Catabolic System of 5-Formylferulic Acid, a Downstream Metabolite of a β-5-Type Lignin-Derived Dimer, in SYK-6.

SYK-6 can assimilate various lignin-derived aromatic compounds, including a β-5-type (phenylcoumaran-type) dimer, dehydrodiconiferyl alcohol (DCA). SYK-6 converts DCA to a stilbene-type intermediate via multiple reaction steps and then to vanillin and 5-formylferulic acid (FFA). Here, we first elucidated the catabolic pathway of FFA, which is the only unknown pathway in DCA catabolism. Then, we identified and characterized the enzyme-encoding genes responsible for this pathway. Analysis of the metabolites revealed that FFA was converted to 5-carboxyferulic acid (CFA) through oxidation of the formyl group, followed by conversion to ferulic acid by decarboxylation. A comprehensive analysis of the aldehyde dehydrogenase genes in SYK-6 indicated that NAD-dependent FerD (SLG_12800) is crucial for the conversion of FFA to CFA. LigW and LigW2, which are 5-carboxyvanillic acid decarboxylases involved in the catabolism of a 5,5-type dimer, were found to be involved in the conversion of CFA to ferulic acid, and LigW2 played a significant role. The gene forms an operon with , and their transcription was induced during growth in DCA.