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Vanillin dehydrogenases are important enzymes involved in the conversion of lignin-derived aromatic aldehydes such as vanillin and p-hydroxybenzaldehyde into its acid form. In a previous study we identified the first fungal vanillin dehydrogenase (VdhA) from the filamentous fungus Aspergillus niger. In this study we heterologous produced VdhA in a bioreactor to obtain sufficient enzyme for a more details analysis of its biochemical properties. This demonstrated that VdhA has a high optimal temperature (50 °C), neutral optimum pH and a broad substrate specificity for aromatic aldehydes. Deletion of vdhA in A. niger resulted in reduced growth on several aromatic aldehydes, largely collating with the in vitro substrate specificity of the enzyme. In addition, we demonstrated that VdhA can convert multiple aromatic aldehydes present in depolymerized lignin, indicating that VdhA can be applied in industrial conversion of lignin fractions to either simplify the composition of the fraction or to reduce the aldehydes that can undergo undesired repolymerization reactions within the mixture.

Carbon catabolite repression enables fungi to utilize the most favourable carbon source in the environment, and is mediated by a key regulator, CreA, in most fungi. CreA-mediated regulation has mainly been studied at high monosaccharide concentrations, an uncommon situation in most natural biotopes. In nature, many fungi rely on plant biomass as their major carbon source by producing enzymes to degrade plant cell wall polysaccharides into metabolizable sugars. To determine the role of CreA when fungi grow in more natural conditions and in particular with respect to degradation and conversion of plant cell walls, we compared transcriptomes of a creA deletion and reference strain of the ascomycete Aspergillus niger during growth on sugar beet pulp and wheat bran. Transcriptomics, extracellular sugar concentrations and growth profiling of A. niger on a variety of carbon sources, revealed that also under conditions with low concentrations of free monosaccharides, CreA has a major effect on gene expression in a strong time and substrate composition dependent manner. In addition, we compared the CreA regulon from five fungi during their growth on crude plant biomass or cellulose. It showed that CreA commonly regulated genes related to carbon metabolism, sugar transport and plant cell wall degrading enzymes across different species. We therefore conclude that CreA has a crucial role for fungi also in adapting to low sugar concentrations as occurring in their natural biotopes, which is supported by the presence of CreA orthologs in nearly all fungi.