The photochemical conversion between the precursors of phycocyanin and phycoerythrin in Tolypothrix tenuis was investigated. It was found that the conversion of phycocyanin-precursor into phycoerythrin-precursor was induced by green light, and the reverse reaction by red light. These reactions proceeded expo nentially, indicating that the photochemical process was accelerated autocatalytically by the reaction-product. The rates of these photochemical reactions were found to be unaltered by varying the incubation temperature (0° to 35°) and the composition of the gas atmosphere (presence or absence of CO2 and of O2 or by an inhibitor of photosynthesis, p-chlorophenyldimethylurea. The action spectra of the photochemical interconversions between precursors of phycobilin chromoproteids were found to be distinctly different from the absorption spectra of chlorophyll and carotenoids. The most effective wavelength for inducing the conversion of phycocyanin- into phycoerythrin-precursor (541 mμ) is near the absorption maximum of phycoerythrin (565 mμ), and that of the reverse reaction (641 mμ) is near the absorption maximum of phycocyanin (620 mμ). Additional data, indicating that the phycobilin chromoproteids themselves do not participate in these processes as light absorber, were also presented. On the basis of these results, a possible mechanism of the photochemical interconversion between the precursors of phycobilin chromoproteids is proposed.
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Fujita et al. (1962) studied this question.