Functional divergence between evolutionary‐related LuxG and Fre oxidoreductases of luminous bacteria

Similarities and Differences in Quorum Sensing-Controlled Bioluminescence between Photobacterium phosphoreum T3 and Vibrio qinghaiensis sp.-Q67

Quorum sensing is a density-dependent mechanism using chemical signal molecules termed autoinducers to regulate diverse biological processes in bacteria, including bioluminescence. However, the correlation between growth and light emission of two typical luminescent bacteria, Photobacterium phosphoreum T3 and Vibrio qinghaiensis sp.-Q67, is still unclear. This study investigates the variations of bioluminescence and the light-emission-involved gene expression of the above two strains, respectively, showing that bioluminescence is population density-dependent. Furthermore, the effect of crude extracts (175, 350, 700 and 1750 mg/L) from the bacterial culture that contains the potential autoinducers on the bioluminescence is explored. At the exponential and the early stationary growth phase, T3 did not exhibit an obvious light intensity and cell density change after adding crude extracts at 175 and 350 mg/L, while the light intensity decreased at 700 and 1750 mg/L, showing a luminescence inhibition. For Q67, the light intensity increased dramatically with crude extract concentration. These results suggest that the bioluminescence process of both T3 and Q67 is controlled by quorum sensing. Furthermore, the different response modes of these two strains to autoinducers imply that the two strains could be applied to different compounds for toxicity assesses.

Mathematical Modeling Approaches for Assessing the Joint Toxicity of Chemical Mixtures Based on Luminescent Bacteria: A Systematic Review

Developments in industrial applications inevitably accelerate the discharge of enormous substances into the environment, whereas multi-component mixtures commonly cause joint toxicity which is distinct from the simple sum of independent effect. Thus, ecotoxicological assessment, by luminescent bioassays has recently brought increasing attention to overcome the environmental risks. Based on the above viewpoint, this review included a brief introduction to the occurrence and characteristics of toxic bioassay based on the luminescent bacteria. In order to assess the environmental risk of mixtures, a series of models for the prediction of the joint effect of multi-component mixtures have been summarized and discussed in-depth. Among them, Quantitative Structure-Activity Relationship (QSAR) method which was widely applied in silico has been described in detail. Furthermore, the reported potential mechanisms of joint toxicity on the luminescent bacteria were also overviewed, including the Trojan-horse type mechanism, funnel hypothesis, and fishing hypothesis. The future perspectives toward the development and application of toxicity assessment based on luminescent bacteria were proposed.

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.