The multisubunit membrane protein complex Photosystem II (PSII) catalyzes one of the key reactions in photosynthesis: the light-driven oxidation of water. Here, we focus on the role of the Psb27 assembly factor, which is involved in biogenesis and repair after light-induced damage of the complex. We show that Psb27 is essential for the survival of cyanobacterial cells grown under stress conditions. The combination of cold stress (30°C) and high light stress (1000 mol of photons ؋ m ؊2 ؋ s ؊1 ) led to complete inhibition of growth in a ⌬psb27 mutant strain of the thermophilic cyanobacterium Thermosynechococcus elongatus, whereas wild-type cells continued to grow. Moreover, Psb27-containing PSII complexes became the predominant PSII species in preparations from wild-type cells grown under cold stress. Two different PSII-Psb27 complexes were isolated and characterized in this study. The first complex represents the known monomeric PSII-Psb27 species, which is involved in the assembly of PSII. Additionally, a novel dimeric PSII-Psb27 complex could be allocated in the repair cycle, i.e. in processes after inactivation of PSII, by 15 N pulse-label experiments followed by mass spectrometry analysis. Comparison with the corresponding PSII species from ⌬psb27 mutant cells showed that Psb27 prevented the release of manganese from the previously inactivated complex. These results indicate a more complex role of the Psb27 protein within the life cycle of PSII, especially under stress conditions. Photosystem II (PSII)2 is the first component of the photosynthetic electron transfer chain located in the thylakoid membranes of cyanobacteria and plant chloroplasts. It catalyzes the light-driven oxidation of water to molecular oxygen (1, 2). The first organisms capable of this reaction were Gram-negative cyanobacteria that evolved 3.5 billion years ago (3).Active cyanobacterial PSII consists of 20 permanent subunits, which were identified in the x-ray structure of PSII complexes isolated from Thermosynechococcus elongatus BP-1 (henceforth T. elongatus) (4). The central D1 and D2 subunits ligate multiple cofactors that facilitate the water-splitting reaction and mediate the electron transfer to plastoquinone B (Q B ). The central antenna proteins CP43 and CP47 capture the light energy, which in turn excites the Chl D1 molecule of P680. This causes a charge separation at the primary donor with the electron subsequently transferred to pheophytin (5). After Chl D1 ϩ is reduced by P D1 , the electron is transferred from pheophytin to Q A and Q B , respectively. In turn, P D1 ϩ is rereduced by electrons from the CaMn 4 cluster and the water-splitting reaction.During the early steps of biogenesis that probably takes place in the cytoplasmic membrane (6) a PSII precomplex is formed consisting of the central heterodimer D1 and D2, cytochrome b 559 , and PsbI (7-9). This precomplex is the smallest known unit capable of charge separation. Before a functional manganese cluster can be assembled, the D1 protein that is expressed as a precursor...

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