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-active 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 [4]. The overall function of the full-length Med25 is efficiently to coordinate the transcriptional activation of RAR/RXR (retinoic acid receptor/retinoic X receptor) in higher eukaryotic cells. Human Med25 consists of several domains with different binding properties, the N-terminal, VWA domain, an SD1 - synapsin 1 - domain from residues 229-381, a PTOV(B) or ACID domain from 395-545, an SD2 domain from residues 564-645 and a C-terminal NR box-containing domain (646-650) from 646-747. This family is the combined PTOV and SD2 domains. the PTOV domain being the domain through which Med25 co-operates with the histone acetyltransferase CBP, but the function of the SD2 domain is unclear [3].

1: The activator-recruited cofactor/Mediator coactivator subunit ARC92 is a functionally important target of the VP16 transcriptional activator. Yang F, DeBeaumont R, Zhou S, Naar AM; Proc Natl Acad Sci U S A. 2004;101:2339-2344. PMID:14983011

2: Purification of a plant mediator from Arabidopsis thaliana identifies PFT1 as the Med25 subunit. Backstrom S, Elfving N, Nilsson R, Wingsle G, Bjorklund S; Mol Cell. 2007;26:717-729. PMID:17560376

3: MED25 is distinct from TRAP220/MED1 in cooperating with CBP for retinoid receptor activation. Lee HK, Park UH, Kim EJ, Um SJ; EMBO J. 2007;26:3545-3557. PMID:17641689

4: 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