Mechanism of the light state transition in photosynthesis. I. Analysis of the kinetics of cytochrome f oxidation in State 1 and State 2 in the red alga, Porphyridium cruentum

Abstract

The kinetics of photooxidation and reduction of cytochrome f were examined spectrophotometrically in the red alga Porphyridium cruentum in light State 1 and light State 2. Experiments were performed on intact cells that had been chemically fixed and stabilized in the light states. The cytochrome f turnover was measured during conditions of linear electron transport driven by both photosystems and during several cyclic reactions mediated by the long-wavelength Photosystem (PS) I. The data show that the rate of photooxidation of cytochrome f increased in State 2 when the cells were activated by subsaturating intensities of green light absorbed primarily by the phycobilisome. No differences in kinetics were found between algae in State 1 or State 2 when they were activated by light absorbed primarily by the chlorophyll of PS I. The results confirm that changes in energy distribution between the two photosystems occur as a result of the light state transition and verify that the redistribution of excitation results in the predicted changes in electron transport.