Transcriptomic changes caused by mutation in xylanase regulator 1 (xyr1) in Trichoderma reesei
Trichoderma reesei is known for its ability to produce large amounts of extracellular proteins and is one of the most important industrially used filamentous fungus. Xylanase regulator 1 (XYR1) as the master regulator is responsible for the activation of cellulase and hemicellulase gene expression, normally under inducing conditions. It has been reported that strains with point mutations in certain areas of xyr1 bypass the carbon catabolite repression, allowing cellulase and hemicellulase production even in the presence of glucose. These mutations also change the profile of produced proteins, shifting it more towards xylanase production, and increase the overall protein production in inducing conditions. However, how these mutations alter the metabolism and other cellular processes to cause these changes remains unclear.  In the present study, our aim was to explore changes caused by a point mutation in xyr1 on transcriptomic and metabolic level to better understand the reasons behind the increased protein production in both repressing glucose and inducing lactose conditions. We observed that the xyr1 mutant strain built more biomass and produced more extracellular proteins during growth on lactose compared to the wild type xyr1 strain. Genes involved in oxidoreductive D-galactose catabolism pathway were upregulated in the xyr1 mutant strain, potentially contributing to the more efficient utilization of lactose. The shift from utilizing lactose to using glucose as carbon source seemed faster in the xyr1 mutant strain. Clustering and enrichment analysis showed over-representation of mitochondria-related Gene Ontology terms in clusters where gene expression was higher in the xyr1 mutant, indicating that mitochondria play a role in the altered metabolic state associated with the xyr1 mutation. Metabolomics revealed that free tyrosine was more abundant in the xyr1 mutant strain in all measured timepoints, whereas multiple fatty acids were less abundant in the mutant strain on glucose.  The results contribute to more in-depth knowledge on T. reesei physiology and aid in finding new targets for improved protein production. Overall design: Strains QM9414 wtXYR1 (cbh1 under synthetic promoter SES, wild type xyr1 under pdc1 promoter, control strain) and mutXYR1 (cbh1 under synthetic promoter SES, mutated (V821F xyr1 under pdc1 promoter) were grown in 250 mL Ambr bioreactors first on batch lactose for about 72 hours until most lactose was depleted and then on glucose feed for about 27 hours. Samples were collected during batch lactose phase at around 52 hours, then during glucose feed at 78 hours and 99 hours from the start of the cultivation. Four biological replicates were included per strain and sample.