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Background  Bacteria belonging to the  Rhodococcus  genus play an important role in the degradation of many contaminants, including methylbenzenes. These bacteria, widely distributed in the environment, are known to be a powerhouse of numerous degradation functions, due to their ability to metabolize a wide range of organic molecules including aliphatic, aromatic, polycyclic aromatic compounds (PAHs), phenols, and nitriles. In accordance with their immense catabolic diversity,  Rhodococcus  spp. possess large and complex genomes, which contain a multiplicity of catabolic genes, a high genetic redundancy of biosynthetic pathways and a sophisticated regulatory network. The present study aimed to identify genes involved in the  o -xylene degradation in  R. opacus  strain R7 through a genome-based approach.    Results  Using genome-based analysis we identified all the sequences in the R7 genome annotated as dioxygenases or monooxygenases/hydroxylases and clustered them into two different trees. The  akb ,  phe  and  prm  sequences were selected as genes encoding respectively for dioxygenases, phenol hydroxylases and monooxygenases and their putative involvement in  o -xylene oxidation was evaluated. The involvement of the  akb  genes in  o -xylene oxidation was demonstrated by RT-PCR/qPCR experiments after growth on  o -xylene and by the selection of the R7–50 leaky mutant. Although the  akb  genes are specifically activated for  o -xylene degradation, metabolic intermediates of the pathway suggested potential alternative oxidation steps, possibly through monooxygenation. This led us to further investigate the role of the  prm  and the  phe  genes. Results showed that these genes were transcribed in a constitutive manner, and that the activity of the Prm monooxygenase was able to transform  o -xylene slowly in intermediates as 3,4-dimethylphenol and 2-methylbenzylalcohol. Moreover, the expression level of  phe  genes, homologous to the  phe  genes of  Rhodococcus  spp. 1CP and UPV-1 with a 90% identity, could explain their role in the further oxidation of  o -xylene and R7 growth on dimethylphenols.    Conclusions  These results suggest that R7 strain is able to degrade  o -xylene by the Akb dioxygenase system leading to the production of the corresponding dihydrodiol. Likewise, the redundancy of sequences encoding for several monooxygenases/phenol hydroxylases, supports the involvement of other oxygenases converging in the  o -xylene degradation pathway in R7 strain.    Electronic supplementary material  The online version of this article (10.1186/s12864-018-4965-6) contains supplementary material, which is available to authorized users.

Genome-based analysis for the identification of genes involved in o-xylene degradation in Rhodococcus opacus R7