Clarke A K, Hurry V M, Gustafsson P, Oquist G
Department of Plant Physiology, University of Umeå, Sweden.
Proc Natl Acad Sci U S A. 1993 Dec 15;90(24):11985-9. doi: 10.1073/pnas.90.24.11985.
The cyanobacterium Synechococcus sp. PCC 7942 possesses a small psbA multigene family that codes for two distinct forms of the photosystem II reaction-center protein D1 (D1:1 and D1:2). We showed previously that the normally predominant D1 form (D1:1) was rapidly replaced with the alternative D1:2 when cells adapted to a photon irradiance of 50 mumol.m-2.s-1 are shifted to 500 mumol.m-2.s-1 and that this interchange was readily reversible once cells were allowed to recover under the original growth conditions. By using the psbA inactivation mutants R2S2C3 and R2K1 (which synthesize only D1:1 and D1:2, respectively), we showed that this interchange between D1 forms was essential for limiting the degree of photoinhibition as well as enabling a rapid recovery of photosynthesis. In this report, we have extended these findings by examining whether any intrinsic functional differences exist between the two D1 forms that may afford increased resistance to photoinhibition. Initial studies on the rate of D1 degradation at three photon irradiances (50, 200, and 500 mumol.m-2.s-1) showed that the rates of degradation for both D1 forms increase with increasing photon flux density but that there was no significant difference between D1:1 and D1:2. Analysis of light-response curves for oxygen evolution for the mutants R2S2C3 and R2K1 revealed that cells with photosystem II reaction centers containing D1:2 have a higher apparent quantum yield (approximately 25%) than cells possessing D1:1. Further studies using chlorophyll a fluorescence measurements confirmed that R2K1 has a higher photochemical yield than R2S2C3; that is, a more efficient conversion of excitation energy from photon absorption into photochemistry. We believe that the higher photochemical efficiency of reaction centers containing D1:2 is causally related to the preferential induction of D1:2 at high light and thus may be an integral component of the protection mechanism within Synechococcus sp. PCC 7942 against photoinhibition.
蓝藻聚球藻属（Synechococcus sp.）PCC 7942拥有一个小的psbA多基因家族，该家族编码两种不同形式的光系统II反应中心蛋白D1（D1:1和D1:2）。我们之前表明，当适应50 μmol·m⁻²·s⁻¹光子辐照度的细胞转移到500 μmol·m⁻²·s⁻¹时，正常情况下占主导的D1形式（D1:1）会迅速被替代形式D1:2取代，并且一旦细胞被允许在原始生长条件下恢复，这种互换很容易逆转。通过使用psbA失活突变体R2S2C3和R2K1（它们分别仅合成D1:1和D1:2），我们表明D1形式之间的这种互换对于限制光抑制程度以及使光合作用快速恢复至关重要。在本报告中，我们通过研究两种D1形式之间是否存在任何内在功能差异来扩展这些发现，这些差异可能提供对光抑制的增强抗性。对三种光子辐照度（50、200和500 μmol·m⁻²·s⁻¹）下D1降解速率的初步研究表明，两种D1形式的降解速率都随光子通量密度的增加而增加，但D1:1和D1:2之间没有显著差异。对突变体R2S2C3和R2K1的放氧光响应曲线分析表明，含有D1:2的光系统II反应中心的细胞比含有D1:1的细胞具有更高的表观量子产率（约25%）。使用叶绿素a荧光测量的进一步研究证实，R2K1比R2S2C3具有更高的光化学产率；也就是说，从光子吸收到光化学的激发能转换更有效。我们认为，含有D1:2的反应中心的较高光化学效率与高光下D1:2的优先诱导存在因果关系，因此可能是聚球藻属PCC 7942中抗光抑制保护机制的一个组成部分。
