Mediator Function and Regulation
This is a picture of the components of the transcription initiation machinery. The mediator complex is the Med/Srb Complex. It has been created by Young Lab.
Overview of the Function
Mediator helps regulate gene-specific transcription and enhances basal transcription. This occurs when Mediator interacts with transcriptional activator proteins and with RNA polymerase II. It interacts with RNA polymerase II to form a holoenzyme. Mediator acts as an interface between gene-specific regulatory proteins and the general transcription apparatus, aka the preinitiation complex. It integrates and transduces positive and negative regulatory information from enhancers and operators to promoters and general transcription factors. It can only work through RNA polymerase II and other proteins though, because Mediator cannot interact with DNA. Mammalian Mediator interacts with transcriptional activator proteins and plays an essential role in transcriptional regulation. (Myers, L. and Kornberg, R.D., 2000)
Basically activation domains interact with specific subunits in the Mediator complex. These subunits are specific to the species of organism. They may also be specific to the tissue that the cell is located in. The Mediator complex then targets RNA polymerase II and either recruits it to the DNA, or helps it either activate or repress transcription. There may be another function of Mediator, or another way that Mediator can help regulate transcription, but it hasn't been identified yet.
Mediator helps recruit RNA polymerase II to the DNA using transcriptional activators. Transcriptional activators and repressors bind to the DNA and Mediator using the tail and middle domains and thus recruiting the RNA polymerase II to the DNA. Mediator binds to RNA polymerase II using the head domain. (Dotson et al., 2000)
Modes of Regulation of Transcription
There are many ways that Mediator can regulate RNA polymerase II and thus regulate transcription. This includes binding to transcriptional activators and either enhancing or repressing transcription. Mediator can undergo a conformational change. It can also stimulate the phosphorylation of the CTD of RNA polymerase II and can stimulate the recruitment of individual proteins of the preinitiation complex.
There are many different transcriptional activators that Mediator interacts with in order to regulate transcription. Specific subunits in Mediator interact with certain transcriptional activators. The GAL11 module of the Saccharomyces cerevisiae yeast found in the Rgr1 subcomplex binds to activators. (Med9, also found in the Rgr1 subcomplex is thought to mediate repression of RNA polymerase II. The Srb4 subcomplex is thought to stimulate basal transcription. (Kang, J.G. et. al. 2001) Some transcriptional activators that Mediator interacts include the thyroid hormone receptor, GAL4, and CREB. In yeast Mediator, GAL4 binds to GAL11 in the tail domain when it undergoes a mutation and becomes GAL11p. It then recruits RNA polymerase II to the GAL4 gene for transcription. In SMCC, CRE binding protein (CREB) binds to cAMP response element (CRE) on the DNA. It then binds to CREB binding protein (CBP) and CBP binds to several subunits in Mediator. The same thing occurs with nuclear receptors such as the thyroid hormone receptor. The thyroid hormone receptor(TR) is mediated when TRAP(thyroid hormone receptor-associated protein) or SMCC binds to it. (Ito, M. and Roeder, R.G. 2001). This is a direct interaction with SMCC. Other nuclear receptors interact indirectly using CBP. Another thing that can regulate transcription is growth factors or stress signals using mitogen-activated protein kinase which interacts indirectly with SMCC using CBP. (Weaver, 2002)
Human Mediator, otherwise known as SMCC or the TRAP complex, can also interact with PC4 and TFIIH to either repress or enhance activator-dependent transcription. Repression of transcription is mediated only with PC4. SMCC enhances transcription by interacting with PC4, and limiting TFIIH. (Gu et al., 1999) Other transcriptional activators that interact with SMCC include p53 and V16 which interact directly with TRAP80. TRAP220 is a subunit that interacts with ligand-dependent nuclear receptors. (Ito et al., 1999)
It is thought that the conformational change that occurs when RNA polymerase II and the CTD are bound to Mediator may also have something to do with regulation. (Dotson et al., 2000) No one is sure why, but it may have to do with the fact that all the domains will be accessible to transcriptional activators. Another way that transcription is regulated is Mediator complex binds to RNA polymerase II and stimulates phosphorylation of the CTD found in Rbp1 by TFIIH. (Jiang et al., 1998) Another way that transcription is regulated is Srb4 helps the TATA-binding protein associate with the TATA box. (Kuras, L. and Struhl, K., 1999)
Works Cited
Dotson, M.R., Yuan, C.X., Roeder, R.G., Myers, L.C., Gustafsson, C.M., Jiang, Y.W., Li, Y., Kornberg, R.D. and Asturias, F.J. (2000). Structural organization of yeast and mammalian mediator complexes. Proc. Natl. Acad. Sci. 97, 14307-14310.
Gu, W., Malik, S., Ito, M., Yuan, C.X., Fondell, J.D., Zhang, X., Martinez, E., Qin, J. and Roeder, R.G. (1999). A novel SRB/MED-containing cofactor complex, SMCC, involved in transcription regulation. Mol. Cell. 4, 97-108.
Ito, M. and Roeder, R.G. (2001). The TRAP/SMCC/Mediator complex and thyroid hormone receptor function. Trends Endocrinol Metab. 3, 127-134.
Ito, M., Yuan, C.X., Malik, S., Gu, W., Fondell, J.D., Yamamura S., Fu, Z.Y., Zhang, X., Qin, J., and Roeder, R.G. (2001). Identity between TRAP and SMCC complexes indicates novel pathways for the function for nuclear receptors and diverse mammalian activators. Mol. Cell. 3, 361-370.
Jiang, Y.W., Veschambre, P., Erdjument-Bromage, H., Tempst, P., Conaway, J.W., Conaway, R.C., and Kornberg, R.D. (1998). Mammalian mediator of transcription regulation and its possible role as an end-point of signal transduction pathways. Proc. Natl. Acad. Sci. 95, 8538-8543.
Kang, J.S., Kim, S.H., Hwang, M.S., Han, S.J., Lee, Y.C. and Kim, Y-J. (2001). The structural and functional organization of the yeast mediator. J. Biol. Chem. 276, 42003-42010.
Kuras, L. and Struhl, K. (1999). Binding of TBP to promoters in vivo is stimulated by activators and requires Pol II holoenzyme. Nature. 399, 609-613.
Myers, L.C., and Kornberg, R.D. (2000). Mediator of transcriptional Regulation. Annu. Rev. Biochem. 69, 729-749.
Weaver, R.F. (2002). Molecular biology. New York: McGraw Hill.