Evidence for the generation of myristylated FMN by bacterial luciferase

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.

The impact of LuxF on light intensity in bacterial bioluminescence

The enzymes involved in bacterial bioluminescence are encoded in the lux operon with a conserved gene order of luxCDABEG. Some photobacterial strains carry an additional gene, termed luxF, which produces the LuxF protein, whose function and influence on bacterial bioluminescence is still uncertain. The LuxF protein binds the flavin derivative 6-(3'-(R)-myristyl)-flavin mononucleotide (myrFMN), which is generated as a side product in the luciferase-catalyzed reaction. This study utilized an Escherichia coli (E. coli) based lux operon expression system where the lux operons of Photobacterium leiognathi subsp. mandapamensis 27561 or of Photobacterium leiognathi subsp. leiognathi 25521, namely luxCDAB(F)EG, were cloned into a single expression vector. Exclusion of luxF gene from the lux operon enabled novel insights into the role of LuxF protein in light emission. E. coli cultures harboring and expressing the genes of the lux operon including luxF gene emit more light than without luxF gene. Furthermore, isolation of the tightly bound flavin derivative revealed the presence of at least three different flavin derivatives. Analysis by UV/Vis absorption and NMR spectroscopy as well as mass spectrometry showed that the flavin derivatives bear fatty acids of various chain lengths. This distribution of FMN derivatives is vastly different to what was found in bioluminescent bacteria and indicates that the luciferase is supplied with a range of aldehyde substrates in E. coli.

The Sensitized Bioluminescence Mechanism of Bacterial Luciferase

After more than one-half century of investigations, the mechanism of bioluminescence from the FMNH₂ assisted oxygen oxidation of an aliphatic aldehyde on bacterial luciferase continues to resist elucidation. There are many types of luciferase from species of bioluminescent bacteria originating from both marine and terrestrial habitats. The luciferases all have close sequence homology, and in vitro, a highly efficient light generation is obtained from these natural metabolites as substrates. Sufficient exothermicity equivalent to the energy of a blue photon is available in the chemical oxidation of the aldehyde to the corresponding carboxylic acid, and a luciferase-bound FMNH-OOH is a key player. A high energy species, the source of the exothermicity, is unknown except that it is not a luciferin cyclic peroxide, a dioxetanone, as identified in the pathway of the firefly and the marine bioluminescence systems. Besides these natural substrates, variable bioluminescence properties are found using other reactants such as flavin analogs or aldehydes, but results also depend on the luciferase type. Some rationalization of the mechanism has resulted from spatial structure determination, NMR of intermediates and dynamic optical spectroscopy. The overall light path appears to fall into the sensitized class of chemiluminescence mechanism, distinct from the dioxetanone types.

Molecular Mechanisms of Bacterial Bioluminescence

Bioluminescence refers to the production of light by living organisms. Bioluminescent bacteria with a variety of bioluminescence emission characteristics have been identified in Vibrionaceae, Shewanellaceae and Enterobacteriaceae. Bioluminescent bacteria are mainly found in marine habitats and they are either free-floating, sessile or have specialized to live in symbiosis with other marine organisms. On the molecular level, bioluminescence is enabled by a cascade of chemical reactions catalyzed by enzymes encoded by the lux operon with the gene order luxCDABEG. The luxA and luxB genes encode the α- and β- subunits, respectively, of the enzyme luciferase producing the light emitting species. LuxC, luxD and luxE constitute the fatty acid reductase complex, responsible for the synthesis of the long-chain aldehyde substrate and luxG encodes a flavin reductase. In bacteria, the heterodimeric luciferase catalyzes the monooxygenation of long-chain aliphatic aldehydes to the corresponding acids utilizing reduced FMN and molecular oxygen. The energy released as a photon results from an excited state flavin-4a-hydroxide, emitting light centered around 490 nm. Advances in the mechanistic understanding of bacterial bioluminescence have been spurred by the structural characterization of protein encoded by the lux operon. However, the number of available crystal structures is limited to LuxAB (Vibrio harveyi), LuxD (Vibrio harveyi) and LuxF (Photobacterium leiognathi). Based on the crystal structure of LuxD and homology models of LuxC and LuxE, we provide a hypothetical model of the overall structure of the LuxCDE fatty acid reductase complex that is in line with biochemical observations.