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Background  Extracellular pH is one of the several environmental factors affecting protein production by filamentous fungi. Regulatory mechanisms ensure that extracellular enzymes are produced under pH-conditions in which the enzymes are active. In filamentous fungi, the transcriptional regulation in different ambient pH has been studied especially in  Aspergilli , whereas the effects of pH in the industrial producer of hydrolytic enzymes,  Trichoderma reesei , have mainly been studied at the protein level. In this study, the pH-dependent expression of  T. reesei  genes was investigated by genome-wide transcriptional profiling and by analysing the effects of deletion of the gene encoding the transcriptional regulator  pac1 , the orthologue of  Aspergillus nidulans pacC  gene.    Results  Transcriptional analysis revealed the pH-responsive genes of  T. reesei,  and functional classification of the genes identified the activities most affected by changing pH. A large number of genes encoding especially transporters, signalling-related proteins, extracellular enzymes and proteins involved in different metabolism-related functions were found to be pH-responsive. Several cellulase- and hemicellulase-encoding genes were found among the pH-responsive genes. Especially, genes encoding hemicellulases with the similar type of activity were shown to include both genes up-regulated at low pH and genes up-regulated at high pH. However, relatively few of the cellulase- and hemicellulase-encoding genes showed direct PACI-mediated regulation, indicating the importance of other regulatory mechanisms affecting expression in different pH conditions. New information was gained on the effects of pH on the genes involved in ambient pH-signalling and on the known and candidate regulatory genes involved in regulation of cellulase and hemicellulase encoding genes. In addition, co-regulated genomic clusters responding to change of ambient pH were identified.    Conclusions  Ambient pH was shown to be an important determinant of  T. reesei  gene expression. The pH-responsive genes, including those affected by the regulator of ambient pH sensing, were identified, and novel information on the activity of genes encoding carbohydrate active enzymes at different pH was gained.    Electronic supplementary material  The online version of this article (doi:10.1186/s12934-015-0247-z) contains supplementary material, which is available to authorized users.

The effects of extracellular pH and of the transcriptional regulator PACI on the transcriptome of Trichoderma reesei