A transcriptional activator, AoXlnR, controls the expression of genes encoding xylanolytic enzymes in Aspergillus oryzae

By deletion across the promoter region of the xynF1 gene encoding the major Aspergillus oryzae xylanase, a 53-bp DNA fragment containing the XlnR binding sequence GGCTAAA as well as two similar sequences was shown to confer xylan inducibility on the gene. Complementary and genomic DNAs encoding the Aspergillus niger xlnR homologous gene, abbreviated AoxlnR, were cloned from A. oryzae and sequenced. AoXlnR comprised 971 amino acids with a zinc binuclear cluster domain at the N-terminal region and revealed 77.5% identity to the A. niger XlnR. Recombinant AoXlnR protein encompassing the zinc cluster region of the N-terminal part bound to both the consensus binding sequence and its cognate sequence, GGCTGA, with an approximately 10 times lower affinity. GGCTA/GA is more appropriate as the XlnR consensus binding sequence. Both sequences functioned independently in vivo in XlnR-mediating induction of the xynF1 gene. This was further confirmed by using an AoxlnR disruptant. Neither the xynF1 nor the xylA gene was expressed in the disruptant, suggesting that the xylan-inducible genes in A. oryzae may also be controlled in the same manner as described for A. niger.