The Lux System
The bacterial lux system is principally composed of five genes - LuxA, LuxB, LuxC, LuxD, and LuxE.
[Operon diagram with their expression in nature, ie, CDABE w/ caption: In their host bacteria, the lux genes are not sequentially ordered, with A and B flanked by C, D, and E.]
The proteins coded by these genes associate to form two enzymatic complexes, with LuxA+LuxB coding for luciferase itself and LuxCDE coding for a fatty acid reductase complex. This reductase complex serves to provide the substrates (fatty aldehydes) for the system’s bioluminescent reaction1. Catalyzed by luciferase, these aldehydes react with FMNH2 and oxygen, emitting a photon and producing a fatty acid, FMN, and water3. FMNH2 is then regenerated by a flavin reductase1.
[Complex diagram w/ elaborating caption: The alpha subunit of luciferase drives enzymatic activity, while the beta subunit offers structural support and stabilizes the alpha subunit as it undergoes conformational changes4.]
To allow for continuous light output, the fatty acid reductase complex recycles the fatty acid product in the luciferase-catalyzed reaction and converts it to a the substrate aldehyde. LuxC, LuxD, and LuxE code for a reductase, transferase, and synthetase, respectively. Together, they associate into a complex consisting of four of each enzyme3.
[Complex diagram w/ elaborating caption: Each enzyme is present in equal proportions in the fully-formed complex. Substrates are recruited by the transferase and moved to a synthetase-reductase complex. These associated enzymes produce a microenvironment that stabilizes reaction intermediates.]
Because the luminescent yield of the system is based on the function of these two enzymatic complexes, modifying the protein levels of each enzyme allows us to control the system’s output.