Dissecting the complex regulation of pentose utilization in Aspergillus niger

• L-Arabitol is the monomeric inducer of AraR in A. niger • D-Xylose is the monomeric inducer of XlnR in A. niger • The induction of AraR and XlnR is chiral-specific in A. niger L-Arabinose and D-xylose are abundantly present in nature as components of plant cell wall polysaccharides. Fungi mainly use the Pentose Catabolic Pathway (PCP) for conversion of these sugars into central carbon metabolism. The PCP has been particularly well studied in Aspergillus niger , by identifying the metabolic genes as well as two transcriptional activators (XlnR and AraR) that control expression of PCP genes as well as genes encoding enzymes releasing D-xylose and L-arabinose from plant polysaccharides. While it has been demonstrated that XlnR is active during growth on D-xylose and AraR on L-arabinose, the actual inducers of these regulators have not been established. In this study, comparative transcriptomics on all intermediate PCP compounds as well as the chiral counterparts of D-xylose and L-arabinose was performed to determine which of them results in induction of AraR and/or XlnR target genes. In addition, mutants that were blocked in each of the metabolic steps of the PCP were analyzed to determine how they affected growth on all PCP intermediates, as well as L-xylose and D-arabinose. The combined results clearly demonstrated that the inducer of AraR is L-arabitol, the first metabolic intermediate from L-arabinose, while XlnR is induced by D-xylose itself, but not by L-xylose.