| Summary: | <i>Rhodococcus</i> sp. P14 was isolated from crude-oil-contaminated sediments, and a wide range of polycyclic aromatic hydrocarbons (PAHs) could be used as the sole source of carbon and energy. A key CYP450 gene, designated as <i>cyp108j1</i> and involved in the degradation of PAHs, was identified and was able to hydroxylate various PAHs. However, the regulatory mechanism of the expression of <i>cyp108j1</i> remains unknown. In this study, we found that the expression of <i>cyp108j1</i> is negatively regulated by a LuxR (helix-turn-helix transcription factors in acyl-homoserine lactones-mediated quorum sensing) family regulator, NarL (nitrate-dependent two-component regulatory factor), which is located upstream of <i>cyp108j1</i>. Further analysis revealed that NarL can directly bind to the promoter region of <i>cyp108j1</i>. Mutational experiments demonstrated that the binding site between NarL and the <i>cyp108j1</i> promoter was the palindromic sequence GAAAGTTG-CAACTTTC. Together, the finding reveal that NarL is a novel repressor for the expression of <i>cyp108j1</i> during PAHs degradation.
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