Light-induced fluorescence changes in Phycomyces : evidence for blue light-receptor associated flavo-semiquinones

Abstract

Light-induced fluorescence changes (LIFCs) were detected in sporangiophores of the blue-light-sensitive fungus Phycomyces blakesleeanus (Burgeff). The LIFCs can be utilized as a spectrophotometric assay for blue-light photoreceptors and for the in vivo characterization of their photochemical primary reactions. Blue-light irradiation of sporangiophores elicited a transient decrease and subsequent regeneration of flavin-like fluorescence emission at 525 nm. The signals recovered in darkness in about 120 min. In contrast to blue light, near-UV (370 nm) caused an increase in the fluorescence emission at 525 nm. Because the LIFCs were altered in a light-insensitive madC mutant with a defective photoreceptor, the fluorescence changes must be associated with early photochemical events of the transduction chain. Action spectra for the fluorescence changes at 525 nm showed major peaks near 470 and 600 nm. Double-pulse experiments involving two consecutive pulses of either blue and near-UV, blue and red, or near-UV and red showed that the responses depended on the sequence in which the different wavelengths were applied. The results indicate a blue-light receptor with intermediates in the near-UV, blue and red spectral regions. We explain the results in the framework of a general model, in which the three redox states of the flavin photoreceptor, the oxidized flavin (Fl), the flavo-semiquinone (FlH*), and the flavo-hydroquinone (FlH2) are each acting as chromophores with their own characteristic photochemical primary reactions. These consist of the photoreduction of the oxidized flavin generating semiquinone, the photoreduction of the semiquinone generating hydroquinone, and the photooxidation of the flavo-hydroquinone regenerating the pool of oxidized flavins. The proposed mechanism represents a photocycle in which two antagonistic photoreceptor forms, Fl and FlH2, determine the pool size of the biological effector molecule, the flavo-semiquinone. The redox changes that are associated with the photocycle are maintained by redox partners, pterins, that function in the near-UV as secondary chromophores.

A Flavin Binding Cryptochrome Photoreceptor Responds to Both Blue and Red Light in Chlamydomonas reinhardtii

“…also as red light sensors, since the absorption of the neutral radical state of flavoproteins, in most cases, extends well beyond 600 nm (Müller et al, 1972). Previous studies suggested a role of the neutral radical in photoreceptors, but only as the signaling state, which is formed from the oxidized state by the absorption of blue light (Galland and Tölle, 2003). Accordingly, illumination of the radical would then lead to depletion of the signaling state.…”

Optospectroscopic Detection of Primary Reactions Associated with the Graviperception of Phycomyces. Effects of Micro- and Hypergravity

“…The data presented in this work can be best explained by the assumption that gravistimulation modulates the pool of these flavo-semiquinones that need to be present in darkness and that are substantially boosted by blue-light irradiation. Evidence for the presence of such flavo-semiquinones in darkness and particularly after blue-light irradiation has been presented recently for P. blakesleeanus (Galland and Tö lle, 2003). We thus propose that graviperception involves and largely depends on the blue-light photoreceptor system.…”

Enhancement of autofluorescence of the brown-rot fungus Piptoporus betulinus by metal ions

“…According to Galland and Tölle (2003), natural fluorescence (as seen in living tissue cultures) is due in large part to substances such as flavins and porphyrins, e.g. chlorophyll.…”