Light-induced absorbance changes in Phycomyces : evidence for cryptochrome-associated flavosemiquinones

Abstract

Light-induced absorbance changes (LIACs), which are associated with early photochemical events of blue-light transduction, were detected in growing zones of Phycomyces sporangiophores. The novel LIACs meet all the essential requirements for a spectrophotometric photoreceptor assay which was previously unavailable for blue-light receptors (cryptochromes). In-vivo absorption spectra of growing zones were derived from reflection spectra which were measured with a novel rapid-scan spectrophotometer. To detect photoreceptor-associated absorbance changes white mutants were employed which lack the interfering bulk pigment beta-carotene. Blue and white light, not however red light, induced in these strains absorbance changes near 460-490 and 600-620 nm. The LIACs were absent in light-insensitive mutants with defects in the genes madA, madB and madC. Because these genes affect photosensory adaptation and the blue-light receptor itself, the novel in-vivo LIACs must be associated with photochemical events which occur early in the transduction chain. The spectral characteristics of the LIACs are in accordance with a blue- and red-light absorbing flavosemiquinone which is generated upon light absorption by an oxidized flavin receptor. It is proposed that the flavosemiquinone functions itself as photoreceptor which mediates several red-light responses of Phycomyces.

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

“…Blue, not however red, light elicits absorbance changes (LIACs) in sporangiophores of P. blakesleeanus that occur at the level of the blue-light photoreceptor. The reaction scheme of the LIACs implies a blue-light induced photoreduction of the oxidized flavin photoreceptor to the flavo-semiquinone state (half-reduced state; Schmidt and Galland, 1999). 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.…”

“…The relative absorbance, A, is calculated from the reflected light according to the following definition: A ¼ R 460 /R 665 , where R 460 is the reflectance at 460 and R 665 the reflectance at 665 nm. If DR << R, this formula replaces the correct definition A ¼ log R 460 /R 665 (see ''Materials and Methods'';Schmidt and Galland, 1999).…”

“…We have found light-induced absorbance changes (LIACs) that occur in the growing zones of sporangiophores in response to blue-light stimulation. These LIACs are specifically associated with the blue-light receptor of P. blakesleeanus and indicate the generation of flavosemiquinone from the oxidized flavin receptor (Schmidt and Galland, 1999). When we analyzed with the same method the gravitropism of the sporangiophore we found in earth-bound experiments gravity-induced absorbance changes (GIACs) that differed substantially from the LIACs .…”

“…rapid-scan spectrometry (Schmidt, 1997;Schmidt and Galland, 1999), to the unicellular fungus, Phycomyces blakesleeanus, which has served in the past mainly as a model system of phototropism (Galland, 2002) and to a lesser extent also of gravitropism (Dennison, 1961;Dennison and Shropshire, 1984;Schimek et al, 1999aSchimek et al, , 1999bGrolig et al, 2000). As a microorganism, P. blakesleeanus possesses several features, such as a short life cycle, amenability to classical and molecular genetics, and the availability of many phototropism and gravitropism mutants, which render it an ideal system to investigate photo-and graviorientation even in the confinements of limited space and time.…”

“…To detect primary responses that are associated with the phototropism and the gravitropism of the sporangiophore of P. blakesleeanus we took advantage of a novel instrument, the rapid-scan spectrometer (RSS; Schmidt 1997; Schmidt and Galland, 1999). We have found light-induced absorbance changes (LIACs) that occur in the growing zones of sporangiophores in response to blue-light stimulation.…”