Identification of mutations causing constitutive expression of galacturonic acid utlization genes in Aspergillus niger

A D-galacturonic acid (GA) specific transcription factor named GaaR was recently identified and shown to be an activator for the expression of genes involved in GA utilization in Botrytis cinerea and in Aspergillus niger. Using a forward genetic screen, we isolated 73 mutants that constitutively express GaaR-controlled genes. Full genome sequencing of five constitutive mutants revealed different mutations in one particular gene encoding a previously uncharacterized protein (NRRL3_08194). Interestingly, this gene is located next to gaaR (NRRL3_08195) in the genome. The protein encoded by NRRL3_08194 shows homology to the repressor of the quinate utilization pathway. The chromosomal clustering, the homology to quinate repressor, and the constitutive phenotype of the isolated mutants suggest that the gene identified is likely to encode a repressor, which we named GaaX. The GaaR/GaaX module and its chromosomal organization is conserved among ascomycetous fungi, and it resembles the quinic acid utilization activator-repressor module in amino-acid sequence and chromosomal organization.