Accumulation of monomer and dimer photosystem (PS) II reaction center core complexes has been analyzed by two-dimensional Blue-native/SDS-PAGE in Synechocystis PCC 6803 wild type and in mutant strains lacking genes psbA, psbB, psbC, psbDIC/DII, or the psbEFLJ operon. In vivo pulse-chase radiolabeling experiments revealed that mutant cells assembled PSII precomplexes only. In ⌬psbC and ⌬psbB, assembly of reaction center cores lacking CP43 and reaction center complexes was detected, respectively. In ⌬psbA, protein subunits CP43, CP47, D2, and cytochrome b 559 were synthesized, but proteins did not assemble. Similarly, in ⌬psbD/C lacking D2, and CP43, the de novo synthesized proteins D1, CP47, and cytochrome b 559 did not form any mutual complexes, indicating that assembly of the reaction center complex is a prerequisite for assembly with core subunits CP47 and CP43. Finally, although CP43 and CP47 accumulated in ⌬psbEFLJ, D2 was neither expressed nor accumulated. We, furthermore, show that the amount of D2 is high in the strain lacking D1, whereas the amount of D1 is low in the strain lacking D2. We conclude that expression of the psbEFLJ operon is a prerequisite for D2 accumulation that is the key regulatory step for D1 accumulation and consecutive assembly of the PSII reaction center complex. The photosystem II (PSII)1 reaction center core (RCC) complex of higher plants, algae, and cyanobacteria can be subdivided into a heterodimer containing D1 and D2, the antenna proteins CP47 and CP43, and a large number of low molecular weight integral membrane proteins including the ␣ and ␤ subunits of cytochrome b 559 (␣ and ␤ cytochrome b 559 ) (1-3). The heterodimer and antenna proteins are essential for binding the prosthetic groups needed for energy and electron transfer (4) as well as for binding the multitude of plastid-encoded small subunits, e.g. Psb-H, -J, -K, -L, and Psb-T, which affect the function of PSII (5-8). Furthermore, plastid-encoded subunit psbZ has been shown to be required for attachment of CP26 during assembly of PSII-LHC supercomplexes, whereas the nucleusencoded subunit psbW was demonstrated to be required for RCC dimer formation (9 -11). The role of plastid-encoded subunits Psb-I, -M, and -N and the nucleus-encoded small subunits Psb-R, and X remains unclear. A striking feature of PSII is the fast turnover of the D1 protein that is believed to be required for PSII repair and restoration of its photochemical activity after photoinactivation (12, 13). Maintaining PSII function may require selective replacement of this central PSII subunit including an efficient apparatus to recognize inactive complexes, and remove damaged and insert a new D1 copy (5, 14, 15). Zhang et al. (16) suggested that D1 replacement in higher plants may occur cotranslationally in a PSII subcomplex consisting of at least D2 and CP47, hence eliminating the need for complete disassembly and de novo assembly from PSII subunits.Cyanobacteria are an excellent model organism to study PSII assembly. The strain used most frequently i...

The role of the Psb28 protein in the structure and function of the photosystem II (PSII) complex has been studied in the cyanobacterium Synechocystis sp. PCC 6803. The protein was localized in the membrane fraction and, whereas most of the protein was detected as an unassembled protein, a small portion was found in the PSII core complex lacking the CP43 antenna (RC47). The association of Psb28 with RC47 was further confirmed by preferential isolation of RC47 from the strain containing a histidine-tagged derivative of Psb28 using nickel-affinity chromatography. However, the affinity-purified fraction also contained a small amount of the unassembled PSII inner antenna CP47 bound to Psb28-histidine, indicating a structural relationship between Psb28 and CP47. A psb28 deletion mutant exhibited slower autotrophic growth than wild type, although the absence of Psb28 did not affect the functional properties of PSII. The mutant showed accelerated turnover of the D1 protein, faster PSII repair, and a decrease in the cellular content of PSI. Radioactive labeling revealed a limitation in the synthesis of both CP47 and the PSI subunits PsaA/PsaB in the absence of Psb28. The mutant cells contained a high level of magnesium protoporphyrin IX methylester, a decreased level of protochlorophyllide, and released large quantities of protoporphyrin IX into the medium, indicating inhibition of chlorophyll (Chl) biosynthesis at the cyclization step yielding the isocyclic ring E. Overall, our results show the importance of Psb28 for synthesis of Chls and/or apoproteins of Chl-binding proteins CP47 and PsaA/PsaB.

The involvement of the PsbI protein in the assembly and repair of the photosystem II (PSII) complex has been studied in the cyanobacterium Synechocystis sp. PCC 6803. Analysis of PSII complexes in the wild-type strain showed that the PsbI protein was present in dimeric and monomeric core complexes, core complexes lacking CP43, and in reaction center complexes containing D1, D2, and cytochrome b-559. In addition, immunoprecipitation experiments and the use of a histidine-tagged derivative of PsbI have revealed the presence in the thylakoid membrane of assembly complexes containing PsbI and either the precursor or mature forms of D1. Analysis of PSII assembly in the psbI deletion mutant and in strains lacking PsbI together with other PSII subunits showed that PsbI was not required for formation of PSII reaction center complexes or core complexes, although levels of unassembled D1 were reduced in its absence. However, loss of PsbI led to a dramatic destabilization of CP43 binding within monomeric and dimeric PSII core complexes. Despite the close structural relationship between D1 and PsbI in the PSII complex, PsbI turned over much slower than D1, whereas high light-induced turnover of D1 was accelerated in the absence of PsbI. Overall, our results suggest that PsbI is an early assembly partner for D1 and that it plays a functional role in stabilizing the binding of CP43 in the PSII holoenzyme.The PSII complex is the multisubunit membrane protein complex catalyzing oxidation of water and reduction of plastoquinone in the thylakoid membranes of higher plants, algae, and cyanobacteria. The membrane-embedded core complex of PSII consists of the D1 and D2 reaction center (RC) subunits, the inner chlorophyll (Chl)-binding antenna proteins, CP47 and CP43, and a number of smaller polypeptides (for review, see Barber, 2006). D1 and D2 form a heterodimer that binds the cofactors involved in primary charge separation and subsequent electron transfer within PSII, while the main purpose of CP47 and CP43 is to deliver energy to the RC for driving electron transfer and, in the case of CP43, to help ligate the CaMn 4 cluster.

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