Rhodococcus 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 cyp108j1 and involved in the degradation of PAHs, was identified and was able to hydroxylate various PAHs. However, the regulatory mechanism of the expression of cyp108j1 remains unknown. In this study, we found that the expression of cyp108j1 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 cyp108j1. Further analysis revealed that NarL can directly bind to the promoter region of cyp108j1. Mutational experiments demonstrated that the binding site between NarL and the cyp108j1 promoter was the palindromic sequence GAAAGTTG-CAACTTTC. Together, the finding reveal that NarL is a novel repressor for the expression of cyp108j1 during PAHs degradation.Int. J. Mol. Sci. 2020, 21, 983 2 of 11 and chrysene can also be used as sole sources of carbon and energy [15]. A common feature of the aerobic Rhodococcus genus is the presence of many types of monooxygenases and dioxygenases. Cytochrome p450 (CYP450) plays an important role in the process. Sylvie et al. characterized spontaneous mutants of Rhodococcus ruber unable to use ethyl tert-butyl ether (ETBE) as the sole source of carbon and energy and found that it was unable to degrade ETBE without a CYP450 gene cluster, whereas the complementation of the mutant using ethRABCD was able to degrade ETBE again, demonstrating the involvement of the Eth CYP450 system in the degradation of ETBE [16].Regulatory proteins and regulated promoters are key elements that control the transcription of catabolic substrates, such as PAHs [17]. The regulator DfdR is one of the LuxR family proteins, which is encoded by a gene in the dfd gene cluster in the dibenzofuran using Rhodococcus sp. strain YK2 and Terrabacter sp. strain YK3. The dfdR gene product affects the promoter activity of the dfdA gene, which is involved in the initial hydroxylation of dibenzofuran [18].Rhodococcus sp. P14 was originally isolated from crude-oil-contaminated sediments and can use a wide range of PAHs and steroids as the sole source of carbon and energy [19][20][21]. Its entire genome has been sequenced [22]. Some hydroxyl products were detected during Rhodococcus sp. P14 metabolism of PAHs, suggesting that oxygenase plays an important role in the degradation process [23,24]. The gene cyp108j1 encoding a CYP450 was identified. Further investigation of the recombinant protein CYP108J1 proved that it is capable of the hydroxylation of a series of PAHs compounds [24].In this study, we analyzed the promoter structure of cyp108j1 and proved that its upstream regulatory NarL has an inhibitory effect on the cyp108j1 expression. Using site-directed mutation, we identified the binding sites of NarL with the promoter cyp108j1. Thes...