A novel cold-adapted nitroreductase from Psychrobactersp. ANT206: Heterologous expression, characterization and nitrobenzene reduction capacity

Abstract

Widely used in multiple industrial processes, nitro-aromatic compounds, especially nitrobenzene, in low temperature environment are considered as heavy pollutants. Among the disposal methods, the bioreduction method has attracted much attention. In this study, a novel cold-adapted nitroreductase gene (psntr) was cloned from Antarctic sea-ice bacteria Psychrobacter sp. ANT206. The psntr gene was 813 bp in length and encoded a protein with flavin mononucleotide (FMN) binding sites. Homology modeling was performed to obtain structural information such as the longer loops and reduced amount of hydrogen bonds, which might be related to the high catalytic efficiency of PsNTR at low temperature. The psntr gene was successfully cloned in cold-shock pCold I vector and transformed to the expression host Escherichia coli (E. coli) BL21 with the induction by isopropyl β-D-thiogalactoside (IPTG) at low temperature (16 °C) for 24 h. The recombinant PsNTR (rPsNTR) was purified using Ni-NTA with the specific activity of 51.59 μmol/min/mg. Interestingly, rPsNTR displayed the highest activity at 25 °C and still maintained 46.9% of the activity at 0 °C. rPsNTR also exhibited the highest activity (136.4%) at 1.0 M NaCl with incredible salt tolerance. The kinetic parameters and substrates specificity analysis demonstrated that rPsNTR could reduce various nitro-aromatic compounds. Moreover, the result of the reduction capability revealed that the recombinant E. coli exhibited a maximum nitrobenzene reduction rate of 3.03 mM/h at 16 °C. These findings revealed that the characteristics of rPsNTR might make it an excellent candidate for the bioreduction of various nitro-aromatic compounds in the low temperature and high-salt wastewater.