LIGHT-DRIVEN CYTOCHROME REACTIONS IN ANACYSTIS AND EUGLENA 

 John M. Olson and Robert M. Smillie 



The basic similarity in cytochrome physiology between Anacys - 

 tis nidulans and Eiiglena gracilis , strain Z, is impressive in 

 view of their gross dissimilarities in size, structure, and pig- 

 ment content. We have investigated the cytochrome reactions by 

 sensitive spectroj^otometric methods in order to gain some in- 

 sight into the patterns of energy transfer from the various light 

 receptors to the reaction centers involved in the two photochemi- 

 cal reactions of green plant photosynthesis and also to elucidate 

 the pathways of photosynthetic electron transfer. The major 

 thrust of this presentation will be the implications of experi- 

 ments on whole cells in which both wavelength and intensity of 

 monochrcmatic actinic light have been systematically varied. 

 Some preliminary observations of the effect of carbonyl cyanide-m 

 chlorophenylhydrazone (CCCP) are presented, and the light-driven 

 reduction of cytochrome b,- in Euglena chloroplast fragments is 

 described. 



INTACT ALGAE 



Two Light Reactions ; The evidence for two essential light re- 

 actions which most clearly laid the precedents for the present 

 work was that obtained by Kokd), Wittv^), and Duysens^^). The 

 light-induced oxidation of c- or f-type cytochromes in green 

 plants was clearly established, and the light-driven reduction by 

 a second photoreaction was demonstrated. We have confirmed the 

 observations by Amesz and Duysens(^) of light-driven cytochrome 

 reactions in Anacystis and have identified the major ccmponent to 

 be cytochrome f-555 on the basis of the alpha trough at 556 mii in 

 light-minus -dark difference spectra. In Euglena the high poten- 

 tial cytochrome -5 52 reacts to light. The effects of the two 

 photochemical reactions on these f-type cytochrcanes are illustra- 

 ted in Figure 1. In both algae far-red light causes a rapid oxi- 

 dation to the steady-state level; but light below a certain crit- 

 ical wavelength causes an initial rapid oxidation followed by a 

 slower reduction to the final steady-state when the proper inten- 

 sity is used. The diphasic kinetics disappear as the light in- 

 tensity is lowered as shown in Figure 2. The curves ("initial 

 peak" and "steady-state") for 0.62 ^ light in Figure 2 are 



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