Björklund, Stefan - Functional Studies of Mediator in Plants

Research

Stefan Björklund standing in front of trees

Plants need to integrate an array of signals to regulate complex patterns of gene expression. This is important since plants have specialized needs to respond to changes in the environment. Regulation of gene expression in plants is important for integration of external signals such as temperature, day length, concentration of different metabolites and light quality in the series of events that ultimately lead to owering. Finally, the regulation of gene expression during specific stages of seed development is an interesting example of tissue and developmental control which is of considerable agricultural importance.

In all eukaryotes, protein-encoding genes are transcribed by RNA polymerase II (pol II). To perform its most basal functions; promoter recognition, melting of the DNA template at the transcription start, formation of the first phosphodiester bonds and promoter escape, pol II requires five so called general transcription factors (GTFs). These GTFs, TBP, TFIIB, E, F, and H are conserved in all eukaryotes and together with pol II they form a preinitiation complex comprising nearly 30 polypeptides. Mediator is a multisubunit complex which functions as a connector between the promoter-bound transcriptional regulators and pol II.

The figure illustrates the eukaryotic transcription initiation complex.A cartoon of a eukaryotic transcription initiation complex consisting of DNA, TBP, TFIIB, E, F and H, Mediator, Pol II and a specific transcription factor binding to an enhancer element.

Surprisingly, Mediator had not been identified in plants until we recently succeeded to purify Mediator from A. thaliana through conventional biochemical purification combined with reversed-phase LC-ESI-MS/MS. This was the first description of Mediator in a plant, and it was evident that it required a biochemical approach since most of the A. thaliana Mediator subunits show very low sequence homology to the corresponding proteins in yeast and metazoans. Plant Mediator is probably structurally conserved, but the amino acid sequences of individual subunits differ considerably when compared to other eukaryotes. Plants also contain a set of unique Mediator subunits, which are likely to be involved in regulation of plant-specific gene expression.

Link to Stefan Björklund's main research page


Key publications

  • Blomberg J, Aguilar X, Brännström K, Rautio L, Olofsson A, Wittung-Stafshede P, and Björklund S. (2012). Interactions between DNA, transcriptional regulator Dreb2a and the Med25 mediator subunit from Arabidopsis thaliana involve conformational changes Nucleic Acids Res. 40:5938-5950
  • Elfving, N., Davoine, C., Benlloch, R., Blomberg, J., Brännström, K., Müller, D., Nilsson, A., Ulfstedt, M., Ronne, H., Wingsle, G., Nilsson, O., and Björklund, S. (2011). The Arabidopsis thaliana Med25 mediator subunit integrates environmental cues to control plant development. Proc. Natl. Acad. Sci. USA. 108(20):8245-8250
  • Shaikhibrahim, Z., Rahaman, H., Wittung-Stafshede, P., and Björklund, S. (2009). Med8, Med18, and Med20 subunits of the Mediator head domain are interdependent upon each other for folding and complex formation. Proc. Natl. Acad. Sci. USA. 106:20728-20733
  • Bäckström, S., Elfving, N., Nilsson, R., Wingsle, G., and Björklund, S. (2007). Purification of a Plant Mediator from Arabidopsis thaliana identifies PFT1 as the Med25 subunit. Molecular Cell 5: 717-729.
  • Hallberg, M., Hu, G-Z., Balciunas, D., Sheikhibrahim, Z., Björklund, S., and Ronne, H. (2006). Functional and physical interactions of the Mediator subunit Med21/Srb7. Mol. Genet Genomics 276:197-210