The Important Function of Mediator Complex in Controlling the Developmental Transitions in Plants
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International Journal of Molecular Sciences Review The Important Function of Mediator Complex in Controlling the Developmental Transitions in Plants Lingjie Zhang and Changkui Guo * State Key Laboratory of Subtropical Silviculture, School of Agriculture and Food Sciences, Zhejiang Agriculture and Forestry University, Hangzhou 311300, China; [email protected] * Correspondence: [email protected] Received: 19 March 2020; Accepted: 11 April 2020; Published: 15 April 2020 Abstract: Developmental transitions in plants are tightly associated with changes in the transcriptional regulation of gene expression. One of the most important regulations is conferred by cofactors of RNA polymerase II including the mediator complex, a large complex with a modular organization. The mediator complex recruits transcription factors to bind to the specific sites of genes including protein-coding genes and non-coding RNA genes to promote or repress the transcription initiation and elongation using a protein-protein interaction module. Mediator complex subunits have been isolated and identified in plants and the function of most mediator subunits in whole life cycle plants have been revealed. Studies have shown that the Mediator complex is indispensable for the regulation of plant developmental transitions by recruiting age-, flowering-, or hormone-related transcription factors. Here, we first overviewed the Mediator subunits in plants, and then we summarized the specific Mediator subunits involved in developmental transitions, including vegetative phase change and floral transition. Finally, we proposed the future directions to further explore their roles in plants. The link between Mediator subunits and developmental transitions implies the necessity to explore targets of this complex as a potential application in developing high quality crop varieties. Keywords: mediator complex; vegetative phase change; floral transition; plants 1. Introduction As sessile organisms, plants are able to adapt to changes around them, including decrease of nutrient and water content, increase of salt content in soil, or changeable temperature and light intensity [1]. At the molecular level, when plants perceive dangerous signals from the environment, the expression of numerous immune-related genes is activated to respond to the harsh environment stresses. The alterations in the regulation of gene expression are controlled by lots of cofactors of RNA polymerase II (RNAP II or Pol II), including the Mediator complex. The regulation of Mediator complex mainly occurs at the transcriptional level, and it is also conserved in eukaryotes. It first recruits transcription factors through protein-protein interaction, and then those transcription factors bind to the specific sites of target genes. The other part of Pol II transcription complex works to act in the transcription process to activate or repress the target genes in plants [2,3]. During this process, the Mediator complex is considered as the most important linker between Pol II and transcription factors (Figure1A). The Mediator also functions in stabilizing the transcription initiation complex, DNA bending, transcription elongation, transcription termination, and chromatin-associated DNA remodeling [4]. Int. J. Mol. Sci. 2020, 21, 2733; doi:10.3390/ijms21082733 www.mdpi.com/journal/ijms Int. J. J. Mol. Mol. Sci. Sci. 20202020,, 2121,, x 2733 FOR PEER REVIEW 2 of 12 Figure 1. Working model and subunit composition of the Mediator complex. (A), Working model of theFigure Mediator 1. Working complex-dependent model and subunit transcriptional composition regulation. of the Mediator Transcription complex. factors A bind, Working to the promotermodel of ofthe genes Mediator and thencomplex-dependent deliver the signal transcriptional to RNA polymerase regulation. II by Transcription interacting with factors specific bind Mediator to the subunit.promoter TFs, of genes transcription and then factors; deliver GTFs, the generalsignal to transcription RNA polymerase factors. II (B by), The interacting illustrations with represent specific theMediator modular subunit. organization TFs, transcription of the Mediator factors; complex GTFs, general in Arabidopsis transcription based factors. on the B interaction, The illustrations map of Mediatorrepresent complexthe modular and theorganization genetic interaction. of the Mediator Mediator complex comprises in Arabidopsis four modules, based including on the Headinteraction (red), Middlemap of (yellow),Mediator Tailcomplex (blue), and and the CDK8 genetic (green), interaction. and four Mediator plant specific comprises subunits four (pink). modules, including Head (red), Middle (yellow), Tail (blue), and CDK8 (green), and four plant specific subunits (pink). The Mediator complex is enormous in size and complex in its composition. It was initially discoveredThe Mediator in yeast (complexSaccharomyces is enormous cerevisiae )in in size 1990 and [5], andcomplex 17 years in its later, composition. the complex It was was identified initially indiscovered the model in plant,yeast (ArabidopsisSaccharomyces thaliana cerevisiae[6].) Thereafter, in 1990 [5], theand Mediator 17 years later, complex the complex from other was eukaryotes identified wasin the analyzed model plant, by bioinformatical Arabidopsis thaliana methods, [6]. X-ray Thereafter, crystallography, the Mediator cryo-electron complex from microscopy other eukaryotes (cryo-EM), andwas cross-linkinganalyzed by bioinformatical mass spectrometry methods, (CXMS) X-ray and crystallography, so on. 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