The photosystem II (PSII) enzyme provides nearly all the oxygen on Earth by catalyzing water oxidation via its oxygen-evolving complex (OEC), a heterocubane metal cluster in the PSII active site. The overall biogenesis of PSII, and especially the mechanism of photoactivation where the OEC is assembled, is currently unclear. To investigate this mechanism, we solved the cryo-EM structure of apo-PSII lacking extrinsic and peripheral subunits from a mesophilic cyanobacterium, Synechocystis sp. PCC 6803. The lumenal surface is negatively charged to attract cations, and the configuration of the OEC-binding site is found to be quite different from that observed in mature PSII. We use these observations to suggest important characteristics of apo-PSII prior to a structural rearrangement involved in photoactivation that has previously remained elusive, giving insight into its mechanism. This provides a platform for future studies that aim to understand OEC assembly using structural and computational techniques.

光系统II（PSII）酶通过其氧气释放复合物（OEC）（PSII活性位点中的杂化金属簇）催化水氧化，从而提供了地球上几乎所有的氧气。目前尚不清楚PSII的整体生物发生，尤其是组装OEC的光活化机制。为了研究这种机制，我们解决了嗜中性蓝藻（Synechochocystis）缺乏外在和外围亚基的apo-PSII的低温EM结构sp。PCC6803。管腔表面带有负电荷以吸引阳离子，并且发现OEC结合位点的构型与成熟PSII中观察到的构型完全不同。我们使用这些观察结果来暗示apo-PSII的重要特征，然后再进行涉及光活化的结构重排，该重排以前一直难以捉摸，从而深入了解其机理。这为将来的研究提供了一个平台，旨在了解使用结构和计算技术的OEC组装。