Ohad Itzhak, Dal Bosco Cristina, Herrmann Reinhold G, Meurer Jörg
Department of Biological Chemistry and Minerva Center of Photosynthesis Research, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Biochemistry. 2004 Mar 2;43(8):2297-308. doi: 10.1021/bi0348260.
The psbEFLJ operon of tobacco plastids encodes four bitopic low molecular mass transmembrane components of photosystem II. Here, we report the effect of inactivation of psbL on the directional forward electron flow of photosystem II as compared to that of the wild type and the psbJ deletion mutant, which is impaired in PSII electron flow to plastoquinone [Regel et al. (2001) J. Biol. Chem. 276, 41473-41478]. Exposure of Delta psbL plants to a saturating light pulse gives rise to the maximal fluorescence emission, Fm(L), which is followed within 4-6 s by a broader hitherto not observed second fluorescence peak in darkness, Fm(D). Conditions either facilitating oxidation or avoiding reduction of the plastoquinone pool do not affect the Fm(L) level of Delta psbL plants but prevent the appearance of Fm(D). The level of Fm(D) is proportional to the intensity and duration of the light pulse allowing reduction of the plastoquinone pool in dark-adapted leaves prior to the activation of PSI and oxidation of plastoquinol. Lowering the temperature decreases the Fm(D) level in the Delta psbL mutant, whereas it increases considerably the lifetime of Q(A)- in the Delta psbJ mutant. The thermoluminescence signal generated by Q(A)-/S(2) charge recombination is not affected; on the other hand, charge recombination of Q(B)-/S(2,3) could not be detected in Delta psbL plants. PSII is highly sensitive to photoinhibition in Delta psbL. We conclude that PsbL prevents reduction of PSII by back electron flow from plastoquinol protecting PSII from photoinactivation, whereas PsbJ regulates forward electron flow from Q(A)- to the plastoquinone pool. Therefore, both proteins contribute substantially to ensure unidirectional forward electron flow from PSII to the plastoquinone pool.
烟草质体的psbEFLJ操纵子编码光系统II的四个双拓扑低分子量跨膜组分。在此，我们报告了与野生型和psbJ缺失突变体相比，psbL失活对光系统II定向正向电子流的影响，psbJ缺失突变体在PSII向质体醌的电子流方面存在缺陷[Regel等人（2001年）《生物化学杂志》276卷，41473 - 41478页]。将ΔpsbL植株暴露于饱和光脉冲会产生最大荧光发射Fm(L)，在4 - 6秒内，随后在黑暗中会出现一个迄今未观察到的更宽的第二荧光峰Fm(D)。促进质体醌库氧化或避免其还原的条件不会影响ΔpsbL植株的Fm(L)水平，但会阻止Fm(D)的出现。Fm(D)的水平与光脉冲的强度和持续时间成正比，该光脉冲能使暗适应叶片中的质体醌库在PSI激活和质体醌醇氧化之前被还原。降低温度会降低ΔpsbL突变体中的Fm(D)水平，而在ΔpsbJ突变体中会显著增加Q(A)* - 的寿命。由Q(A)* - /S(2)电荷复合产生的热发光信号不受影响；另一方面，在ΔpsbL植株中未检测到Q(B)* - /S(2,3)的电荷复合。PSII在ΔpsbL中对光抑制高度敏感。我们得出结论，PsbL通过阻止质体醌醇的反向电子流对PSII的还原，保护PSII免受过光灭活，而PsbJ调节从Q(A)* - 到质体醌库的正向电子流。因此，这两种蛋白质都对确保从PSII到质体醌库的单向正向电子流做出了重大贡献。
