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

Abstract

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 strain hyphae also lyse and have thinner cell walls. This pleiotropic phenotype is associated with differential expression of 36 predicted transcription factor genes, one of which, rpnR, was inactivated in this study. Sporulation, biomass, and secretome complexity were not affected in the ΔrpnR deletion strain of the fungus. In contrast, ribosomal subunit expression and protein secretion into the medium were reduced when A. niger was grown on xylose. Moreover, the ΔrpnR strain showed decreased resistance to H 2 O 2 and the proteotoxic stress-inducing agent dithiothreitol. Taking the data together, RpnR is involved in proteotoxic stress resistance and impacts protein secretion when A. niger is grown on xylose. IMPORTANCE Aspergillus niger secretes a large amount and diversity of industrially relevant enzymes into the culture medium. This makes the 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 the 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. The gene is thus an interesting target to improve enzyme production in A. niger.

Characterizing the role of the zinc finger transcription factor AcrpnR in governing development in Aspergillus cristatus

“…However, the ΔAcrpnR strain produced a significantly reduced number of conidia, while OE::AcrpnR produced a large number of conidia, indicating that AcrpnR promoted asexual sporulation. This is in contrast with the A. niger rpnR deletion strain, in which there was no difference in spore production compared with the WT [20]. The ability of AcrpnR to promote asexual development was reported for the first time in Aspergillus.…”

“…A previous study on A. niger rpnR showed that rpnR was involved in stress resistance [20]. The ΔrpnR strain decreased resistance to DTT and H 2 O 2 [20].…”

“…The involvement of the rpn gene in the stress response was also observed in A. niger. The ΔrpnR strain of A. niger showed decreased resistance to H 2 O 2 and DTT [20]. Unlike A. niger, AcrpnR did not seem to play important roles in the H 2 O 2 response, and the ΔAcrpnR and S6 and Table 1).…”

“…A previous study on A. niger rpnR showed that rpnR was involved in stress resistance [20]. The ΔrpnR strain decreased resistance to DTT and H 2 O 2 [20]. We also compared the sensitivity of the WT, ΔAcrpnR, ΔAcrpnR-C, and OE::AcrpnR strains to DTT, H 2 O 2 , menadione, and SDS.…”

“…The putative protein ODM18596.1 was 62.6% identical to RpnR of A. niger (An08g06850; entry number: A2QRQ4) and 53.8% identical to SilG of A. nidulans (AN0709; entry number: Q5BFH1). Based on phylogenetic analysis and the existing reports [19,20], the GME2916 gene was named AcrpnR.…”