Bach Institute of Biochemistry, Research Center of Biotechnology, Russian Academy of Sciences, Moscow, 119071, Russia.
Faculty of Biology, Lomonosov Moscow State University, Moscow, 119991, Russia.
Biochemistry (Mosc). 2021 Oct;86(10):1181-1191. doi: 10.1134/S0006297921100023.
Imbalanced light absorption by photosystem I (PSI) and photosystem II (PSII) in oxygenic phototrophs leads to changes in interaction of photosystems altering the linear electron flow. In plants and green algae, this imbalance is mitigated by a partial migration of the chlorophyll a/b containing light-harvesting antenna between the two photosystem core complexes. This migration is registered as fluorescence changes of the pigment apparatus and is termed the reverse transitions between States 1 and 2. By contrast, the molecular mechanism of State 1/2 transitions in phycobilisome (PBS)-containing photosynthetics, cyanobacteria and red algae, is still insufficiently understood. The suggested hypotheses - PBS movement along the surface of thylakoid membrane between PSI and PSII complexes, reversible PBS detachment from the dimeric PSII complex, and spillover - have some limitations as they do not fully explain the accumulated data. Here, we have recorded changes in the stationary fluorescence emission spectra of red algae and cyanobacteria in States 1/2 at room temperature, which allowed us to offer an explanation of the existing contradictions. The change of room temperature fluorescence of chlorophyll belonged to PSII was revealed, while the fluorescence of PBS associated with the PSII complexes remained during States 1/2 transitions at the stable level. Only the reversible dissociation of PBS from the monomeric PSI was revealed earlier which implied different degree of surface contact of PBS with the two photosystems. The detachment of PBS from the PSI corresponds to ferredoxin oxidation as electron carrier and the increase of cyclic electron transport in the pigment apparatus in State I.
在产氧光合作用生物中，光系统 I (PSI)和光系统 II (PSII)的吸光不平衡会导致两个光系统之间相互作用的改变，从而改变线性电子流。在植物和绿藻中，这种不平衡通过叶绿素 a/b 含量较高的光捕获天线在两个光系统核心复合物之间的部分迁移得到缓解。这种迁移被记录为色素装置的荧光变化，被称为状态 1 和状态 2 之间的反向转变。相比之下，含藻胆体（PBS）的光合作用生物、蓝细菌和红藻中状态 1/2 转变的分子机制仍未得到充分理解。建议的假说 - PBS 在 PSI 和 PSII 复合物之间沿类囊体膜表面的运动、PBS 从二聚 PSII 复合物上的可逆脱落以及溢色 - 都有一些局限性，因为它们不能完全解释积累的数据。在这里，我们在室温下记录了红藻和蓝细菌在状态 1/2 下的固定荧光发射光谱的变化，这使我们能够对现有的矛盾提供解释。揭示了室温下 PSII 叶绿素荧光的变化，而与 PSII 复合物相关的 PBS 荧光在状态 1/2 转变过程中保持稳定水平。之前只揭示了 PBS 从单体 PSI 的可逆解离，这意味着 PBS 与两个光系统的表面接触程度不同。PBS 从 PSI 的脱离与铁氧还蛋白氧化作为电子载体相对应，并且在状态 I 中色素装置的循环电子传递增加。
