Lignin is a major lock for lignocellulose valorization in biorefineries, prompting a need to find new ligninolytic systems. Termites are efficient lignocellulose degraders, and a large part of this ...ability comes from its anaerobic gut microbiome. However, the potential of termite gut microbiomes to degrade lignin under anaerobic conditions has yet to be elucidated. By applying wet chemistry, multidimensional NMR spectroscopy, and quantitative 13C-IS py-GC-MS, we determined the chemical and structural characteristics of wheat straw digested by the gut microbiomes of higher termites Nasutitermes ephratae, Nasutitermes lujae, Microcerotermes parvus, and Termes hospes implemented in anaerobic bioreactors. Interestingly, all gut microbiomes managed to remove lignin (up to 37%), although hemicellulose (mean 51%) and cellulose (mean 41%) were degraded more efficiently. Important structural differences, indicative of ligninolytic action, were discerned in the residual lignin. For the studied termite gut microbiomes, a slight decrease of S/G ratio was observed upon digestion, whereas lignin’s interunit linkages and Cα-oxidized moieties, including benzaldehyde and hydroxypropiovanillone/syringone substructures, accumulated. Additionally, tricin terminal units were clearly depleted, along with decreases in p-coumarate and ferulate pendant units. At least 80% of the observed delignification is estimated to be due to the removal of “true” lignin, with the remainder being explained by the removal of hydroxycinnamic acids and tricin. Collectively, our findings suggest a partial deconstruction of lignin, with “peripheral” lignin subunits being preferentially targeted. The present work thus provides new insights into the anaerobic deconstruction of lignin by termite gut microbiomes.
There is a well-established paradigm for degradation of protocatechuic acid via the β-ketoadipate pathway in a range of soil bacteria. In this study, we have found the existence of a second pathway ...for degradation of protocatechuic acid in
Rhodococcus jostii
RHA1, via hydroxyquinol (benzene-1,2,4-triol), which establishes a metabolic link between protocatechuic acid and hydroxyquinol. The presence of this pathway in a lignin-degrading
Agrobacterium
sp. strain suggests the involvement of the hydroxyquinol pathway in the metabolism of degraded lignin fragments.
Deletion of the
pcaHG
genes, encoding protocatechuate 3,4-dioxygenase in
Rhodococcus jostii
RHA1, gives a gene deletion strain still able to grow on protocatechuic acid as the sole carbon source, indicating a second degradation pathway for protocatechuic acid. Metabolite analysis of wild-type
R. jostii
RHA1 grown on medium containing vanillin or protocatechuic acid indicated the formation of hydroxyquinol (benzene-1,2,4-triol) as a downstream product. Gene cluster ro01857-ro01860 in
Rhodococcus jostii
RHA1 contains genes encoding hydroxyquinol 1,2-dioxygenase and maleylacetate reductase for degradation of hydroxyquinol but also putative mono-oxygenase (ro01860) and putative decarboxylase (ro01859) genes, and a similar gene cluster is found in the genome of lignin-degrading
Agrobacterium
species. Recombinant
R. jostii
mono-oxygenase and decarboxylase enzymes in combination were found to convert protocatechuic acid to hydroxyquinol. Hence, an alternative pathway for degradation of protocatechuic acid via oxidative decarboxylation to hydroxyquinol is proposed.
IMPORTANCE
There is a well-established paradigm for degradation of protocatechuic acid via the β-ketoadipate pathway in a range of soil bacteria. In this study, we have found the existence of a second pathway for degradation of protocatechuic acid in
Rhodococcus jostii
RHA1, via hydroxyquinol (benzene-1,2,4-triol), which establishes a metabolic link between protocatechuic acid and hydroxyquinol. The presence of this pathway in a lignin-degrading
Agrobacterium
sp. strain suggests the involvement of the hydroxyquinol pathway in the metabolism of degraded lignin fragments.
Deletion of the
genes, encoding protocatechuate 3,4-dioxygenase in
RHA1, gives a gene deletion strain still able to grow on protocatechuic acid as the sole carbon source, indicating a second ...degradation pathway for protocatechuic acid. Metabolite analysis of wild-type
RHA1 grown on medium containing vanillin or protocatechuic acid indicated the formation of hydroxyquinol (benzene-1,2,4-triol) as a downstream product. Gene cluster ro01857-ro01860 in
RHA1 contains genes encoding hydroxyquinol 1,2-dioxygenase and maleylacetate reductase for degradation of hydroxyquinol but also putative mono-oxygenase (ro01860) and putative decarboxylase (ro01859) genes, and a similar gene cluster is found in the genome of lignin-degrading
species. Recombinant
mono-oxygenase and decarboxylase enzymes in combination were found to convert protocatechuic acid to hydroxyquinol. Hence, an alternative pathway for degradation of protocatechuic acid via oxidative decarboxylation to hydroxyquinol is proposed.
There is a well-established paradigm for degradation of protocatechuic acid via the β-ketoadipate pathway in a range of soil bacteria. In this study, we have found the existence of a second pathway for degradation of protocatechuic acid in
RHA1, via hydroxyquinol (benzene-1,2,4-triol), which establishes a metabolic link between protocatechuic acid and hydroxyquinol. The presence of this pathway in a lignin-degrading
sp. strain suggests the involvement of the hydroxyquinol pathway in the metabolism of degraded lignin fragments.
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