On the in vivo redox state of flavin-containing photosensory receptor proteins

Measured values of the redox midpoint potential of flavin-containing photoreceptor proteins range from physiologically very negative values, i.e., < -300 mV (compared to the calomel electrode) for some LOV domains, to slightly positive values for some cryptochromes. The actual intracellular redox potential of several key physiological electron-transfer intermediates, like the nicotinamide dinucleotides, particularly in chemoheterotrophic bacteria, may be varying beyond these two values, and are subject to physiological- and environmental regulation. The photochemical activity of photoreceptor proteins containing their flavin chromophore in the reduced, and in the fully oxidized form, is very different. We therefore have addressed the question whether or not the functioning of these flavin-containing photosensory receptors in vivo is subject to redox regulation. Here we (1) provide further evidence for the overlap of the ranges of the redox midpoint potential of the flavin in a specific photoreceptor protein and the redox potential of key intracellular redox-active metabolites, and (2) demonstrate that the redox state and photochemical activity of LOV domains can be recorded in vivo in Escherichia coli. Significantly, so far in vivo reduction of LOV domains under physiological conditions could not be detected. The implications of these observations are discussed.