Proposed Involvement of a Soluble Methane Monooxygenase Homologue in the Cyclohexane-dependent Growth of a New Brachymonas Species

High-throughput mRNA differential display (DD) was used to identify genes induced by cyclohexane in Brachymonas petroleovorans CHX, a recently isolated beta-proteobacterium that grows on cyclohexane. Two metabolic gene clusters were identified multiple times in independent reverse transcription polymerase chain reactions (RT-PCR) in the course of this DD experiment. These clusters encode genes believed to be required for cyclohexane metabolism. One gene cluster (8 kb) encodes the subunits of a multicomponent hydroxylase related to the soluble butane of Pseudomonas butanovora and methane monooxygenases (sMMO) of methanotrophs. We propose that this butane monooxygenase homologue carries out the oxidation of cyclohexane into cyclohexanol during growth. A second gene cluster (11 kb) contains almost all the genes required for the oxidation of cyclohexanol to adipic acid. Real-time PCR experiments confirmed that genes from both clusters are induced by cyclohexane. The role of the Baeyer-Villiger cyclohexanone monooxygenase of the second cluster was confirmed by heterologous expression in Escherichia coli.