Discovery and Functional Analysis of a Salicylic Acid Hydroxylase from Aspergillus niger.

Salicylic acid plays an important role in the plant immune response, and its degradation is therefore important for plant-pathogenic fungi. However, many nonpathogenic microorganisms can also degrade salicylic acid. In the filamentous fungus , two salicylic acid metabolic pathways have been suggested. The first pathway converts salicylic acid to catechol by a salicylate hydroxylase (ShyA). In the second pathway, salicylic acid is 3-hydroxylated to 2,3-dihydroxybenzoic acid, followed by decarboxylation to catechol by 2,3-dihydroxybenzoate decarboxylase (DhbA). cleaves the aromatic ring of catechol catalyzed by catechol 1,2-dioxygenase (CrcA) to form ,-muconic acid. However, the identification and role of the genes and characterization of the enzymes involved in these pathways are lacking. In this study, we used transcriptome data of grown on salicylic acid to identify genes ( and ) involved in salicylic acid metabolism. Heterologous production in followed by biochemical characterization confirmed the function of ShyA and CrcA. The combination of ShyA and CrcA demonstrated that -muconic acid can be produced from salicylic acid. In addition, the roles of , , and were studied by creating deletion mutants which revealed the role of these genes in the fungal metabolism of salicylic acid. Nonrenewable petroleum sources are being depleted, and therefore, alternative sources are needed. Plant biomass is one of the most abundant renewable sources on Earth and is efficiently degraded by fungi. In order to utilize plant biomass efficiently, knowledge about the fungal metabolic pathways and the genes and enzymes involved is essential to create efficient strategies for producing valuable compounds such as ,-muconic acid. ,-Muconic acid is an important platform chemical that is used to synthesize nylon, polyethylene terephthalate (PET), polyurethane, resins, and lubricants. Currently, ,-muconic acid is mainly produced through chemical synthesis from petroleum-based chemicals. Here, we show that two enzymes from fungi can be used to produce ,-muconic acid from salicylic acid and contributes in creating alternative methods for the production of platform chemicals.