An evolutionary conserved d-galacturonic acid metabolic pathway operates across filamentous fungi capable of pectin degradation

Transcriptome analysis of Aspergillus niger transfer cultures grown on galacturonic acid media identified a highly correlating cluster of four strongly induced hypothetical genes linked with a subset set of genes encoding pectin degrading enzymes. Three of the encoded hypothetical proteins now designated GAAA to GAAC are directly involved in further galacturonic acid catabolism. Functional and biochemical analysis revealed that GAAA is a novel d-galacturonic acid reductase. Two non-allelic Aspergillus nidulans strains unable to utilize galacturonic acid are mutated in orthologs of gaaA and gaaB, respectively. The A. niger gaaA and gaaC genes share a common promoter region. This feature appears to be strictly conserved in the genomes of plant cell wall degrading fungi from subphylum Pezizomycotina. Combined with the presence of homologs of the gaaB gene in the same set of fungi, these strongly suggest that a common d-galacturonic acid utilization pathway is operative in these species.