Coinducible Catabolism of 1-Naphthol via Synergistic Regulation of the Initial Hydroxylase Genes in Sphingobium sp. Strain B2

Abstract

1-Naphthol, a widely used raw material for organic synthesis, is also a well-known organic pollutant. Due to its high toxicity, 1-naphthol is rarely used by microorganisms as the sole carbon source for growth. In this study, catabolism of 1-naphthol by Sphingobium sp. strain B2 was found to be greatly enhanced by additional supplementation with primary carbon sources (e.g., glucose, maltose and sucrose), and 1-naphthol was even used as the carbon source for growth when strain B2 cells had been pre-induced by both 1-naphthol and glucose. A distinct two-component flavin-dependent monooxygenase NdcA1A2 was found to be responsible for the initial hydroxylation of 1-naphthol to 1,2-dihydroxynaphthalene, a more toxic compound. Transcriptional levels of ndcA1A2 genes were significantly up-regulated when strain B2 cells were cultured with both 1-naphthol and glucose as compared to cells cultured with sole 1-naphthol or glucose. Two transcriptional regulators, the activator NdcS and the inhibitor NdcR were found to play key roles in the synergistic regulation of the transcription of the 1-naphthol initial catabolic genes ndcA1A2. Importance Co-metabolism is a widely observed phenomenon, especially in the field of microbial catabolism of highly toxic xenobiotics. However, the mechanisms of co-metabolism are ambiguous and the roles of the obligately co-existing growth substrates remain largely unknown. In this study, we revealed that the roles of the co-existing primary carbon sources (e.g. glucose) in the enhanced catabolism of the toxic compound 1-naphthol in Sphingobium sp. strain B2 was not solely because they were used as growth substrates to support cell growth, but more importantly they acted as “co-inducers” to interact with two transcriptional regulators, the activator NdcS and the inhibitor NdcR, to synergistically regulate the transcription of the 1-naphthol initial catabolic genes ndcA1A2. Our findings provide new insights into the co-metabolic mechanism of highly toxic compounds in microorganisms.

Co-inducible Catabolism of 2-Naphthol Initiated by Hydroxylase CehC1C2 in Rhizobium sp. X9 Removed Its Ecotoxicity

“…This interesting regulatory mechanism, which is similar to a double-lock switch, indicated that the catabolism of 1-naphthol is only initiated when the cells have a sufficient carbon source to maintain basic metabolism. 19 In this study, we demonstrated that CehC1C2 could catalyze 2-naphthol to produce 1,2-DHN and that 2naphthol could activate the transcription of the gene cluster. These results complemented the co-inducible catabolism of naphthol compounds by the cehC1C3C2/ndcA1BA2 cluster.…”

“…NdcS (CehR3) was predicted to be an activator that rapidly promoted the level of transcription of ndcA1BA2 ( cehC1C3C2 ) in the presence of 1-naphthol. This interesting regulatory mechanism, which is similar to a double-lock switch, indicated that the catabolism of 1-naphthol is only initiated when the cells have a sufficient carbon source to maintain basic metabolism . In this study, we demonstrated that CehC1C2 could catalyze 2-naphthol to produce 1,2-DHN and that 2-naphthol could activate the transcription of the gene cluster.…”

“…Similarly, 1,2-dihydroxynaphthalene (1,2-DHN) was proven to be the central intermediate of the microbial degradation of naphthalene compounds. Naphthalene, 1-naphthol, naphthalene–sulfonic acid, carbaryl, , and napropamide were all degraded via 1,2-DHN. In this study, the product and enzymatic characteristics of two-component hydroxylase CehC1C2 on 2-naphthol were verified.…”

“…Two transcriptional regulators, the activator NdcS (activated by 1-naphthol) and the repressor NdcR (repressed by glucose or its metabolites), were found to play key roles in the synergistic regulation of the transcription of the 1-naphthol initial catabolic genes ndcA1A2. 19 A bioinformatic analysis revealed that cehR3, cehR2, and cehC1C2 shared extremely high amino acid similarity and had the same gene distribution as ndcS, ndcR, and ndcA1A2 (Figure S2 of the Supporting Information). The monooxygenase CehC1 shares 99.62% identity with NdcA1.…”

Oxygenases as Powerful Weapons in the Microbial Degradation of Pesticides

“…McbC, a one-component flavin-dependent monooxygenase identified from Pseudomonas sp. strain C4 ortho-hydroxylates 1-naphthol (an intermediate of the insecticide carbaryl and the herbicide napropamide) to 1,2-dihydroxynaphthalene (41,83,110) (Supplemental Figure 3h). Other structural analogs of 1-naphthol, such as 4-chloro-1-naphthol and 5-amino-1-naphthol, can also be hydroxylated by McbC.…”