The pathway intermediate 2-keto-3-deoxy-L-galactonate mediates the induction of genes involved in D-galacturonic acid utilization in Aspergillus niger

In Aspergillus niger, the enzymes encoded by gaaA, gaaB, and gaaC catabolize D-galacturonic acid (GA) consecutively into L-galactonate, 2-keto-3-deoxy-L-galactonate, pyruvate, and L-glyceraldehyde, while GaaD converts L-glyceraldehyde to glycerol. Deletion of gaaB or gaaC results in severely impaired growth on GA and accumulation of L-galactonate and 2-keto-3-deoxy-L-galactonate, respectively. Expression levels of GA-responsive genes are specifically elevated in the Delta gaaC mutant on GA as compared to the reference strain and other GA catabolic pathway deletion mutants. This indicates that 2-keto-3-deoxy-L-galactonate is the inducer of genes required for GA utilization.