The reaction center (RC) of photosystem II (PSII), which is composed of D1, D2, PsbI, and cytochrome b559 subunits, forms at an early stage of PSII biogenesis. However, it is largely unclear how these components assemble to form a functional unit. In this work, we show that synthesis of the PSII core proteins D1/D2 and formation of the PSII RC is blocked specifically in the absence of ONE-HELIX PROTEIN1 (OHP1) and OHP2 proteins in Arabidopsis (Arabidopsis thaliana), indicating that OHP1 and OHP2 are essential for the formation of the PSII RC. Mutagenesis of the chlorophyll-binding residues in OHP proteins impairs their function and/or stability, suggesting that they may function in the binding of chlorophyll in vivo. We further show that OHP1, OHP2, and HIGH CHLOROPHYLL FLUORESCENCE244 (HCF244), together with D1, D2, PsbI, and cytochrome b559, form a complex. We designated this complex the PSII RC-like complex to distinguish it from the RC subcomplex in the intact PSII complex. Our data imply that OHP1, OHP2, and HCF244 are present in this PSII RC-like complex for a limited time at an early stage of PSII de novo assembly and of PSII repair under highlight conditions. In a subsequent stage of PSII biogenesis, OHP1, OHP2, and HCF244 are released from the PSII RC-like complex and replaced by the other PSII subunits. Together with previous reports on the cyanobacterium Synechocystis, our results demonstrate that the process of PSII RC assembly is highly conserved among photosynthetic species. PSII is a multisubunit pigment-protein complex present in the thylakoid membranes of oxygenic photosynthetic organisms (Nelson and Yocum, 2006; Nelson and Junge, 2015). It captures light energy to extract electrons from water and drive the transfer of electrons to plastoquinone, resulting in the production of oxygen (Nelson and Junge, 2015). A series of structural analyses revealed that the PSII structure is highly conserved among cyanobacteria and chloroplasts, except for the light-harvesting system (Umena et al., 2011; Su et al., 2017). The photochemical reaction center (RC) is the smallest unit having photochemical activity in PSII and is composed of D1, D2, PsbI, as well as the cytochrome (Cyt) b559 aand b-subunits (PsbE and PsbF, respectively). In addition, the PSII RC contains several cofactors, including six chlorophylls, two pheophytins a, the plastoquinones Q A and Q B , one heme, and one b-carotene molecule (Nelson and Yocum, 2006), which are required for energy transfer, charge separation, and electron transfer. Surrounding the PSII RC are the CP47 and CP43 subunits, which bind chlorophyll a and b-carotene molecules. CP43 and D1 also provide ligands for the CaMn 4 cluster that is essential for water oxidation in the oxygen-evolving complex of PSII (Shen, 2015). In addition to these core subunits, PSII contains at least 11 subunits with low molecular mass (Shi et al., 2012). The phycobilisomes and chlorophyll a/b-binding complex (LHCII) associated with PSII core complexes in cyanobacteria and chloroplasts, respectively...

Chloroplast thylakoid protein rubredoxin 1 (RBD1) in Chlamydomonas and its cyanobacterial homolog RubA contain a rubredoxin domain. These proteins have been proposed to participate in the assembly of photosystem II (PSII) at early stages. However, the effects of inactivation of RBD1 on PSII assembly in higher plants are largely unclear. Here, we characterized an Arabidopsis rbd1 mutant in detail. A drastic reduction of intact PSII complex but relatively higher levels of assembly intermediates including PSII RC, pre-CP47, and pre-CP43 were found in rbd1. Polysome association and ribosome profiling revealed that ribosome recruitment of psbA mRNA is specifically reduced. Consistently, in vivo protein pulse-chase labeling showed that the rate of D1/pD1 synthesis is significantly reduced in rbd1 compared with WT. Moreover, newly synthesized mature D1 and pD1/D2 can assemble into the PSII reaction center (RC) complex but further formation of larger PSII complexes is nearly totally blocked in rbd1. Our data imply that RBD1 is not only required for the formation of a functional PSII core complex during the early stages of PSII assembly but may also be involved in the translation of D1 in higher plants.