MED16 and MED23 of Mediator are coactivators of lipopolysaccharide- and heat-shock-induced transcriptional activators

Abstract

Transcriptional activators interact with diverse proteins and recruit transcriptional machinery to the activated promoter. Recruitment of the Mediator complex by transcriptional activators is usually the key step in transcriptional activation. However, it is unclear how Mediator recognizes different types of activator proteins. To systematically identify the subunits responsible for the signal-and activator-specific functions of Mediator in Drosophila melanogaster, each Mediator subunit was depleted by RNA interference, and its effect on transcriptional activation of endogenous as well as synthetic promoters was examined. The depletion of some Mediator gene products caused general transcriptional defects, whereas depletion of others caused defects specifically related to activation. In particular, MED16 and MED23 were required for lipopolysaccharide-and heat-shock-specific gene expression, respectively, and their activator-specific functions appeared to result from interaction with specific activators. The corequirement of MED16 for other forms of differentiation-inducing factor-induced transcription was confirmed by microarray analysis of differentiation-inducing factor (DIF)-and MED16-depleted cells individually. These results suggest that distinct Mediator subunits interact with specific activators to coordinate and transfer activator-specific signals to the transcriptional machinery. Environmental signals induce specific cellular responses. Cell surface receptors and their individual intracellular signaling pathways provide most of the specificity needed to evoke the appropriate cellular responses by activating particular transcription factors (1, 2). For example, heat-shock treatment induces the expression of molecular chaperons (3-5). In response to heat shock, trimerized heat-shock factor (HSF) proteins bind to heat-shock promoters and recruit the Mediator complex to drive the arrested RNA polymerase II into productive elongation. On the other hand, lipopolysaccharide (LPS) treatment of SL2 cells induces a cellular innate immune response mainly manifested by the expression of antimicrobial peptides (AMPs) (6, 7). Nuclear transport and binding of Drosophila NF-B homologues Relish, differentiationinducing factor, and dorsal to the AMP promoters in response to LPS induces transcriptional activation of the AMP genes (8, 9). Despite the difference in the environmental triggers and their signaling pathways, both types of activator require the Mediator complex to activate their target genes.Diverse transcriptional activation processes have been shown to require the Mediator complex. We have reported previously that the MED17 subunit of the Mediator complex is required for transcriptional activation of Drosomycin when Toll is activated (10). MED23 and MED1 have been shown to mediate transcriptional activation of E1A (11, 12) and ligand-bound nuclear receptors (13,14), respectively, in the mouse. In addition, a recent study revealed that MED15 is a critical mediator of transforming growth factor type ␤͞activin͞Nod...

The Arabidopsis Mediator subunit MED16 regulates iron homeostasis by associating with EIN3/EIL1 through subunit MED25

“…Subunits in the tail module directly mediate interactions with transcription factors regulating different processes (Kim et al, 2004;Yang et al, 2006;Casamassimi and Napoli, 2007). Increasing evidence has shown that subunits in other topological positions participate in development regulation in a complex and precise manner.…”

Mediator complex association with constitutively transcribed genes in yeast

“…Previous studies in yeast, Drosophila, and human cells have indicated direct interaction of Mediator subunits with enhancerbound activators, pointing to a general role of Mediator as a coactivator (32,33). If Mediator plays a functional role at RP genes and other constitutively transcribed genes in yeast, we would therefore expect its presence to depend on the activators for those genes.…”

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

“…Of the three Arabidopsis Med15 paralogs, AtMed15_1 is not represented on the chip and the other two did not show appreciable differential expression in reproductive stages with respect to leaf and root. Med16 is a coactivator of lipopolysaccharide-and heat shock-induced transcriptional activator in Drosophila (Kim et al, 2004) and is known as SFR6 in Arabidopsis. SFR6 is part of a complex network regulating the photoperiodic pathway and circadian clock gene expression in response to external and internal changes such as light, metabolic status, and temperature (Knight et al, 2008).…”