Multiple-subunit genes of the aromatic-ring-hydroxylating dioxygenase play an active role in biphenyl and polychlorinated biphenyl degradation in Rhodococcus sp. strain RHA1.

A gram-positive strong polychlorinated biphenyl (PCB) degrader, Rhodococcus sp. strain RHA1, can degrade PCBs by cometabolism with biphenyl or ethylbenzene. In RHA1, three sets of aromatic-ring-hydroxylating dioxygenase genes are induced by biphenyl. The large and small subunits of their terminal dioxygenase components are encoded by bphA1 and bphA2, etbA1 and etbA2, and ebdA1 and ebdA2, respectively, and the deduced amino acid sequences of etbA1 and etbA2 are identical to those of ebdA1 and ebdA2, respectively. In this study, we examined the involvement of the respective subunit genes in biphenyl/PCB degradation by RHA1. Reverse transcription-PCR and two-dimensional polyacrylamide gel electrophoresis analyses indicated the induction of RNA and protein products of etbA1 and ebdA1 by biphenyl. Single- and double-disruption mutants of etbA1, ebdA1, and bphA1 were constructed by insertional inactivation. The 4-chlorobiphenyl (4-CB) degradation activities of all the mutants were lower than that of RHA1. The results indicated that all of these genes are involved in biphenyl/PCB degradation. Furthermore, we constructed disruption mutants of ebdA3 and bphA3, encoding ferredoxin, and etbA4, encoding ferredoxin reductase components. The 4-CB degradation activities of these mutants were also lower than that of RHA1, suggesting that all of these genes play a role in biphenyl/PCB degradation. The substrate preferences of etbA1A2/ebdA1A2- and bphA1A2-encoded dioxygenases for PCB congeners were examined using the corresponding mutants. The results indicated that these dioxygenase isozymes have different substrate preferences and that the etbA1A2/ebdA1A2-encoded isozyme is more active on highly chlorinated congeners than the bphA1A2-encoded one.