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. The ubiquitous expression of Med27 mRNA suggests a universal requirement for Med27 in transcriptional initiation. Loss of Crsp34/Med27 decreases amacrine cell number, but increases the number of rod photoreceptor cells [4].

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: Dynamic regulation of pol II transcription by the mammalian Mediator complex. Malik S, Roeder RG; Trends Biochem Sci. 2005;30:256-263. PMID:15896744

3: Structure-system correlation identifies a gene regulatory Mediator submodule. Lariviere L, Seizl M, van Wageningen S, Rother S, van de Pasch L, Feldmann H, Strasser K, Hahn S, Holstege FC, Cramer P; Genes Dev. 2008;22:872-877. PMID:18381891

4: Differential roles of transcriptional mediator complex subunits Crsp34/Med27, Crsp150/Med14 and Trap100/Med24 during zebrafish retinal development. Durr K, Holzschuh J, Filippi A, Ettl AK, Ryu S, Shepherd IT, Driever W; Genetics. 2006;174:693-705. PMID:16582438