Contrary to the phosphorolytic degradation of D-galacturonic acid in bacteria and plants, non-phosphorolytic metabolism of D-galacturonic acid has been shown to occur in Aspergillus species (Uitzetter et al., 1985). This short pathway begins with the conversion of D-galacturonic acid to galactonate by an aldoketo reductase. Subsequently a dehydratase or racemase modifies galactonate to 2-keto-3-deoxygalactonate and an aldolase splits 2-keto-3-deoxygalactonate into pyruvate and glyceraldehyde. Although, A. nidulans mutants defective in the racemase and aldolase activities, known as gaaB and gaaA, respectively, were previously reported (Uitzetter et al., 1985), the genes encoding these activities have not been described yet. Transcriptome analysis of Aspergillus niger cultures grown on D-galacturonic acid, enabled the identification of a cluster of co-expressed genes that, among others, encode the necessary putative aldoketo reductase, racemase and aldolase. Sequence analysis of the genes orthologous to the A. niger gaaA and gaaB from the two A. nidulans D-galacturonate non utilizing mutants demonstrated the presence of point mutations in the coding regions of both A. nidulans gaaA and gaaB leading to the production of non-functional proteins and thus to the mutant phenotype. Comparative analysis of the A. niger gaaA and gaaB genes in publicly available fungal genomes revealed the presence of conserved orthologs to both genes in various distantly related fungi which suggests the presence of a common utilization pathway of D-galacturonic acid in fungi. Reference: Uitzetter JH, Bos CJ, Visser J., J Gen Microbiol. 1986 132: 1167-72.
Full conference title:
23rd Fungal Genetics Conference
- Fungal Genetics Conference 23rd (2002)