Uncovering CAZyme Induction in the Thermophilic Fungus Thermoascus Aurantiacus

Abstract Background: Filamentous fungi are excellent lignocellulose degraders, which they achieve through producing carbohydrate active enzymes (CAZymes). CAZyme production is highly orchestrated and the application of –omics methods such as RNA-Seq has greatly expanded understanding of this important biotechnological process. The thermophilic fungus Thermoascus aurantiacus secretes high amounts of highly active thermostable enzymes that enable saccharifications at higher temperatures; however, the genome-wide response to CAZyme induction is not understood. Results: A fed-batch system with plant biomass-derived sugars D-xylose, L-arabinose and cellobiose established that these sugars induce CAZyme expression in T. aurantiacus. The C5 sugars induced both cellulases and hemicellulases, while cellobiose specifically induced cellulases. A minimal medium formulation was developed to enable RNA-seq studies of T. aurantiacus with these inducers. It was found that D-xylose and L-arabinose strongly induced a wide variety of CAZymes, auxiliary activity (AA) enzymes and carbohydrate esterases (CEs), while cellobiose facilitated lower expression of mostly cellulase genes. Furthermore, putative orthologues of different unfolded protein response genes were up-regulated during the C5 sugar feeding together with genes in the C5 sugar assimilation pathways. Conclusion: This work has identified two additional CAZyme inducers for T. aurantiacus, L-arabinose and cellobiose, along with D-xylose. A combination of biochemical assays and RNA-seq measurements established that C5 sugars induce a suite of cellulases and hemicellulases, providing a path to produce a broad spectrum thermotolerant enzymatic mixture for deconstruction of plant biomass.