Activating intrinsic CAZymes of the smut fungus Ustilago maydis for the degradation of plant cell wall components

33 The microbial conversion of plant biomass to valuable products in a consolidated bioprocess 34 could greatly increase the ecologic and economic impact of a biorefinery. Current strategies 35 for hydrolysis of plant material mostly rely on the external application of carbohydrate-active 36 enzymes (CAZymes). Alternatively, production organisms can be engineered to secrete 37 CAZymes to reduce the reliance on externally added enzymes. Plant-pathogenic fungi have a 38 vast repertoire of hydrolytic enzymes to sustain their lifestyle, but expression of the 39 corresponding genes is usually highly regulated and restricted to the pathogenic phase. Here, 40 we present a new strategy in using the biotrophic smut fungus Ustilago maydis for the 41 degradation of plant cell wall components by activating its intrinsic enzyme potential during 42 axenic growth. This fungal model organism is fully equipped with hydrolytic enzymes, and 43 moreover, it naturally produces value-added substances such as organic acids and 44 biosurfactants. To achieve the deregulated expression of hydrolytic enzymes during the 45 industrially relevant yeast-like growth in axenic culture, the native promoters of the respective 46 genes were replaced by constitutively active synthetic promoters. This led to an enhanced 47 conversion of xylan, cellobiose, and carboxymethyl cellulose to fermentable sugars. 48 Moreover, a combination of strains with activated endoglucanase and β-glucanase increased 49 the release of glucose from carboxymethyl cellulose and regenerated amorphous cellulose, 50 suggesting that mixed cultivations could be a means for degrading more complex substrates in 51 the future. In summary, this proof-of-principle demonstrates the potential applicability of 52 activating the expression of native CAZymes from phytopathogens in a biocatalytic process. 53 54 Importance 55 This study describes basic experiments that aim at the degradation of plant cell wall 56 components by the smut fungus Ustilago maydis. As a plant pathogen this fungus contains a 57 3 on S etem er 3, 2017 by gest ht://aem .sm .rg/ D ow nladed fom