Degradation of Photosystem I Reaction Center Proteins During Photoinhibition in Vitro
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Photoinhibition
Degradation
Photoinhibition
Photosynthetic efficiency
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P680
Photoinhibition
Pheophytin
Oxygen evolution
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We have studied photoinhibition of photosynthesis in the cyanobacterium Synechococcus sp. PCC 7942, which possesses two distinct forms of the photosystem II reaction-center protein D1 (D1:1 and D1:2). We report here that when cells adapted to a growth irradiance of 50 mumol.m-2.s-1 are exposed to an irradiance of 500 mumol.m-2.s-1, the normally predominant D1 form (D1:1) is rapidly replaced with the alternative D1:2. This interchange is not only complete within the first hour of photoinhibition but is also fully reversible once cells are returned to 50 mumol.m-2 x s-1. By using a mutant that synthesizes only D1:1, we show that the failure to replace D1:1 with D1:2 during photoinhibition results in severe loss of photosynthetic activity as well as a diminished capacity to recover after the stress period. We believe that this interchange between D1 forms may constitute an active component in a protection mechanism unique among photosynthetic organisms that enables cyanobacteria to effectively cope with and recover from photoinhibition.
Photoinhibition
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Photoinhibition
Oxygen evolution
Oxygen-evolving complex
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Isolation of photosystem-II reaction centres from pea leaves after photoinhibitory treatment at low temperature (0-1 degrees C) has provided evidence for the mechanism of degradation of the D1 protein in vivo. These isolated reaction centres did not appear to be spectrally distinct from preparations obtained from control leaves that had not been photoinhibited. Breakdown fragments of both the D1 and D2 proteins were, however, found in preparations isolated from photoinhibited leaves, and showed similarities with those detected when isolated reaction centres were exposed to acceptor-side photoinhibition. Analyses of the origin of D1 fragments indicated that the primary cleavage site of this protein was between transmembrane helices IV and V indicative of the acceptor-side mechanism for photoinhibition. The origins of other D1 protein fragments indicate that some donor-side photoinhibition may also have occurred in vivo under the conditions employed. We have shown that the spectral and functional integrating of the isolated photosystem II reaction centre complex is resistant to proteolytic cleavage by trypsin. Use of a more non-specific protease (subtilisin), however, caused significant destabilisation of the special pair of chlorophylls constituting the primary electron donor, P680, with a consequential loss of functional activity. Thus, it is possible that specific cleavage of photosystem-II reaction-centre proteins may occur in vivo following photoinhibitory damage without a significant change in structural integrity, a conclusion supported by the finding that photodamaged and normal reaction centres were isolated together.
Photoinhibition
P680
Cleavage (geology)
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Light energy
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Photoinhibition on soybean is studied, and the result shows that the photosynthetic rate of mesophyll cells is inhibited as the light intensity increased and the treatment time prolonged. The photoinhibition extent is different as the growth conditions different, it is easier to occur under conditions of low light intensity than that of high light intensity. The damage of photoinhibition is located mainly in PSⅡ. When the photosynthetic rate of leaves is photoinhibited, the quantum yield decreased. Phohoinhibition of photosynthesis is a reversible process. When it returns to low light intensity, however, the relative photosynthetic rate can not recover completely.
Photoinhibition
Light intensity
Intensity
Luminous intensity
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Photoinhibition
P680
Oxygen evolution
Oxygen-evolving complex
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