Development and utilization of new O2-independent bioreporters

Fluorescent proteins have revolutionized science since their discovery in 1962. They have enabled imaging experiments to decipher the function of proteins, cells and organisms, as well as gene regulation. GFP and all its derivatives are now standard tools in cell biology, immunology, molecular biology and microbiology laboratories around the world. A common feature of these proteins is their O2-dependent maturation allowing fluorescence, which precludes their use in anoxic contexts. In this work, we report the development and in cellulo characterization of genetic circuits encoding the O2-independent KOFP-7 protein, a flavin-binding fluorescent protein. We have optimized the genetic circuit for high bacterial fluorescence at population and single-cell level, implemented this circuit in various plasmids differing in host range, and quantified their fluorescence under both aerobic and anaerobic conditions. Finally, we showed that KOFP-7 based constructions can be used to produce fluorescing cells of V. diazotrophicus, a facultative anaerobe, demonstrating the usefulness of the genetic circuits for various anaerobic bacteria. These genetic circuits can thus be modified at will, both to solve basic and applied research questions, opening a highway to shed light on the obscure anaerobic world. ImportanceFluorescent proteins are used since decades, and have allowed major discoveries in biology in a wide variety of fields, and are used in environmental as well as clinical contexts. GFP and all its derivatives share a common feature: they rely on the presence of O2 for protein maturation and fluorescence. This dependency precludes their use in anoxic environments. Here, we constructed a series of genetic circuits allowing production of KOFP-7, an O2-independant Flavin-Binding Fluorescent Protein. We demonstrated that Escherichia coli cells producing KOFP-7 are fluorescent, both at the population and single-cell levels. Importantly, we showed that, unlike cells producing GFP, cells producing KOFP-7 are fluorescent in anoxia. Finally, we demonstrated that Vibrio diazotrophicus NS1, a facultative anaerobe, is fluorescent in the absence of O2 when KOFP-7 is produced. Altogether, the development of new genetic circuits allowing O2-independent fluorescence will open new perspective to study anaerobic processes.