State transitions, cyclic and linear electron transport and photophosphorylation in Chlamydomonas reinhardtii

The relationship between state transitions and the kinetic properties of th e electron transfer chain has been studied in Chlamydomonas reinhardtii. Th e same turnover rate of cytochrome f was found in state 1 and 2. However, w hile DBMIB was inhibitory in both states, DCMU was effective only in state 1. These observations suggest that linear electron transport was active onl y in state 1, while a cyclic pathway around photosystem (PS) I operated in state 2. The reversible shift from linear to cyclic electron transport was modulated by changes of PSII antenna size, which inactivated the linear pat hway, and by oxygen, which inhibited the cyclic one. Attainment of state 2, under anaerobiosis in the dark, was associated with the decline of the ATP /ADP ratio in the cells and the dark reduction of the intersystem carriers. Upon illumination of the cells, the ATP/ADP ratio increased in a few secon ds to the aerobic level. Then, several minutes later, the F-m returned to t he state 1 level, and O-2 evolution was reactivated. This suggests that ATP , though required for photosynthesis, is not the rate-limiting factor in th e reactivation of photosynthetic O-2 evolution, which is rather controlled by the redox state of the electron carriers. (C) 1999 Elsevier Science B.V. All rights reserved.