LuxG Is a Functioning Flavin Reductase for Bacterial Luminescence

The 'luxG' gene is part of the 'lux' operon of marine luminous bacteria. 'luxG' has been proposed to be a flavin reductase that supplies reduced flavin mononucleotide (FMN) for bacterial luminescence. However, this role has never been established because the gene product has not been successfully expressed and characterized. In this study, 'luxG' from 'Photobacterium leiognathi' TH1 was cloned and expressed in 'Escherichia coli' in both native and C-terminal His6-tagged forms. Sequence analysis indicates that the protein consists of 237 amino acids, corresponding to a subunit molecular mass of 26.3 kDa. Both expressed forms of 'LuxG' were purified to homogeneity, and their biochemical properties were characterized. Purified 'LuxG' is homodimeric and has no bound prosthetic group. The enzyme can catalyze oxidation of NADH in the presence of free flavin, indicating that it can function as a flavin reductase in luminous bacteria. NADPH can also be used as a reducing substrate for the 'LuxG' reaction, but with much less efficiency than NADH. With NADH and FMN as substrates, a Lineweaver-Burk plot revealed a series of convergent lines characteristic of a ternary-complex kinetic model. From steady-state kinetics data at 4°C pH 8.0, Km for NADH, Km for FMN, and kcat were calculated to be 15.1 μM, 2.7 μM, and 1.7 s‾¹, respectively. Coupled assays between LuxG and luciferases from 'P. leiognathi' TH1 and Vibrio campbellii also showed that LuxG could supply FMNH‾ for light emission in vitro. A 'luxG' gene knockout mutant of 'P. leiognathi' TH1 exhibited a much dimmer luminescent phenotype compared to the native 'P. leiognathi' TH1, implying that 'LuxG' is the most significant source of FMNH‾ for the luminescence reaction in vivo.