Blue light-induced absorbance changes were measured from differentially centrifuged membrane fractions from dark-grown coleoptiles of Zea mays L., and mycelia from an albino mutant of Neurospora crassa Actinic irradiation caused changes in absorbance consistent with a flavinmediated reduction of a b-type cytochrome. Both corn and Neurospora showed similar light-minus-dark difference spectra, dose response curves, and kinetics of dark recovery after irradiation. The photoreducible cytochrome system from Neurospora showed the same distribution as the activity of a sodium-stimulated adenosine triphosphatase, thought to be a plasma membrane marker, in differential centrifugation experiments. The fraction showing the absorbance change did not co-sediment with the mitochondria, nor with the endoplasmic reticulum. Comparison of absorption spectra of fully oxidized, partially reduced, and fully reduced preparations showed that approximately a 30% reduction of the cytochromes involved with the process was needed to obtain the lightinduced absorbance changes.There is currently a substantial body of literature concerning the effects of blue light on biological processes in higher plants and fungi. Only recently has any real progress been made toward the identification of the blue light photoreceptor associated with these effects (see ref. 6). Poff and Butler (16) ation in that report was the separation of soluble from pelletable components. Mufioz and Butler (15) did report light-inducible Cyt reduction in soluble extracts of Neurospora, but they studied supernatant rather than pelletable activity, observed reduction both of Cyt b and c, and had to add FMN or FAD in order to observe the reaction. It seems likely that they were studying the same phenomenon as Schmidt and Butler (20) who found photoreduction of Cyt c in the presence of added flavins in a model system. Widell and Bjorn (24) have reported light-induced A changes in another higher plant system, intact wheat coleoptiles, but the responses measured were not the same as those monitored by Briggs (5) or Munioz and Butler (15).The purposes of the study reported here were: (a) to isolate and characterize partially purified membrane fractions of Neurospora with a photoreduction system similar to that found in intact mycelia by Munoz and Butler; (b) to study further the blue light-induced A changes found in corn membrane fractions and compare them to similar changes found in Neurospora; and (c) to determine qualitatively the redox state necessary for observation of these changes. A preliminary account of these studies has appeared elsewhere (3, 7). To prepare the mycelia, approximately 70-ml liquid suspensions were made of the 7-day-old agar cultures, and added to a 2,000-ml Erlenmeyer flask, filled with 1,000 ml of liquid medium (the same as solid medium except dextrose instead of sucrose, and no agar). The cultures were grown aerobically in the dark at 30 C for 24 hr at 250 rpm in an incubating rotary shaker (New Brunswick Scientific,.
MATERIALS AND METHODSA...