A Large Gene Cluster in Burkholderia xenovorans Encoding Abietane Diterpenoid Catabolism

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Abstract
Abietane diterpenoids are defense compounds synthesized by trees that are abundant in natural environments and occur as significant pollutants from pulp and paper production. Burkholderia xenovorans LB400 has diverse catabolic capabilities and represents an important group of heterotrophic bacteria in soil environments. The genome sequence of LB400 revealed homologs of the dit genes of Pseudomonas abietaniphila BKME-9, which encode abietane diterpenoid degradation. LB400 grew on abietic acid (AbA), dehydroabietic acid (DhA), palustric acid (PaA), and 7-oxo-DhA. A Xeotron microarray set, with probes for 8450 of the estimated 9000 LB400 genes, was used to compare the transcriptomes of LB400 growing on DhA versus on succinate. On DhA, 97 genes were upregulated, 43 of which were within an 80-kb cluster located on the 1.47-Mbp megaplasmid of LB400. Upregulated genes in this cluster encode a permease, a ring-hydroxylating dioxygenase system (DitA), a ring-cleavage dioxygenase (DitC), a P450 monooxygenase (DitQ), and enzymes catalyzing beta-oxidation-type reactions. Disruption of the ditA1 gene, encoding the alpha-subunit of DitA, abolished growth on these abietanes. Analyses of the metabolism of abietanes by cell suspensions of wild-type LB400 and the ditA1 mutant indicate a convergent pathway, with 7-oxo-DhA as a common intermediate for ring hydroxylation by DitA. Also, 7-oxo-PaA was identified as a metabolite of both AbA and PaA. Sequence analysis indicates that genes encoding this pathway have been horizontally transferred among Betaproteobacteria and Gammaproteobacteria.

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