LIGHT-INDUCED ABSORBANCE CHANGES IN PHYCOMYCES PHOTOMUTANTS

Abstract— Blue-light-induced absorbance changes indicating the reduction of a b -type cytochrome by a flavin photoreceptor have been reported elsewhere for Dictyostelium, Neurospora , and Phycomyces . On the basis of action spectra alone these absorbance changes were associated with the blue-light physiological photoresponses in the latter two organisms. We have investigated phototropically abnormal mutants of Phycomyces in order to determine if there were common defects in blue-light-induced absorbance changes and photophysiology. However, mutants representing four genes, madA, madB, madC , and madE , the first two of which are associated with early steps in the sensory pathway, showed blue-light-induced absorbance changes identical to those in a phototropically normal strain. A pigment kinetics model, which was used to interpret dose-response data, indicated a rather low quantum yield of 0.015 for the light-induced absorbance changes. Similar blue-light induced absorbance changes were observed in HeLa cells. These cells are derived from a human cervical carcinoma and are thus very unlikely to possess any natural photophysiology. Our results question the relevance of these blue-light-induced absorbance changes regarding the blue-light receptor for physiological responses in various organisms.