Mechanism of photoinhibition in vivo. A reversible light-induced conformational change of reaction center II is related to an irreversible modification of the D1 protein.

The light-induced inactivation of the photochemical reaction center II (RCII) of oxygenic chloroplasts (photoinhibition) was investigated in cells and isolated thylakoids of the green alga Chlamydomonas reinhardtii. The process is resolved into a reversible conformational change followed by an irreversible modification of RCII D1 protein. The light-induced changes in vivo persisted in isolated thylakoids. The first step is characterized by (i) destabilization of the secondary acceptor semiquinone anion, Q-B, bound to the D1 protein. This is demonstrated by a reduction in the activation energy of S2,3Q-B charge recombination as measured by the thermoluminescence technique; and (ii) a rise in the intrinsic fluorescence and a decrease of the maximal fluorescence. Unoccupancy of the QB site by plastoquinone partially protected RCII against the light-induced destabilization of Q-B. The extent of charge separation (P+680Q-A) was not affected. However, the slow phase (microsecond) of P+680 dark reduction increased, and the amplitude of signal II was reduced by 20-30%, indicating that in a fraction of RCII, electron donation from Z to P+680 was impaired without losing primary photochemistry. This modification correlates with the irreversible change in D1 protein resulting in the formation of a trypsin-resistant fragment of 16 kDa detected in D1 isolated from light-exposed cells. The change in the Q-B stability could allow charge equilibration with QA and thus explain the rise in the intrinsic fluorescence level and reduction of electron flow to plastoquinone. The change in the lifetime of P+680 can account for further reduction in electron flow (photo-inhibition). The irreversible light-dependent modification of D1 may serve as the signal for its degradation and replacement by a newly synthesized molecule (turnover).