Combination of six enzymes of a marine Novosphingobium converts the stereoisomers of β-O-4 lignin model dimers into the respective monomers

Lignin, an aromatic polymer of phenylpropane units joined predominantly by β-O-4 linkages, is the second most abundant biomass component on Earth. Despite the continuous discharge of terrestrially produced lignin into marine environments, few studies have examined lignin degradation by marine microorganisms. Here, we screened marine isolates for β-O-4 cleavage activity and determined the genes responsible for this enzymatic activity in one positive isolate. Novosphingobium sp. strain MBES04 converted all four stereoisomers of guaiacylglycerol-β-guaiacyl ether (GGGE), a structural mimic of lignin, to guaiacylhydroxypropanone as an end metabolite in three steps involving six enzymes, including a newly identified Nu-class glutathione-S-transferase (GST). In silico searches of the strain MBES04 genome revealed that four GGGE-metabolizing GST genes were arranged in a cluster. Transcriptome analysis demonstrated that the lignin model compounds GGGE and (2-methoxyphenoxy)hydroxypropiovanillone (MPHPV) enhanced the expression of genes in involved in energy metabolism, including aromatic-monomer assimilation, and evoked defense responses typically expressed upon exposure to toxic compounds. The findings from this study provide insight into previously unidentified bacterial enzymatic systems and the physiological acclimation of microbes associated with the biological transformation of lignin-containing materials in marine environments.