Plant polysaccharides represent a virtually unlimited feedstock for the generation of biofuels and other commodities. However, the extraordinary recalcitrance of plant polysaccharides toward breakdown necessitates a continued search for enzymes that degrade these materials efficiently under defined conditions. Activity-based protein profiling provides a route for the functional discovery of such enzymes in complex mixtures and under industrially relevant conditions. Here, we show the detection and identification of β-xylosidases and endo -β-1,4-xylanases in the secretomes of Aspergillus niger , by the use of chemical probes inspired by the β-glucosidase inhibitor cyclophellitol. Furthermore, we demonstrate the use of these activity-based probes (ABPs) to assess enzyme–substrate specificities, thermal stabilities, and other biotechnologically relevant parameters. Our experiments highlight the utility of ABPs as promising tools for the discovery of relevant enzymes useful for biomass breakdown.

Identifying and characterizing the enzymes responsible for an observed activity within a complex eukaryotic catabolic system remains one of the most significant challenges in the study of biomassdegrading systems. The debranching of both complex hemicellulosic and pectinaceous polysaccharides requires the production of α-Larabinofuranosidases among a wide variety of coexpressed carbohydrate-active enzymes. To selectively detect and identify α-Larabinofuranosidases produced by fungi grown on complex biomass, potential covalent inhibitors and probes which mimic α-L-arabinofuranosides were sought. The conformational free energy landscapes of free α-L-arabinofuranose and several rationally designed covalent α-Larabinofuranosidase inhibitors were analyzed. A synthetic route to these inhibitors was subsequently developed based on a key Wittig− Still rearrangement. Through a combination of kinetic measurements, intact mass spectrometry, and structural experiments, the designed inhibitors were shown to efficiently label the catalytic nucleophiles of retaining GH51 and GH54 α-L-arabinofuranosidases. Activity-based probes elaborated from an inhibitor with an aziridine warhead were applied to the identification and characterization of α-L-arabinofuranosidases within the secretome of A. niger grown on arabinan. This method was extended to the detection and identification of α-L-arabinofuranosidases produced by eight biomass-degrading basidiomycete fungi grown on complex biomass. The broad applicability of the cyclophellitol-derived activity-based probes and inhibitors presented here make them a valuable new tool in the characterization of complex eukaryotic carbohydrate-degrading systems and in the high-throughput discovery of α-Larabinofuranosidases.

New cyclophellitol-derived activity-based probes enable the sensitive detection and identification of cellulases.

Objective With the aim to decipher the mechanisms involved in the transcriptional regulation of feruloyl esterase encoded by faeB, a genetic screen was performed to isolate A. niger mutants displaying inducer-independent expression from the faeB promoter. Result PfaeB-amdS and PfaeB-lux dual reporter strains were constructed and used to isolate trans-acting mutants in which the expression of both reporters was increased, based on the ability to grow on acetamide plates and higher luciferase activity, respectively. The genetic screen on the non-inducing carbon source D-fructose yielded in total 111 trans-acting mutants. The genome of one of the mutants was sequenced and revealed several SNPs, including a point mutation in the creA gene encoding a transcription factor known to be involved in carbon catabolite repression. Subsequently, all mutants were analyzed for defects in carbon catabolite repression by determining sensitivity towards allyl alcohol. All except four of the 111 mutants were sensitive to allyl alcohol, indicating that the vast majority of the mutants are defective in carbon catabolite repression. The creA gene of 32 allyl alcohol sensitive mutants was sequenced and 27 of them indeed contained a mutation in the creA gene. Targeted deletion of creA in the reporter strain confirmed that the loss of CreA results in constitutive expression from the faeB promoter. Conclusion Loss of function of CreA leads to low but inducer-independent expression from the faeB promoter in A. niger.

The AraR transcription factor of Aspergillus niger encodes a Zn(II) 2 Cys 6 transcription factor required for the induction of genes encoding arabinolytic enzymes. One of the target genes of AraR is abfA , encoding an arabinofuranosidase. The expression of abfA as well as other L-arabinose-induced genes in A. niger requires the presence of L-arabinose or its derivative L-arabitol as an inducer to activate AraR-dependant gene expression. In this study, mutants were isolated that express L-arabinose-induced genes independently of the presence of an inducer under derepressing conditions. To obtain these mutants, a reporter strain was constructed in a ΔcreA background containing the L-arabinose-responsive promoter ( PabfA ) fused to the acetamidase ( amdS ) gene. Spores of the ΔcreA PabfA-amdS reporter strain were UV-mutagenized and mutants were obtained by their ability to grow on acetamide without the presence of inducer. From a total of 164 mutants, 15 mutants were identified to contain transacting mutations resulting in high arabinofuranosidase activity in the medium after growth under non-inducing conditions. Sequencing of the araR gene of the 15 constitutive mutants revealed that 14 mutants carried a mutation in AraR. Some mutations were found more than once and in total nine different point mutations were identified in AraR. The AraR N806I point mutation was reintroduced into a parental strain and confirmed that this point mutation leads to inducer-independent expression of AraR target genes. The inducer independent of L-arabinose-induced genes in the AraR N806I mutant was found to be sensitive to carbon catabolite repression, indicating that the CreA-mediated carbon catabolite repression is dominant over the AraR N806I mutant allele. These mutations in AraR provide new opportunities to improve arabinase production in industrial fungal strains. Electronic supplementary material The online version of this article (10.1007/s00253-019-09777-0) contains supplementary material, which is available to authorized users.