Smolinski Sharon L, Tokmina-Lukaszewska Monika, Holland Junia M, Guo Zhanjun, Kisgeropoulos Effie, Bothner Brian, King Paul W, Lubner Carolyn E
Biosciences Center, National Renewable Energy Lab, Golden, Colorado, USA.
Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana, USA.
J Biol Chem. 2025 May 14;301(7):110233. doi: 10.1016/j.jbc.2025.110233.
Cyanobacterial photosystem I (PSI) can undergo modifications that adjust photosynthetic electron transport in response to fluctuations in environmental and cellular conditions. We recently reported that PSI isolated from Synechocystis sp. PCC 6803 (S. 6803) strains lacking a peripheral oxygen reduction reaction (ORR1) pathway demonstrated altered P photooxidation capacity, changes in spectral properties, and a higher proportion of monomers. These changes in PSI were augmented when cells were grown under higher photon flux, which creates a greater energy imbalance at PSI. We have shown that the modified PSI is functional in photochemical charge separation and ferredoxin reduction reactions. Thus, we hypothesized that monomerization of PSI was caused by changes in the environment of PsaL, which is known to be essential for stabilizing trimers. To test our hypothesis, we isolated PSI monomers and trimers from ORR1 and wild-type (WT) strains. The electron paramagnetic resonance (EPR) spectra of reduced PSI demonstrated the presence of intact F and F [4Fe-4S] clusters, consistent with measurements of functional charge separation and electron transport. Limited proteolysis followed by mass spectrometric analysis showed altered accessibility of PsaL in the ORRI PSI monomers compared to WT monomers, and included regions associated with chlorophyll and carotenoid binding, and in functional interactions with adjacent subunits. In addition, ORR1 PSI monomers had spectral changes compared to WT PSI due to differences in carotenoid compositions. Collectively, these findings reveal new insights into how microbes adjust PSI structure and photochemistry to mitigate photodamage in response to changes in electron utilization by downstream chemical reactions.
蓝藻光系统I（PSI）可发生修饰，以响应环境和细胞条件的波动来调节光合电子传递。我们最近报道，从缺乏外周氧还原反应（ORR1）途径的聚球藻属PCC 6803（S. 6803）菌株中分离出的PSI表现出改变的P光氧化能力、光谱特性变化以及更高比例的单体。当细胞在更高光子通量下生长时，PSI的这些变化会加剧，这会在PSI处产生更大的能量不平衡。我们已经表明，修饰后的PSI在光化学电荷分离和铁氧还蛋白还原反应中起作用。因此，我们假设PSI的单体化是由PsaL环境的变化引起的，已知PsaL对于稳定三聚体至关重要。为了验证我们的假设，我们从ORR1和野生型（WT）菌株中分离出PSI单体和三聚体。还原型PSI的电子顺磁共振（EPR）光谱表明存在完整的F和F [4Fe-4S]簇，这与功能电荷分离和电子传输的测量结果一致。有限蛋白酶解后进行质谱分析表明，与WT单体相比，ORRI PSI单体中PsaL的可及性发生了改变，包括与叶绿素和类胡萝卜素结合相关的区域，以及与相邻亚基的功能相互作用区域。此外，由于类胡萝卜素组成的差异，ORR1 PSI单体与WT PSI相比有光谱变化。总的来说，这些发现揭示了微生物如何调整PSI结构和光化学以减轻光损伤，以响应下游化学反应中电子利用变化的新见解。
