Benzene dioxygenase (BDO; EC 184.108.40.206) from Pseudomonas putida ML2 dihydroxylates benzene to produce cis-1,2-dihydroxy-cyclohexa-3,5-diene. As well as oxidising benzene and toluene, cell-free extracts of Escherichia coli JM109 expressing recombinant BDO oxidised cyclohexene, 1-methylcyclohexene and 3-methylcyclohexene. In an attempt to construct a novel metabolic pathway for the degradation of cyclohex-ene (via an initial BDO-mediated dihydroxylation of cyclohexene), cis-1,2-cyclohexanediol-degrading bacteria were isolated by enrichment culture. The bedC1C2BA genes encoding BDO (under the control of the tac promoter) were sub-cloned into pLAFR5, successfully conjugated into seven of the Gram-negative cis-1,2-cyclo-hexanediol-degrading isolates and stably maintained and expressed in three of them. However, despite their ability to grow on cis-1,2-cyclohexanediol as sole carbon source, express an active BDO and oxidise cyclohexene, none of the three strains was able to grow on cyclohexene as sole carbon source. Analysis revealed that BDO oxidised cyclohexene to a mixture of two products, a monohydroxylated (2-cyclohexen-1-ol) product and a dihydroxylated (cis-1,2-cyclohexanediol) product; and failure to grow on cyclohexene was attributed to the toxicity of metabolic intermediates accumulating from the 2-cyclohexen-1-ol metabolism.
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