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Dibenzothiophene (DBT), a model compound for sulfur-containing organic molecules found in fossil fuels, can be desulfurized to 2-hydroxybiphenyl (2-HBP) by Rhodococcus sp. strain IGTS8. Complementation of a desulfurization (dsz) mutant provided the genes from Rhodococcus sp. strain IGTS8 responsible for desulfurization. A 6.7-kb TaqI fragment cloned in Escherichia coli-Rhodococcus shuttle vector pRR-6 was found to both complement this mutation and confer desulfurization to Rhodococcus fascians, which normally is not able to desulfurize DBT. Expression of this fragment in E. coli also conferred the ability to desulfurize DBT. A molecular analysis of the cloned fragment revealed a single operon containing three open reading frames involved in the conversion of DBT to 2-HBP. The three genes were designated dszA, dszB, and dszC. Neither the nucleotide sequences nor the deduced amino acid sequences of the enzymes exhibited significant similarity to sequences obtained from the GenBank, EMBL, and Swiss-Prot databases, indicating that these enzymes are novel enzymes. Subclone analyses revealed that the gene product of dszC converts DBT directly to DBT-sulfone and that the gene products of dszA and dszB act in concert to convert DBT-sulfone to 2-HBP.
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PMID:Sequence and molecular characterization of a DNA region encoding the dibenzothiophene desulfurization operon of Rhodococcus sp. strain IGTS8. 757 82

Dibenzothiophene (DBT), a model of organic sulfur compound in petroleum, is microbially desulfurized to 2-hydroxybiphenyl (2-HBP), and the gene operon dszABC was required for DBT desulfurization. The final step in the microbial DBT desulfurization is the conversion of 2'-hydroxybiphenyl-2-sulfinate (HBPSi) to 2-HBP catalyzed by DszB. In this study, DszB of a DBT-desulfurizing bacterium Rhodococcus erythropolis KA2-5-1 was overproduced in Escherichia coli by coexpression with chaperonin genes, groEL/groES, at 25 degrees C. The recombinant DszB was purified to homogeneity and characterized. The optimal temperature and pH for DszB activity were 35 degrees C and about 7.5, respectively. The K(m) and k(cat) values for HBPSi were 8.2 microM and 0.123.s(-1), respectively. DszB has only one cysteine residue, and the mutant enzyme completely lost the activity when the cysteine residue was changed to a serine residue. This result together with experiments using inhibitors showed that the cysteine residue contributes to the enzyme activity. DszB was also inhibited by a reaction product, 2-HBP (K(i)=0.25 mM), and its derivatives, but not by the other reaction product, sulfite. The enzyme showed a narrow substrate specificity: only 2-phenylbenzene sulfinate except HBPSi served as a substrate among the aromatic and aliphatic sulfinates or sulfonates tested. DszB was thought to be a novel enzyme (HBPSi desulfinase) in that it could specifically cleave the carbon-sulfur bond of HBPSi to give 2-HBP and sulfite ion without the aid of any other proteinic components and coenzymes.
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PMID:A novel enzyme, 2'-hydroxybiphenyl-2-sulfinate desulfinase (DszB), from a dibenzothiophene-desulfurizing bacterium Rhodococcus erythropolis KA2-5-1: gene overexpression and enzyme characterization. 1214 52

Removal of sulfur from petroleum can be accomplished by various means. One method is to use microorganisms, such as bacteria. In the present study, strain Rhodococcus sp. SDUZAWQ was employed to test the effects of various concentrations of dibenzothiophene (DBT) and sulfate, had on this removal process. Desulfurization was accomplished, using Basal Salts Medium (BSM), supplemented with 0.2mmol/L DBT and different concentrations of Na2SO4. Growth of SDUZAWQ was pronounced, even when the concentration of DBT was increased to 6mmol/L. Furthermore, it should be noted that the end product of DBT desulfurization, 2 hydroxybiphenyl (2-HBP), was detected as well. This finding was significant because it demonstrated the bacteria' s ability to withstand high concentrations of organosulfur. Dibenzothiophene was utilized when both DBT and Na2SO4 were present in the culture medium. Additionally, 2-HBP was produced. These data are in contrast to previous studies that indicated that DBT could not be metabolized by Rhodococcus sp. in the presence of sulfate. Finally, cloning and sequencing of the gene cluster dszABC, its upstream regulatory sequence and dszD, demonstrated that they share 99%, 100% and 100% with those of R. erythropolis IGTS8, respectively.
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PMID:[The innate ability of Rhodococcus sp. SDUZAWQ to tolerate sulfur in petroleum]. 1624 74

