Catabolic System of Syringic Acid, a Key Intermediate of Lignin-Derived Aromatic Compounds, via a Novel Linear Pathway in Pseudomonas sp. NGC7.

Syringic acid (SA) is a key intermediate in the bacterial catabolism of syringyl lignin-derived aromatic compounds. However, bacterial SA catabolism remains largely unknown. Here, we investigated the SA catabolic system in Pseudomonas sp. NGC7, which catabolizes various lignin-derived aromatic monomers. Pathway analysis and gene disruption studies revealed that SA undergoes O demethylation by VanA1B1 to 3-O-methylgallic acid, which is subsequently catabolized through a linear pathway, involving MgaAB, MgaC, MgaD, GalD, GalB, and GalC into the TCA cycle, unlike the branching pathway found in sphingomonad strains. vanA1B1, mgaAB, mgaC, and mgaD constitute an operon whose transcription is regulated by the GntR-type transcriptional repressor VanR2. In contrast, galBCD constitutes an operon with the formaldehyde dehydrogenase gene fdhA2, and this operon is regulated by the LysR-type transcriptional activator GalR. FdhA2 is involved in the detoxification of formaldehyde, a byproduct of SA catabolism. These findings contribute to the valorization of syringyl lignin-containing biomass.