Comparative Transcriptome Analysis Reveals the Mechanism Underlying 3,5-Dibromo-4-Hydroxybenzoate Catabolism via a New Oxidative Decarboxylation Pathway

The compound 3,5-dibromo-4-hydroxybenzoate (DBHB) is both anthropogenically released into and naturally produced in the environment, and its environmental fate is of great concern. Aerobic and anaerobic reductive dehalogenations are the only two reported pathways for DBHB catabolism. In this study, a new oxidative decarboxylation pathway for DBHB catabolism was identified in a DBHB-utilizing strain, Pigmentiphaga sp. strain H8. The genetic determinants underlying this pathway were elucidated based on comparative transcriptome analysis and subsequent experimental validation. A gene cluster comprising orf420 to orf426, with transcripts that were about 33- to 4,400-fold upregulated in DBHB-induced cells compared with those in uninduced cells, was suspected to be involved in DBHB catabolism. The gene odcA (orf420), which is essential for the initial catabolism of DBHB, encodes a novel NAD(P)H-dependent flavin monooxygenase that mediates the oxidative decarboxylation of DBHB to 2,6-dibromohydroquinone (2,6-DBHQ). The substrate specificity of the purified OdcA indicated that the 4-hydroxyl group and its ortho-halogen(s) are important for hydroxylation of the C-1 site carboxyl group by OdcA. 2,6-DBHQ is then ring cleaved by the dioxygenase OdcB (Orf425) to 2-bromomaleylacetate, which is finally transformed to β-ketoadipate by the maleylacetate reductase OdcC (Orf426). These results provide a better understanding of the molecular mechanism underlying the catabolic diversity of halogenated para-hydroxybenzoates.IMPORTANCE Halogenated hydroxybenzoates (HBs), which are widely used synthetic precursors for chemical products and common metabolic intermediates from halogenated aromatics, exert considerable adverse effects on human health and ecological security. Microbial catabolism plays key roles in the dissipation of halogenated HBs in the environment. In this study, the discovery of a new catabolic pathway for 3,5-dibromo-4-hydroxybenzoate (DBHB) and clarification of the genetic determinants underlying the pathway broaden our knowledge of the catabolic diversity of halogenated HBs in microorganisms. Furthermore, the NAD(P)H-dependent flavin monooxygenase OdcA identified in Pigmentiphaga sp. strain H8 represents a novel 1-monooxygenase for halogenated para-HBs found in prokaryotes and enhances our knowledge of the decarboxylative hydroxylation of (halogenated) para-HBs.