Abstract
Biodesulfurization (BDS) is an environmentally friendly desulfurizing process with the potential of replacing or adding to the current expensive technologies for sulfur removal from fossil fuels. The BDS, however, still suffers from low biocatalyst activity. One reason is repression of dsz promoter transcription in presence of inorganic sulfate that impedes translation of Dsz enzymes required for desulfurization pathway. One approach to solve this problem is replacing the native promoter with a new promoter that is no longer repressed. In this study, dsz genes from desulfurizing strain Rhodococcus sp. FUM94 was cloned in an alkane responsive promoter, pCom8, and expressed in Escherichia coli BL21 (DE3) as a host. The recombinant was not susceptible to inorganic sulfate in the culture medium. Desulfurizing activity of recombinant strain versus wild type indicated that in a sulfate containing medium, BDS yield of recombinant increased from 16.0% ± 0.9 to 34.0% ± 1.9% when dibenzothiophene (DBT) concentration (dissolved in ethanol) increased from 25 to 100 ppm. Also, 2-hydroxy biphenyl (2-HBP) production rate improved 8.5-fold (from 0.302 ± 0.020 to 2.57 ± 0.14 mmol 2-HBP (kg DCW)-1 h-1) at the same DBT concentration range. This is while no 2-HBP production was detected in FUM94 biphasic reaction. In a sulfate-free medium, wild type strain demonstrated desulfurization activity, but decreasing with the increase of DBT concentration dissolved in n-tetradecane. Whereas, the recombinant strain demonstrated increasing desulfurizing activity in a sulfate-containing high DBT concentration environment. Overall, the result of this molecular manipulation can be considered as a step forward toward commercialization of BDS technology.
Highlights
Emission of certain hazardous pollutants such as sulfur oxides into the atmosphere resulting from the combustion of petroleum causes environmental impacts such as air pollution, and human health problems (Dube et al, 2014)
Despite the economic benefits of BDS over rival technologies for sulfur removal, this technology suffers from a major obstacle, i.e., low biocatalyst activities that prevents it from being used as a commercial technology (Kilbane, 2006)
One reason for the limited activity is that Dsz enzymes are sulfate-starvation-induced proteins (Tanaka et al, 2002) and desulfurization activity is repressed by preferred sulfur source, including inorganic sulfate and the sulfur containing amino acids methionine and cysteine (Piddington et al, 1995; Matsui et al, 2002)
Summary
Emission of certain hazardous pollutants such as sulfur oxides into the atmosphere resulting from the combustion of petroleum causes environmental impacts such as air pollution, and human health problems (Dube et al, 2014). One reason for the limited activity is that Dsz enzymes are sulfate-starvation-induced proteins (Tanaka et al, 2002) and desulfurization activity is repressed by preferred sulfur source, including inorganic sulfate and the sulfur containing amino acids methionine and cysteine (Piddington et al, 1995; Matsui et al, 2002). Shavandi et al (2009) utilized the lac promoter for self-cloning of dsz genes in Gordonia alkanivorans RIPI90A It resulted in 2.67-fold desulfurization activity of recombinant resting cells in comparison to native strain. All reported desulfurization improvements in the above-mentioned investigations were attributed to replacement of native promoter with a known promoter from other sources
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