Aspergillus oryzae pathways that convert phenylalanine into the flavor volatile 2-phenylethanol

The filamentous fungus Aspergillus oryzae RIB40 produced 2-phenylethanol (PE) when cultured in minimum medium containing l-phenylalanine as a sole source of nitrogen. The fungus accumulated less PE in the absence of l-phenylalanine, indicating that it converted l-phenylalanine to PE. The PE production associated with fungal glucose consumption was repressed by exogenous ammonium, indicating that nitrogen-metabolite repression controls the pathway that produces PE. We identified the A. oryzae ppdA gene that is expressed at high levels in the presence of exogenous l-phenylalanine and its encoded protein was an active phenylpyruvate decarboxylase. The fungal genome encodes predicted aminotransferases of phenylalanine and PE dehydrogenases, which, together with PpdA, are likely to constitute an Erlich pathway similar to that in Saccharomyces cerevisiae that produces PE. We also identified an A. oryzae aromatic amino acid decarboxylase (AadA) that converted l-phenylalanine to phenylethylamine (PEA), and phenylalanine-inducible PEA oxidase activity in fungal cell extracts, and found that both constitute an alternative pathway through which PEA generates PE. Incubating fungal cultures with l-[2H8] phenylalanine to distinguish PE produced by these pathways, indicated that the fungus produced PE by both pathways, but to a greater extent by the Erlich pathway. Gene disruption of ppdA and aadA showed that both pathways participate in the fungal conversion of l-phenylalanine to PE.