Trichoderma reesei XYR1 activates cellulase gene expression via interaction with the Mediator subunit TrGAL11 to recruit RNA polymerase II.

The ascomyceteTrichoderma reeseiis a highly prolific cellulase producer. While XYR1 (Xylanase regulator 1) has been firmly established to be the master activator of cellulase gene expression inT.reesei, its precise transcriptional activation mechanism remains poorly understood. In the present study, TrGAL11, a component of the Mediator tail module, was identified as a putative interacting partner of XYR1. Deletion ofTrgal11markedly impaired the induced expression of most (hemi)cellulase genes, but not that of the major beta-glucosidase encoding genes. This differential involvement of TrGAL11 in the full induction of cellulase genes was reflected by the RNA polymerase II (Pol II) recruitment on their core promoters, indicating that TrGAL11 was required for the efficient transcriptional initiation of the majority of cellulase genes. In addition, we found that TrGAL11 recruitment to cellulase gene promoters largely occurred in an XYR1-dependent manner. Althoughxyr1expression was significantly tuned down without TrGAL11, the binding of XYR1 to cellulase gene promoters did not entail TrGAL11. These results indicate that TrGAL11 represents a directin vivotarget of XYR1 and may play a critical role in contributing to Mediator and the following RNA Pol II recruitment to ensure the induced cellulase gene expression. Author summary As a model cellulolytic fungus,T.reeseiis capable of rapidly producing a large quantity of (hemi)cellulases when appropriate substrates are present. This outstanding characteristic has madeT.reeseia prominent producer of cellulase in industry and also a model organism for studying eukaryotic gene expression. The expression of these hydrolytic enzymes encoding genes inT.reeseiis precisely regulated at a transcriptional level and controlled by a suite of transcription factors. Among others, the transcription activator XYR1 has been firmly established to be absolutely necessary for activating the expression of almost all cellulase genes. However, the precise mechanism it acts remains largely unknown. In eukaryotes, the multisubunit Mediator complex has been shown to be critical for expression of most, if not all, protein-coding genes by conveying regulatory information to the basal transcription machinery. Here, we find that XYR1 interacts with the Mediator tail module subunit, TrGAL11, which contributes to cellobiohydrolase (cbh) and endoglucanase (eg)genes but not beta-glucosidase (bgl) genes expression. Thus, the induced XYR1 binding to cellulase gene promoters led to TrGAL11 and RNA Pol II recruitment to these promoters. These results show that TrGAL11 represents a directin vivotarget of XYR1 and provide evidence for not only the evolutionarily conserved function of Mediator, but also for the existence of some subtle difference in its action to mediate gene expression in different eukaryotes.