In the search for the photoreceptor in photocontrolled phycoerythrin formation, photoreversible absorption changes of chromoproteins in vivo and in vitro were studied with the blue-green alga Tolypothrix tenuis. Neither intact cells nor crude extracts of soluble proteins showed any significant absorption changes which were reversibly induced by green and red light. However, the photoresponse was observable when the crude protein extracts were treated with the chaotropic reagent guanidine-HCl (0.4 M, for 1 hr in the dark). Isolated phycocyanin and allophycocyanin also showed the same photoresponse after the guanidine-HCl treatment. The difference spectrum (green minus red) of guanidine-HCl-treated phycocyanin was almost identical with that shown by phycochrome a of Björn and Björn (3), and the allophycocyanin showed the same difference spectrum as those of phycochrome c of Björn and Björn and the photoreversible pigment isolated by Scheibe (7). Urea at a concentration higher than 1 M or alkaline incubation (pH 8.5) also showed the same effect. The results were interpreted as indicating that phycocyanin and allophycocyanin obtain the ability for photoresponsiveness when their protein conformation, probably around the chromophore site, is modified.
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Ohki et al. (1979) studied this question.