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Fig. 2. Central role of the Hog1p MAPK in osmotic-stress-induced transcription complexes. The transcription complexes formed upon osmotic stress at a Hot1p- (a), Sko1p- (b) and Msn2/4p- (c) regulated gene promoter are shown. (a) Hot1p does not bind to the STL1 promoter in non-stressed conditions. In response to osmotic stress, Hot1p and Hog1p associate interdependently with the STL1 promoter, DNA-bound Hot1p anchoring Hog1p in position. Although Hog1p phosphorylates Hot1p, this is not required for Hot1p/Hog1p recruitment or transcription. Promoter-tethered Hog1p recruits the Rpd3p-Sin3p histone deacetylase complex, which is required for entry of the RNA polymerase II (pol II) holoenzyme complex and transcription. Hog1p also plays a more direct role in targeting the pol II holoenzyme by interacting with, and perhaps phosphorylating/activating, components of the complex. (b) In unstressed conditions, the Sko1p-Cyc8p-Tup1p repressor complex binds to the GRE2 promoter and inhibits transcription. Osmotic-stress-activated Hog1p enters the nucleus and is tethered to the GRE2 promoter by Sko1p. Phosphorylation of Sko1p by Hog1p converts the Sko1p-Cyc8p-Tup1p repressor into an activator, which recruits SAGA and SWI/SNF chromatin-modifying complexes. These, along with the Rpd3p-Sin3p histone deacetylase complex recruited by Hog1p, alter chromatin structure, thus allowing pol II holoenzyme binding and transcriptional activation. As in the case of the STL1 promoter, Hog1p directly interacts with and perhaps phosphorylates pol II holoenzyme components, aiding stable recruitment and perhaps activation of the general transcription machinery. (c) Osmotic stress leads to the nuclear translocation of Msn2/4p, which binds to the CTT1 promoter and recruits Hog1p. Hog1p then functions in transcription complex assembly as it does at the STL1 promoter, but with the additional direct recruitment of Hot1p.
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