Biodegradation of malachite green by strain Pseudomonas sp. K9 and cloning of the tmr2 gene associated with an ISPpu12

Hydrolase CehA and a Novel Two-Component 1-Naphthol Hydroxylase CehC1C2 are Responsible for the Two Initial Steps of Carbaryl Degradation in Rhizobium sp. X9

Carbaryl is a widely used carbamate pesticide in agriculture. The strain Rhizobium sp. X9 possesses the typical carbaryl degradation pathway in which carbaryl is mineralized via 1-naphthol, salicylate, and gentisate. In this study, we cloned a carbaryl hydrolase gene cehA and a novel two-component 1-naphthol hydroxylase gene cehC1C2. CehA mediates carbaryl hydrolysis to 1-naphthol and CehC1, an FMNH₂ or FADH₂-dependent monooxygenase belonging to the HpaB superfamily, and hydroxylates 1-naphthol in the presence of reduced nicotinamide-adenine dinucleotide (FMN)/flavin adenine dinucleotide (FAD), and the reductase CehC2. CehC1 has the highest amino acid similarity (58%) with the oxygenase component of a two-component 4-nitrophenol 2-monooxygenase, while CehC2 has the highest amino acid similarity (46%) with its reductase component. CehC1C2 could utilize both FAD and FMN as the cofactor during the hydroxylation, although higher catalytic activity was observed with FAD as the cofactor. The optimal molar ratio of CehC1 to CehC2 was 2:1. The K_m and K_cat/K_m values of CehC1 for 1-naphthol were 74.71 ± 16.07 μM and (8.29 ± 2.44) × 10^-4 s^-1·μM^-1, respectively. Moreover, the enzyme activities and substrate spectrum between CehC1C2 and previously reported 1-naphthol hydroxylase McbC were compared. The results suggested that McbC had a higher 1-naphthol hydroxylation activity, while CehC1C2 had a broader substrate spectrum.

Bioremediation of malachite green by cyanobacterium Synechococcus elongatus PCC 7942 engineered with a triphenylmethane reductase gene

Malachite green is a carcinogenic dye that has been detected in fish tissues and freshwater. Here we evaluated the malachite green decoloring ability of a photoautotrophic cyanobacterium, Synechococcus elongatus PCC 7942 (Synechococcus), that lives in freshwater. Results show that 99.5% of the dye was removed by Synechococcus through bioabsorption and bioaccumulation; however, the dye was not degraded or chemically modified. Next, we established an engineered Synechococcus strain to degrade the dye after uptake. The triphenylmethane reductase gene katmr was heterologously expressed, resulting in high production of a soluble recombinant protein. The engineered strain showed advanced decoloring abilities at a low cell density and in stressful environments. It degraded malachite green into the smaller molecules 4-methylaminobenzoic acid and 4-hydroxyl-aniline. After treatment with the engineered cyanobacterium, the growth of wheat seeds was fully recovered in the presence of malachite green. These results demonstrate the potential application of the engineered Synechococcus as a photosynthetic cell factory for the removal of malachite green from wastewater.

Organic compounds stimulate horizontal transfer of antibiotic resistance genes in mixed wastewater treatment systems

Domestic wastewater treatment plants as a reservoir of antibiotic resistance genes (ARGs) have received much attention, but the effect of dyes on the propagation of ARGs has rarely been investigated. In this study, we investigated the differences in distributions of ARGs and microbial communities using high-throughput qPCR and 16S rRNA gene sequencing, respectively, between mixed (dyeing and domestic) wastewater and domestic sewage. The relative abundance of ARGs in inflows of mixed wastewater (IW2 and IW3) was higher than that of domestic wastewater (IW1). The relative abundance of mobile genetic elements in the inflow of textile dyeing wastewater (IDW3) was 3- to 13-fold higher than that in other samples. Moreover, in IDW3, some distinct high abundance ARGs, particularly operons encoding efflux pumps (such as acrR-01, acrB-01 and acrF), were significantly correlated with Streptococcus of the Firmicutes. To explore why the abundance of ARGs was relatively high in mixed wastewater, six representative types of organic compounds in textile dyeing wastewater were used to test the effect on plasmid-based conjugative transfer from E. coli HB101 to E. coli NK5449. These six compounds all facilitated the transfer of resistance-carrying RP4 plasmid, and the highest transfer frequency (approximately 10^-5-10^-3) was over 4- to 200-fold higher than that in the control group (approximately 10^-6-10^-5). These results illustrated that the six common residual compounds, particularly low-dose substances in IDW3, could facilitate the dissemination of ARGs in aquatic environments. More importantly, this study revealed for the first time that dyeing contaminants influenced horizontal gene transfer (HGT) of ARGs.

Genetic characterization of plasmid-associated triphenylmethane reductase in Listeria monocytogenes

The enzyme triphenylmethane reductase (TMR) reduces toxic triphenylmethane dyes into colorless, nontoxic derivatives, and TMR-producing microorganisms have been proposed as bioremediation tools. Analysis of the genome of Listeria monocytogenes H7858 (1998-1999 hot dog outbreak) revealed that the plasmid (pLM80) of this strain harboring a gene cassette (bcrABC) conferring resistance to benzalkonium chloride (BC) and other quaternary ammonium disinfectants also harbored a gene (tmr) highly homologous to TMR-encoding genes from diverse Gram-negative bacteria. The pLM80-associated tmr was located two genes downstream of bcrABC as part of a putative IS1216 composite transposon. To confirm the role of tmr in triphenylmethane dye detoxification, we introduced various tmr-harboring fragments of pLM80 in a pLM80-cured derivative of strain H7550, from the same outbreak as H7858, and assessed the resistance of the constructs to the triphenylmethane dyes crystal violet (CV) and malachite green. Transcriptional and subcloning data suggest that the regulation of TMR is complex. Constructs harboring fragments spanning bcrABC and tmr were CV resistant, and in such constructs tmr transcription was induced by sublethal levels of either BC or CV. However, constructs harboring only tmr and its upstream intergenic region could also confer resistance to CV, albeit at lower levels. Screening a panel of BC-resistant L. monocytogenes strains revealed that all those harboring bcrABC and adjacent pLM80 sequences, including tmr, were resistant to CV and decolorized this dye. The findings suggest a potential role of TMR as a previously unknown adaptive attribute for environmental persistence of L. monocytogenes.

Identification of two combined genes responsible for dechlorination of 3,5,6-trichloro-2-pyridinol (TCP) in Cupriavidus pauculus P2

Dehalogenation is an important mechanism for degrading and detoxifying halogenated aromatics in microbes. However, the biochemical and molecular mechanisms of dehalogenation of 3,5,6-trichloro-2-pyridinol (TCP) are still unknown. In this study, a novel 6012 bp gene cluster was cloned from TCP-degrading strain P2, which was responsible for the dehalogenation of TCP. The cluster included a monooxygenase gene (tcpA1), a flavin reductase gene (tcpB1), tcpR1, orf1 and orf2. TcpA1 and TcpB1 were indispensable for the dehalogenation of TCP. They worked together to catalyze the dehalogenation of three chlorine of TCP, and generated a more readily biodegradable product of 3,6-dihydroxypyridine-2,5-dione. TcpA1 displayed the highest activity against TCP at 40°C and at pH 8.0. Cu(2+), Zn(2+), and Hg(2+) significantly inhibited enzyme activity. To the best of our knowledge, this is the first report on a gene cluster responsible for TCP degradation.