FlbA-Regulated Gene rpnR Is Involved in Stress Resistance and Impacts Protein Secretion when Aspergillus niger Is Grown on Xylose

Proteins are secreted throughout the mycelium of Aspergillus niger except for the sporulating zone. A link between sporulation and repression of protein secretion was underlined by the finding that inactivation of the sporulation gene flbA results in mycelial colonies that secrete proteins throughout the colony. However, ΔflbA hyphae also lyse and have thinner cell walls. This pleiotropic phenotype is associated with differential expression of 36 predicted transcription factor genes, of which rpnR was inactivated in this study. Sporulation, biomass, and secretome complexity were not affected in the deletion strain ΔrpnR of the fungus. In contrast, ribosomal subunit expression and protein secretion into the medium were reduced when A. niger was grown on xylose. Moreover, ΔrpnR showed decreased resistance to H2O2 and the proteotoxic stress-inducing agent dithiothreitol. Taken together, RpnR is involved in proteotoxic stress resistance and impacts protein secretion when A. niger is grown on xylose.IMPORTANCEAspergillus niger secretes a high amount and diversity of industrially relevant enzymes into the culture medium. This makes this fungus a widely used industrial cell factory. For instance, carbohydrate-active enzymes of A. niger are used in biofuel production from lignocellulosic feedstock. These enzymes represent a major cost factor in this process. Higher production yields could substantially reduce these costs and therefore contribute to a more sustainable economy and less dependence on fossil fuels. Enzyme secretion is inhibited in A. niger by asexual reproduction. The sporulation protein FlbA is involved in this process by impacting expression of 36 predicted transcription factor genes. Here we show that one of these predicted transcriptional regulators, RpnR, regulates protein secretion and proteotoxic stress resistance. This gene is thus an interesting target to improve enzyme production in A. niger.