Reporter proteins for in vivofluorescence without oxygen

Fluorescent reporter proteins such as green fluorescent protein are valuable noninvasive molecular tools for in vivo real-time imaging of living specimens. However, their use is generally restricted to aerobic systems, as the formation of their chromophores strictly requires oxygen. Starting with blue-light photoreceptors from Bacillus subtilis and Pseudomonas putida that contain light-oxygen-voltage–sensing domains, we engineered flavin mononucleotide–based fluorescent proteins that can be used as fluorescent reporters in both aerobic and anaerobic biological systems.