Studies of the blue light receptor in Phycomyces

Abstract

A large number of organisms possess a variety of responses to blue and long-wavelength ultraviolet light. Riboflavin and [beta]-carotene have long been considered the major candidates for the photoreceptor mediating these so-called blue-light physiological responses. The fungus Phycomyces possesses several responses to blue light, among which are induction of [beta]-carotene synthesis in the mycelium and sporangiophore growth and tropism. In this thesis, it is conclusively demonstrated that a carotenoid is not the photoreceptor for the responses of Phycomyces sporangiophores to blue light. The contention that the photoreceptor is riboflavin is further strengthened by action spectral evidence consistent with the direct excitation of the photochemically active flavin triplet state. Light-induced cytochrome optical absorbance changes are investigated in in vivo preparations of Phycomyces; an action spectrum determination shows such absorbance changes to be mediated by a flavin photoreceptor. However, no evidence linking these absorbance changes to possible steps in the sensory transduction pathway was found. Indeed, the findings that the flavin-mediated cytochrome absorbance changes occur with a low quantum yield in Phycomyces and that they also occur in cells from a human cervical carcinoma speak against their possible relevance to the physiological blue-light receptor. Electron paramagnetic resonance spectroscopy and fluorescence lifetime spectroscopy are also investigated as possible probes of the flavin photoreceptor. However, the widespread occurrence of riboflavin in cellular roles other than photoreception makes it difficult to separate out that particular flavin which functions as the physiological blue-light receptor. It represents a case of a photoreceptor which is at once ubiquitous and elusive. Finally, blue-light-induced synthesis of [beta]-carotene is investigated in the wildtype and in several sensory mutants of Phycomyces.