The FlbA-regulated predicted transcription factor rpnR of Aspergillus niger is involved in protein secretion and stress resistance. Aspergillus niger

Proteins are secreted throughout the mycelium of Aspergillus niger except for the sporulating zone. Inactivation of flbA results in colonies that do not reproduce asexually and secrete proteins throughout the colony. Moreover, ΔflbA hyphae lyse and have thinner cell walls. This pleiotropic phenotype is associated with differential expression of 38 transcription factor genes, of which rpnR was inactivated in this study. Biomass formation and sporulation was not affected in strain ΔrpnR but protein secretion was severely reduced. Moreover, ΔrpnR showed decreased resistance to H2O2 and the unfolded protein response inducing agent dithiothreitol. The secretomes of ΔrpnR and wild type consisted of 342 and 367 proteins, respectively. Secretion of several CAZymes and proteins involved in cell wall remodeling was more strongly reduced when compared to other proteins within the secretome of ΔrpnR. Whole genome expression analysis of xylose-grown colonies showed that RpnR activates expression of ribosomal subunits as well as small secreted proteins. On the other hand, it inhibited expression of the genes encoding the arabinofuranosidases AbfA, AbfB and AbfE, as well as genes related to amino acid metabolism, xenobiotics biodegradation and metabolism, lipid metabolism, and metabolism of cofactors and vitamins. Taken together, RpnR is involved in protein synthesis and may also be involved in cell wall architecture. Both functions could explain the reduced protein secretion and proteotoxic stress resistance in the deletion strain. Overall design: mRNA profiles from ΔrpnR and the control strain MA234.1 were generated by Illumina Hiseq using duplicate samples from liquid shaken cultures.