Figure 1.

De novo assembly of PSII in Arabidopsis. The major steps include: (1) assembly of precursor D1-PsbI (pD1-PsbI) and D2-cytochrome b₅₅₉ (D2-Cyt b₅₅₉) precomplexes, (2) assembly of the minimal reaction-center complex (RC), which lacks CP47 and CP43, (3) assembly of the reaction-center complex (RC47a) that contains CP47 but lacks CP43, (4) incorporation of LMM subunits, such as PsbH, PsbM, PsbT, and PsbR, to form RC47b, (5) incorporation of CP43, along with LMM subunit PsbK, to form the OEC-less PSII core monomer, (6) assembly of the oxygen-evolving complex (OEC) and additional LMM subunits, such as PsbW and PsbZ, to form the PSII core monomer, and (7) dimerization and formation of the PSII-light-harvesting complex II (LHCII) supercomplex. Proteins that are involved in these steps are listed. Although RBD1 promotes PSII assembly and/or PSII stability, it is not depicted in this figure because it is not clear which step(s) of de novo PSII assembly this protein is involved in. Letters (B, C, D1, D2, E, F, H, I, K, M, O, P, Q, R, T, W, Z) in rectangles represent PSII proteins PsbB (i.e., CP47), PsbC (i.e., CP43), D1, D2, PsbE, PsbF, PsbH, PsbI, PsbK, PsbM, PsbO, PsbP, PsbQ, PsbR, PsbT, PsbW, and PsbZ, respectively. Abbreviations: D2-Cyt b₅₅₉, D2-cytochrome b₅₅₉ precomplex; LHCII, light-harvesting complex II; OEC, oxygen-evolving complex; pD1, precursor D1; pD1-PsbI, precursor D1-PsbI precomplex; PSII, Photosystem II; RC, PSII minimal reaction-center complex; RC47a, PSII reaction-center complex with CP47, without PsbM, PsbH, PsbT, or PsbR; RC47b, PSII reaction-center complex with CP47, PsbM, PsbH, PsbT, and PsbR. For simplicity, only one name is shown for proteins with multiple names (e.g., “THF1” for THF1/PSB29).