The Hydroxyquinol Degradation Pathway in Rhodococcus jostii RHA1 and <i>Agrobacterium</i> Species Is an Alternative Pathway for Degradation of Protocatechuic Acid and Lignin Fragments

Abstract

Deletion of the <i>pcaHG</i> genes, encoding protocatechuate 3,4-dioxygenase in <i>Rhodococcus jostii</i> 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 <i>R. jostii</i> 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 <i>Rhodococcus jostii</i> 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 <i>Agrobacterium</i> species. Recombinant <i>R. jostii</i> 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.<b>IMPORTANCE</b> 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 <i>Rhodococcus jostii</i> 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 <i>Agrobacterium</i> sp. strain suggests the involvement of the hydroxyquinol pathway in the metabolism of degraded lignin fragments.