Purification of a Plant Mediator from Arabidopsis thaliana Identifies PFT1 as the Med25 Subunit

Abstract

Mediator, a central coregulator of transcription, has been identified as a large protein complex in eukaryotes ranging from yeast to man. It is therefore remarkable that Mediator has not yet been identified within the plant kingdom. Here we identify Mediator in a plant, Arabidopsis thaliana. The plant Mediator subunits typically show very low homology to other species, but our biochemical purification identifies 21 conserved and six A. thaliana-specific Mediator subunits. Most notably, we identify the A. thaliana proteins STRUWWELPETER (SWP) and PHYTOCHROME AND FLOWERING TIME 1 (PFT1) as the Med14 and Med25 subunits, respectively. These findings show that specific plant Mediator subunits are linked to the regulation of specialized processes such as the control of cell proliferation and the regulation of flowering time in response to light quality. The identification of the plant Mediator will provide new tools and insights into the regulation of transcription in plants.

The Mediator Complex in Plants: Structure, Phylogeny, and Expression Profiling of Representative Genes in a Dicot (Arabidopsis) and a Monocot (Rice) during Reproduction and Abiotic Stress

“…2). These include more paralogs for six Meds (Med15, as well as orthologs for Med26, Med30, and the kinase module subunits not identified earlier in the biochemical screen (Bäckströ m et al, 2007). We have identified SENSITIVE TO FREEZING6 (SFR6) as Med16, a nucleus-localized protein with roles in cold acclimation (Knight et al, 2009) as well as photoperiodic and clock gene expression (Knight et al, 2008).…”

“…Med subunits have been biochemically identified in several fungi and metazoans but only for one plant, Arabidopsis (Bäckströ m et al, 2007). In this study, putative Meds have been identified in seven groups of the plant kingdom by Hidden Markov Model (HMM) methods.…”

“…We have identified SENSITIVE TO FREEZING6 (SFR6) as Med16, a nucleus-localized protein with roles in cold acclimation (Knight et al, 2009) as well as photoperiodic and clock gene expression (Knight et al, 2008). In addition, two of the Arabidopsis Med33 subunits (AT2G48110 and AT3G23590; Bäckströ m et al, 2007) are annotated as REDUCED EPIDERMAL FLUORESCENCE4 (REF4) and REF4-RELATED1, respectively (The Arabidopsis Information Resource [TAIR]). The dominant mutations in REF4 lead to reduced accumulation of phenylpropanoid end products and affect plant growth Stout et al, 2008).…”

“…Although the exact function of REF4 has not been ascertained, the authors argue against the function of REF4 as a TF, owing to putative membranespanning domains in the protein. However, REF4 is not a part of any membrane proteome (Stout et al, 2008;TAIR) and has been isolated as a component of the Med complex in a biochemical screen (Bäckströ m et al, 2007), making it a legitimate transcriptional regulator. On the other hand, the rice counterparts of Med5/33 have been annotated as "structural constituent of ribosome" in the Rice Genome Annotation Project (RGAP) database.…”

“…To bring uniformity, Bourbon and coworkers (2004) proposed a common unified nomenclature for different Med subunits across species. Biochemical purification of the first plant Med complex in Arabidopsis (Arabidopsis thaliana) identified 21 conserved and six Arabidopsis-specific Med subunits (Bäckströ m et al, 2007). The study identified STRUW-WELPETER (SWP) as Med14 and PHYTOCHROME AND FLOWERING TIME1 (PFT1) as Med25 subunits.…”