Gabriel et al. Biotechnol Biofuels (2021) 14:169 https://doi.org/10.1186/s13068-021-02018-5 RESEARCH CAZymes from the thermophilic fungus Thermoascus aurantiacus are induced by C5 and C6 sugars Raphael Gabriel 1,2,3 , Rebecca Mueller 1,2,3 , Lena Floerl 1,2,4,5 , Cynthia Hopson 1,2,6 , Simon Harth 1,2,7 , Timo Schuerg 1,2 , Andre Fleissner 3,8 and Steven W. Singer 1,2* Abstract Background: Filamentous fungi are excellent lignocellulose degraders, which they achieve through producing carbohydrate active enzymes (CAZymes). CAZyme production is highly orchestrated and gene expression analysis has greatly expanded understanding of this important biotechnological process. The thermophilic fungus Thermoascus aurantiacus secretes highly active thermostable enzymes that enable saccharifcations at higher temperatures; how- ever, the genome-wide measurements of gene expression in response to CAZyme induction are 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 hemicellu- lases, while cellobiose specifcally induced cellulases. A minimal medium formulation was developed to enable gene expression 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 diferent 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 identifed 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 paths to produce broad spectrum thermotolerant enzy- matic mixtures. Keywords: CAZy, Filamentous fungi, Thermoascus aurantiacus, Transcriptomics, Cellulase gene expression © The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativeco mmons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Introduction Carbohydrate active enzymes (CAZymes) are vital for the conversion of plant polysaccharides to biofuels and bio-based chemicals [1]. Cellulose and hemicellulose are the most abundant polysaccharides in plant biomass and thus are an immense untapped carbon pool for biotech- nological applications. Trough using CAZymes such as cellulases and xylanases, lignocellulosic plant biomass can be deconstructed into simple sugars that can be fur- ther converted into biofuels and bioproducts [2]. Te thermophilic fungus Termoascus aurantiacus is a notable host for thermostable CAZyme production [3]. Te enzymes of this fungus were found to be more heat stable and efective at deconstructing lignocellu- lose than enzymes from other thermophilic fungi and demonstrated the release of sugars from pre-treated bio- mass at comparable levels to the commercial enzymatic mixture CTec2 at 50 °C. Notably, the enzymatic mixture from T. aurantiacus lost only half of its activity during Open Access Biotechnology for Biofuels *Correspondence: swsinger@lbl.gov 1 Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, CA 9720, USA Full list of author information is available at the end of the article