Construction and characterization of hybrid luciferases coded by lux genes from Xenorhabdus luminescens and Vibrio fischeri

Differential transfers of reduced flavin cofactor and product by bacterial flavin reductase to luciferase

It is believed that the reduced FMN substrate required by luciferase from luminous bacteria is provided in vivo by NAD(P)H-FMN oxidoreductases (flavin reductases). Our earlier kinetic study indicates a direct flavin cofactor transfer from Vibrio harveyi NADPH-preferring flavin reductase P (FRP(H)) to the luciferase (L(H)) from the same bacterium in the in vitro coupled luminescence reaction. Kinetic studies were carried out in this work to characterize coupled luminescence reactions using FRP(H) and the Vibrio fischeri NAD(P)H-utilizing flavin reductase G (FRG(F)) in combination with L(H) or luciferase from V. fischeri (L(F)). Comparisons of K(m) values of reductases for flavin and pyridine nucleotide substrates in single-enzyme and luciferase-coupled assays indicate a direct transfer of reduced flavin, in contrast to free diffusion, from reductase to luciferase by all enzyme couples tested. Kinetic mechanisms were determined for the FRG(F)-L(F) and FRP(H)-L(F) coupled reactions. For these two and the FRG(F)-L(H) coupled reactions, patterns of FMN inhibition and effects of replacement of the FMN cofactor of FRP(H) and FRG(F) by 2-thioFMN were also characterized. Similar to the FRP(H)-L(H) couple, direct cofactor transfer was detected for FRG(F)-L(F) and FRP(H)-L(F). In contrast, despite the structural similarities between FRG(F) and FRP(H) and between L(F) and L(H), direct flavin product transfer was observed for the FRG(F)-L(H) couple. The mechanism of reduced flavin transfer appears to be delicately controlled by both flavin reductase and luciferase in the couple rather than unilaterally by either enzyme species.

Formation of active bacterial luciferase between interspecific subunits in vivo

Interspecific complementation between luxAs and luxBs from Vibrio harveyi, Vibrio fischeri, Photobacterium leiognathi and Xenorhabdus luminescens was examined in vivo. The individual genes from these species were cloned on different compatible plasmids or amplified by PCR and brought together to yield cis combinations without extraneous DNA. The beta subunits from V. harveyi and X. luminescens form active enzyme only with alpha subunits from one of these species. All other combinations yield active enzymes. The lack of activity of the V. harveyi and X. luminescens beta subunits with the alpha subunits from V. fischeri and P. leiognathi results from a lack of association. This was shown by in vivo competition in which these beta subunits were overproduced in comparison with the beta and alpha of V. fischeri. No reduction in light was found. Overall, the in vivo results parallel those found in vitro using isolated denatured subunits and renaturation by removal of the denaturant.