Catabolic System of Syringic Acid, a Key Intermediate of Lignin-Derived Aromatic Compounds, via a Novel Linear Pathway in <i>Pseudomonas</i> 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 <i>Pseudomonas</i> 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-<i>O</i>-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. <i>vanA1B1</i>, <i>mgaAB</i>, <i>mgaC</i>, and <i>mgaD</i> constitute an operon whose transcription is regulated by the GntR-type transcriptional repressor VanR2. In contrast, <i>galBCD</i> constitutes an operon with the formaldehyde dehydrogenase gene <i>fdhA2</i>, 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.