Rhodococcus Multiple-Enzyme and Parallel-Degradation System for Aromatic Compounds

Aromatic hydrocarbons are aerobically degraded by a variety of bacteria via enzyme systems including ring-hydroxylating dioxygenases and monooxygenases and extradiol and intradiol ring-cleavage dioxygenases. The degradation enzyme genes of gram-negative bacteria usually constitute a single gene cluster. Multiple gene clusters of degradation enzyme genes were found in the gram-positive Rhodococcus strain RHA1. Multiple isozyme genes with 2,3-dihydroxybiphenyl dioxygenase activity have been reported in several biphenyl/PCB-degrading Rhodococcus strains including RHA1, suggesting the multiple-enzyme pathways in Rhodococcus strains. Transcriptional induction of multiple isozymes and functional involvement of multiple isozymes in degradation of not only biphenyl and PCBs but benzene and alkylbenzenes have been reported in RHA1. A couple of two-component regulatory systems are responsible for transcriptional induction of multiple isozymes. This multiple-enzyme pathway is thought to support the strong PCB degradation activity and the wide range of degradation activity of RHA1. The multiple-enzyme system in combination with the regulatory system with a broad inducing-substrate spectrum appears to provide the parallel-degradation system conferring the tough degradation activity toward a broad range of aromatics, suggesting the advantage of the multiple-enzyme system.