Kraft lignin decomposition by lignin-derived aromatic compound degrader Rhodococcus sp. DK17

Rhodococcus sp. DK17 has been previously isolated from oil-contaminated soil and studied for its ability to degrade various monocyclic alkylbenzenes. This study investigated the decomposition of kraft lignin (genes, enzymes, and metabolic pathways) by DK17, using whole-genome sequencing data, as a potential biocatalyst for biotechnological lignin valorization. DK17 used kraft lignin and its main degradative metabolites, such as vanillin and vanillic acid, as growth substrates. High-performance liquid chromatography revealed that DK17 converted dehydrodivanillin (a representative lignin model compound). Quantitative polymerase chain reaction of mRNAs from DK17 cells induced in the presence of lignin showed that the putative genes coding for two copies of dye-decolorizing peroxidases (dypB1 and dypB2) were upregulated 1.6- and 2.4-fold after 5 and 24 h of induction, respectively, compared with glucose-induced cells. Vanillic acid induced dypB1 and dypB2 at lower levels than lignin by 1.4- and 1.6-fold after 5 and 24 h of induction, respectively. Computational homology analysis using the DypB1 and DypB2 protein sequences also predicted their initial roles in lignin decomposition. The duplicated dyp genes are believed to allow DK17 to achieve prolonged and continuous initial lignin decomposition, cleaving C-C and C-O-C linkages in the main lignin structure, the arylglycerol-β-aryl ether. Based on the above data, DK17 appears to initiate oxidative lignin decomposition using DyPs, producing smaller metabolites, such as vanillin and vanillic acid, which could be accumulated as value-added bioproducts (in metabolically engineered mutant strains) or further degraded for cell growth (in wild-type strains) via an ortho-ring cleavage pathway.