Stefan Björklund - Functional Studies of Mediator in Plants

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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 flowering. 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.
stefan_bjorklund_portrait

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.

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.
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 homol- ogy 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.

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Key publications

Balciunas, D., Hallberg, M., Björklund, S., and Ronne, H. (2003) Functional interactions within yeast mediator and evi- dence of differential subunit modifications.J. Biol. Chem. 278:3831-3839.

Hallberg, M., Polozkov, G.V., Hu, G.Z., Beve, J., Gustafsson, C.M., Ronne, H., and Björklund, S. (2005) Site-specific Srb10- dependent phosphorylation of the yeast mediator subunit Med2 regulates gene expression from the 2-microm plas- mid. Proc. Natl. Acad. Sci. U. S. A. 101:3370-3375 (2004)

Björklund S., and Gustafsson C.M. The yeast mediator complex and its regulation. Trends. Biochem. Sci. 30:240- 244.

Hallberg, M., Hu, G-Z., Balciunas, D., Sheikhibrahim, Z., Björklund, S., and Ronne, H. (2006) Functional and physical inter- actions of the Mediator subunit Med21/Srb7. Mol. Genet Genomics 276:197-210

Bäckström, S., Elfving, N., Nilsson, R., Wingsle, G., and Björklund, S. (2007) Purification of a Plant Mediator from Arabi- dopsis thaliana identifies PFT1 as the Med25 subunit. Mo- lecular Cell 5: 717-729.

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May 2013

Wednesday, May 22, 2013
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