This study examines the effects of ecologically important levels of ultraviolet B radiation on protein D1 turnover and stability and lateral redistribution of photosystem II. It is shown that ultraviolet B light supported only limited synthesis of protein D1, one of the most important components of photosystem II, whereas it promoted significant degradation of proteins D1 and D2. Furthermore, dephosphorylation of photosystem II subunits was specifically elicited upon exposure to ultraviolet B light. Structural modifications of photosystem II and changes in its lateral distribution between granum membranes and stroma-exposed lamellae were found to be different from those observed after photoinhibition by strong visible light. In particular, more complete dismantling of photosystem II cores was observed. Altogether, the data reported here suggest that ultraviolet B radiation alone fails to activate the photosystem II repair cycle, as hypothesized for visible light. This failure may contribute to the toxic effect of ultraviolet B radiation, which is increasing as a consequence of depletion of stratospheric ozone.An increased level of ultraviolet B radiation (280 -320 nm) reaching the Earth's surface has been observed as a consequence of depletion of stratospheric ozone. This phenomenon, first described over the Antarctic Circle, is now extending significantly even to temperate regions (1). It has been shown that even low levels of ultraviolet B light may harm most biological organisms, land plants being particularly sensitive to this radiation (2, 3). The molecular basis of ultraviolet B-induced damage is not completely understood. Apart from DNA damage (4) and other effects such as activation of genes involved in the phenylpropanoid pathway (5, 6), ultraviolet B light impairs photosynthesis. PSII 1 is the most sensitive protein complex of the photosynthetic electron transfer chain (7-9). Damage to the manganese cluster associated with oxygen-evolving activity (10 -12), redox-active tyrosines (12, 13), P 680 (the primary donor in photosystem II) (14), and bound (15, 16) and unbound (10, 15) plastoquinone molecules has been reported. As a consequence of damage, loss of photosynthesis and degradation of reaction center proteins D1 and D2 occur. Studies carried out using in vitro systems have shown that irradiation with ultraviolet B light brings about a loss of protein D1, paralleled by the appearance of an immunodetectable C-terminal fragment of 20 kDa (17). This fragment is produced by cleavage in the second transmembrane ␣-helix (17) in a reaction depending on the presence of manganese ions bound at the catalytic site(s) of the PSII donor side (10). Degradation of protein D2 upon ultraviolet B exposure occurs both in vitro (18) and in vivo (19), although the appearance of specific degradation fragments of protein D2 has so far been reported only in vitro (18). Since protein cleavage was observed only when the Q A site was at least partially active, a role for the bound quinone in the ultraviolet B-induced degradat...

The effect of visible light on photosystem II reaction centre D1 protein in plants treated with ultraviolet-B light was studied. It was found that a 20 kDa C-terminal fragment of D1 protein generated during irradiation with ultraviolet-B light was stable when plants were incubated in the dark, but was degraded when plants were incubated in visible light. In this condition the recovery of photosynthetic activity was also observed. Even a low level of white light was sufficient to promote both further degradation of the fragment and recovery of activity. During this phase, the D1 protein is the main synthesized thylakoid polypeptide, indicating that other photosystem II proteins are recycled in the recovery process. Although both degradation of the 20 kDa fragment and resynthesis of D1 are light-dependent phenomena, they are not closely related, as degradation of the 20 kDa fragment may occur even in the absence of D1 synthesis. Comparing chemical and physical factors affecting the formation of the fragment in ultraviolet-B light and its degradation in white light, it was concluded that the formation of the fragment in ultraviolet-B light is a photochemical process, whereas the degradation of the fragment in white light is a protease-mediated process.

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The relationship between state transitions and photoinhibition has been studied in Chlamydomonas reinhardtii cells. In State 2, photosystem II activity was more inhibited by light than in State 1. In State 2, however, the D1 subunit was not degraded, whereas a substantial degradation was observed in State 1. These results suggest that photoinhibition occurs via the generation of an intermediate state in which photosystem II is inactive but the D1 protein is still intact. The accumulation of this state is enhanced in State 2, because in this State only cyclic photosynthetic electron transport is active, whereas there is no electron flow between photosystem II and the cytochrome b 6 f complex (Finazzi, G., Furia, A., Barbagallo, R. P., and Forti, G. (1999) Biochim. Biophys. Acta 1413, 117-129). The activity of photosystem I and of cytochrome b 6 f as well as the coupling of thylakoid membranes was not affected by illumination under the same conditions. This allows repairing the damages to photosystem II thanks to cell capacity to maintain a high rate of ATP synthesis (via photosystem I-driven cyclic electron flow). This capacity might represent an important physiological tool in protecting the photosynthetic apparatus from excess of light as well as from other a-biotic stress conditions. The photochemical utilization of absorbed light is a critical step in the photosynthetic process. Because harvesting of light, photochemistry, and electron transfer occur on widely different scales of time, a correct balance among these different processes is required to optimize the efficiency of CO 2 fixation.When light is absorbed in excess of what can actually be utilized by photochemistry, damage to the photosynthetic apparatus may be induced. Impairment of both photosystem I (PSI) 1 (1) and photosystem II (PSII) (2) has been described, and this loss of activity has been termed photoinhibition (3). It has been also shown that the degradation of the PSII reaction center D1 subunit is a major consequence of photoinhibition (2).Some mechanisms contribute to protecting the photosynthetic apparatus from an excess of light (4, 5). The first is the so-called energy-dependent quenching, qE, i.e. the increased thermal dissipation in the PSII antennae that follows the generation of the electrochemical proton gradient across the thylakoid membranes. It is supposed to protect the reaction center from the consequences of a strong illumination by reducing the amount of energy present in the antenna protein complexes (6).The second one (6) is state transitions, a phenomenon that has been discovered in Chlorella pyrenoidosa (7) and in Porphyridium cruentum (8). It is a mechanism to balance light utilization between the two photosystems that is based on the reversible transfer of a fraction of the light-harvesting complex II (LHCII) from PSII to PSI (reviewed in Refs 9 -11). It is also supposed to protect PSII from photoinhibition inasmuch as it can decrease the size of its antenna.The migration of LHCII to PSI (State 1-State 2 transition) re...