Szeged Biological Research Centre, Institute of Plant Biology, Temesvári krt. 62, Szeged 6726, Hungary.
Doctoral School of Biology, University of Szeged, Közép fasor 52, Szeged 6726, Hungary.
Plant Physiol. 2022 Jun 1;189(2):827-838. doi: 10.1093/plphys/kiac130.
In cyanobacteria, phycobilisomes (PBS) serve as peripheral light-harvesting complexes of the two photosystems, extending their antenna size and the wavelength range of photons available for photosynthesis. The abundance of PBS, the number of phycobiliproteins they contain, and their light-harvesting function are dynamically adjusted in response to the physiological conditions. PBS are also thought to be involved in state transitions that maintain the excitation balance between the two photosystems. Unlike its eukaryotic counterpart, PSI is trimeric in many cyanobacterial species and the physiological significance of this is not well understood. Here, we compared the composition and light-harvesting function of PBS in cells of Synechocystis sp. PCC 6803, which has primarily trimeric PSI, and the ΔpsaL mutant, which lacks the PsaL subunit of PSI and is unable to form trimers. We also investigated a mutant additionally lacking the PsaJ and PsaF subunits of PSI. Both strains with monomeric PSI accumulated significantly more allophycocyanin per chlorophyll, indicating higher abundance of PBS. On the other hand, a higher phycocyanin:allophycocyanin ratio in the wild type suggests larger PBS or the presence of APC-less PBS (CpcL-type) that are not assembled in cells with monomeric PSI. Steady-state and time-resolved fluorescence spectroscopy at room temperature and 77 K revealed that PSII receives more energy from the PBS at the expense of PSI in cells with monomeric PSI, regardless of the presence of PsaF. Taken together, these results show that the oligomeric state of PSI impacts the excitation energy flow in Synechocystis.
在蓝藻中，藻胆体（PBS）作为两个光系统的外围光捕获复合物，扩展了它们的天线大小和可用于光合作用的光子波长范围。PBS 的丰度、它们所含的藻胆蛋白数量以及它们的光捕获功能会根据生理条件进行动态调整。PBS 还被认为参与了维持两个光系统之间激发平衡的状态转换。与真核生物的对应物不同，许多蓝藻物种中的 PSI 是三聚体，其生理意义尚不清楚。在这里，我们比较了具有主要三聚体 PSI 的 Synechocystis sp. PCC 6803 细胞和缺失 PSI 的 PsaL 亚基且无法形成三聚体的 ΔpsaL 突变体中 PBS 的组成和光捕获功能。我们还研究了一个额外缺失 PSI 的 PsaJ 和 PsaF 亚基的突变体。这两种具有单体 PSI 的菌株每叶绿素积累的藻蓝蛋白显著更多，表明 PBS 的丰度更高。另一方面，野生型中较高的藻蓝蛋白：藻红蛋白比表明 PBS 较大或存在 APC 缺失的 PBS（CpcL 型），这些 PBS 无法在单体 PSI 的细胞中组装。室温下和 77 K 下的稳态和时间分辨荧光光谱表明，无论是否存在 PsaF，单体 PSI 细胞中的 PBS 会从 PSI 中获取更多的能量，从而为 PSII 提供能量。总之，这些结果表明 PSI 的寡聚状态会影响 Synechocystis 中的激发能量流。
