Mediator is a large complex of up to 33 proteins that is conserved from plants through 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. Med13 is part of the ancillary kinase module, together with Med12, CDK8 and CycC, which in yeast is implicated in transcriptional repression, though most of this activity is likely attributable to the CDK8 kinase. The large Med12 and Med13 proteins are required for specific developmental processes in Drosophila, zebrafish, and Caenorhabditis elegans but their biochemical functions are not understood [4]. This domain is also identified as an RNaseH domain of the medPIWI PIWI/Argonaute module. medPIWI is the core domain found in the Med13 protein. The medPIWI module in Med13 is predicted to bind double-stranded nucleic acids, triggering the experimentally-observed conformational switch in the CDK8 subcomplex which regulates the Mediator complex [5].

1: TRAP230/ARC240 and TRAP240/ARC250 Mediator subunits are functionally conserved through evolution. Samuelsen CO, Baraznenok V, Khorosjutina O, Spahr H, Kieselbach T, Holmberg S, Gustafsson CM; Proc Natl Acad Sci U S A 2003;100:6422-6427. PMID:12738880

2: 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

3: 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

4: Pygopus activates Wingless target gene transcription through the mediator complex subunits Med12 and Med13. Carrera I, Janody F, Leeds N, Duveau F, Treisman JE; Proc Natl Acad Sci U S A. 2008;105:6644-6649. PMID:18451032

5: Two novel PIWI families: roles in inter-genomic conflicts in bacteria and Mediator-dependent modulation of transcription in eukaryotes. Burroughs AM, Iyer LM, Aravind L; Biol Direct. 2013;8:13. PMID:23758928