Biological dusulfurizaion of petroleum feedstocks and products may offer an attractive alternative to reduce sulfur oxide emissions that cause serious environmental pollution. Dibenzothiophene (DBT), a model of organic sulfur compound in petroleum, can be microbially desulfurized without degradation of the organic structure by 4S pathway. Three desulfurization enzymes (DszA, DszB and DszC) and flavin reductase (DszD) are involved in sulfur-specific DBT desulfurization. DszA and DszC are FMNH2-dependent monoxygenases, FMNH2 is provided from the freely diffusible FMNH2 pool in the cell, and is replenished by DszD. So, co-expression of the desulfurization enzymes and flavin reductase can enhance the rate of sulfur removal. In the present work two incompatible plasmids: pBADD and paN2 were constructed. The paN2 allows Escherichia coli to liberate the sulfur of DBT and DBTs and pBADD produces a flavin reductase. They were co-expressed in Escherichia coli B121 (DE3). The soluble products of DszA, DszB, DszC and DszD accounted for 7.6%, 3.5%, 3.1% and 18% of the total proteins in co-expressed system. The desulfurization rate of lysate of E. coli BL21- pBADD + paN2 is 12.03 micromol/(h x mg) Dsz protein and about 5.4-fold of that of E. coli BL2-paN2. Experiment were also conducted using resting cell with the 0.6 wt% DBT in n-hexadecane as model diesel oil. After 24 hours reaction, 0.42 mmol/L (about 84%) DBT was converted to 2-HBP by E. coli BL21- pBADD + paN2, however, there was only 0.08 mmol/L (about 16%) DBT was desulfurized by E. coli BL2-paN2. The maximum desulfurization rate of E.coli BL21-pBADD + paN2 is about 67 micromol/h. The result shows that DszD enhances the rate of 2-HBP production when co-expressed in vivo with the desulfurization enzymes.
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PMID:[Co-expression of Rhodococcus sp. DS-3 dszABC and dszD gene with incompatible plasmids in Escherichia coli]. 1673 91

In this study, complicated model sulfur compounds in crude oil were biodesulfurized in a batch process by microbial consortium enriched from oil contaminated soil. Dibenzothiophene (DBT) was selected as model sulfur compounds. Ultrasonic radiation was used to pre-oxidize the model sulfur compounds before the biodesulfurization (BDS) process. The enhancement mechanism of ultrasound pre-oxidation (UPO) on the biodesulfurization of DBT was investigated. The effects of initial conditions on the biodesulfurization of DBT in UPO/BDS system such as solution initial pH, DBT initial concentration, sulfur source, biocatalyst initial concentration, and incubation temperature were discussed. The results show that the application of UPO before BDS procedure significantly improved the efficiency of the biodesulfurization and allowed sulfur removal in shorter time through oxidizing DBT to DBT sulfone, resulting in shortening the "4S" pathway for biodesulfurization from 4 steps to 2 steps, enhancement in reaction velocity and enzyme-substrate affinity as well as reduction in substrate inhibition. The concentration of 2-HBP increased fast with the use of ultrasound pre-oxidation, which was dependent on solution initial pH, DBT initial concentration, sulfur source, biocatalyst initial concentration, and incubation temperature.
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PMID:Investigations in enhancement biodesulfurization of model compounds by ultrasound pre-oxidation. 3082 8