Stabilization of states I and II by p-benzoquinone treatment of intact cells of Chlamydomonas reinhardtii

Abstract Intact cells of the unicellular green alga C. reinhardtii were treated with p -benzoquinone after adaptation to state I by far-red illumination or to state II by an incubation under anaerobic conditions. This reagent can be regarded both as an oxidant of the plastoquinones and a protein cross-linker. When algae placed in state I then treated with p -benzoquinone were transferred to state II conditions, and conversely, they retained the fluorescence yield characteristic of their initial state. We could thus obtain intact cells blocked in either state I or II with a similar oxidation state of the electron transfer chain. Analysis of the concentration dependences for state fixation by p -benzoquinone indicated that its reduction prevented state fixation, as expected from a cross-linking action mechanism. We present evidence for such a cross-linking action at the thylakoid membrane level. However, we could rule out the possibility that state fixation occurred through LHC immobilization due to an extensive protein cross-linking, since p -benzoquinone treatment did not prevent protein lateral diffusion along the thylakoid membranes. In contrast, it did prevent changes in LHC phosphorylation. We conclude that the inactivation of the enzymes (kinase and phosphatase) controlling LHC phosphorylation changes is responsible for state fixation. We discuss the mechanism of such enzyme inactivation by p -benzoquinone in terms of intramolecular cross-links.