Cloning and expression of the flavin reductase LuxG from Photobacterium leiognathi YL and its improvement for NADH detection
In the present study, we aimed to purify and characterize LuxG obtained from Photobacterium leiognathi YL and examine its improvement for NADH detection. To this end, we cloned and expressed the putative luxG gene of P. leiognathi YL in the Escherichia coli BL21 strain. The product of luxG is a flavin reductase that consists of 206 amino acids, corresponding to a subunit molecular mass of ∼26 kDa. Phylogenetic analysis demonstrated that P. leiognathi YL LuxG has a rather distant evolutionary relationship with Frase I of Aliivibrio fischeri and Frp of Vibrio harveyi, but a close evolutionary relationship with Fre from Escherichia coli, which are all enzymes related to oxido-reductase. Further comparison shows that the changes in the functionally conserved sites may contribute to the functional divergence of LuxG and Fre. LuxG could supply reduced flavin mononucleotide (FMN) for bacterial luminescence by catalyzing the oxidation of nicotinamide adenine dinucleotide hydrogen (NADH). Based on this, a coupled pure enzyme bioluminescent system was established and used for NADH detection. The NADH samples with concentrations of 0.1–1 nM were used to validate the linear relationship, and it was found that the logarithmic deviations were less than 3%, which showed more sensitive and stable results than the NADH detection by recombinant E. coli including the exogenously expressed luciferase and intrinsic Fre. Investigation of P. leiognathi YL LuxG would provide a basic understanding of its evolution, and structural and functional properties, which might contribute to the development of a NADH detection kit in the future.