Multiple Mediator Subunits Impact Metabolism

Dolan et al. examine how a complex that regulates gene expression alters the production of phenylpropanoids

By Whitney Dolan and Clint Chapple

Background: Plants produce a vast array of compounds known as phenylpropanoids from the amino acid phenylalanine. Phenylpropanoids are important to plant growth and fitness. Previously, we identified an Arabidopsis mutant named ref4-3 that has reduced levels of phenylpropanoids and stunted growth. The ref4-3 mutation occurs in the gene encoding the MED5 subunit of the Mediator complex, which is made up of approximately 28 different proteins and is an important regulator of gene expression. Mediator influences gene expression by interacting with other proteins, including transcription factors and RNA Polymerase II.

Question: Ultimately, we would like to understand how the ref4-3 mutation alters the function of Mediator and how that leads to a change in plant growth and phenylpropanoid accumulation. To begin, we asked, “What other proteins are required for the function of ref4-3?

Findings: We introduced random mutations into ref4-3 seeds and, in following generations, looked for plants that had restored growth and/or phenylpropanoid production. This would indicate that a protein required for the function of ref4-3 had been disrupted. When we sequenced the genomes of these mutants, we found that all of the mutations that suppressed the function of ref4-3 occurred in three other subunits of the Mediator complex. RNAseq analysis of the ref4-3 suppressors, the ref4-3 mutant, and wild-type plants showed that, although in the ref4-3 mutant approximately 30% of the genome was misregulated, in the ref4-3 suppressors, normal gene expression was largely restored.

Next steps: In the future, we hope to purify Mediator from these different Arabidopsis mutant lines so that we can determine how these mutations affect the composition of the complex and its ability to interact with other proteins.

Dolan, W.L., Dilkes, B.P., Stout, J.M., Bonawitz, N.D., and Chapple, C.

(2017). Mediator Complex Subunits MED2, MED5, MED16, and MED23 Genetically Interact in the Regulation of Phenylpropanoid Biosynthesis. Plant Cell