Abstract

Plants produce an abundant supply of many aromatic compounds including those derived from the aromatic polymer lignin, which comprises approximately 25% of biomass on land, and quinic acid, which can comprise up to 10% of the weight of dry leaf litter. Many aromatic compounds have industrial applications, e.g. as building blocks for plastics, biofuels, drugs, preservatives, flavoring agents, or antioxidants. Many fungal species are able to degrade aromatic compounds and utilize them as a carbon source. While aromatic metabolism is well understood in bacteria, the central pathways remain poorly understood in fungi and the genes involved largely unknown and lacking proper annotation. In this study, we have used data from comparative transcriptomics and the characterization of knockout mutants by growth phenotype and metabolic analysis to determine the genes and enzymes involved in each step in the catabolism of quinic acid and its derivative 3,4-dihydroxybenzoic acid in the filamentous fungus Aspergillus niger. The genes encoding quinate dehydrogenase, protocatechuate-3,4-dioxygenase, 3-carboxy-cis,cis-muconate cyclase, 3-carboxymuconolactone hydrolase/decarboxylase, β-ketoadipate CoA transferase, and β-ketoadipyl-CoA transferase in Aspergillus niger were determined to be NRRL3_08520, NRRL3_01405, NRRL3_02586, NRRL3_01409, NRRL3_01886, and NRRL3_01526, respectively. Two quinic acid transporters, NRRL3_11036 and NRRL3_05631, were also identified. NRRL3_05631 is a previously uncharacterized gene which functions at a lower pH than NRRL3_11036. Continued research will determine the entire metabolic network formed by the catabolic pathways of the other central aromatic intermediates, providing a comprehensive understanding of fungal aromatic metabolism for the first time. Knowledge of these pathways and the genes involved can lead to improved production of various valuable compounds, utilization of lignin, bioremediation or tolerance of toxic aromatic compounds.