175151

2: A set of consensus mammalian mediator subunits identified by multidimensional protein identification technology. Sato S, Tomomori-Sato C, Parmely TJ, Florens L, Zybailov B, Swanson SK, Banks CA, Jin J, Cai Y, Washburn MP, Conaway JW, Conaway RC; Mol Cell. 2004;14:685-691. PMID:15175163

3: Transcription mechanisms. Blackwell TK, Walker AK; WormBook. 2006;5:1-16. PMID:18050436

4: Cooperative activity of cdk8 and GCN5L within Mediator directs tandem phosphoacetylation of histone H3. Meyer KD, Donner AJ, Knuesel MT, York AG, Espinosa JM, Taatjes DJ; EMBO J. 2008;27:1447-1457. PMID:18418385

5: RNA polymerase II elongation factors of Saccharomyces cerevisiae: a targeted proteomics approach. Krogan NJ, Kim M, Ahn SH, Zhong G, Kobor MS, Cagney G, Emili A, Shilatifard A, Buratowski S, Greenblatt JF; Mol Cell Biol 2002;22:6979-6992. PMID:12242279

6: Dual roles for Spt5 in pre-mRNA processing and transcription elongation revealed by identification of Spt5-associated proteins. Lindstrom DL, Squazzo SL, Muster N, Burckin TA, Wachter KC, Emigh CA, McCleery JA, Yates JR 3rd, Hartzog GA; Mol Cell Biol 2003;23:1368-1378. PMID:12556496