Chloroplast-encoded small subunits of the three multiprotein complexes in photosynthetic electron transport

Photosystem II is a large pigment-protein complex catalyzing water oxidation and initiating electron transfer processes across the thylakoid membrane. In addition to large protein subunits, many of which bind redox cofactors, photosystem II particles contain a number of low molecular weight polypeptides whose function is only poorly defined. Here we have investigated the function of one of the smallest polypeptides in photosystem II, PsbJ. Using a reverse genetics approach, we have inactivated the psbJ gene in the tobacco chloroplast genome. We show that, although the PsbJ polypeptide is not principally required for functional photosynthetic electron transport, plants lacking PsbJ are unable to grow photoautotrophically. We provide evidence that this is due to the accumulation of incompletely assembled water-splitting complexes, which in turn causes drastically reduced photosynthetic performance and extreme hypersensitivity to light. Our results suggest a role of PsbJ for the stable assembly of the water-splitting complex of photosystem II and, in addition, support a control of photosystem I accumulation through photosystem II activity. Photosystem II (PSII)1 is a large cofactor-protein complex consisting of at least 17 protein subunits (Ref. 1; for review, see e.g. Ref. 2). The PSII reaction center is formed by a heterodimer of two pigment-binding proteins, D1 and D2, which, in photosynthetic eukaryotes, are encoded by the chloroplast psbA and psbD genes, respectively. The photochemical reaction carried out by the reaction center converts the energy of a photon into a separation of charge and, in this way, initiates electron flow. Around the reaction center, the outer parts of PSII are assembled, the inner and outer antennae funneling absorbed light energy to the catalytic core and the oxygen-evolving complex splitting water into protons, electrons, and dioxygen (reviewed in Refs. 3 and 4).In addition to the well-studied large protein subunits, purified PSII particles contain a number of low molecular weight polypeptides, many of which are encoded by the plastid genome of photosynthetically active eukaryotes (5). Most of these small subunits do not bind redox cofactors and, hence, are unlikely to participate directly in electron transfer reactions. It is generally assumed that they rather function as photosystem-assembling or stabilizing factors. However, in many cases, molecular evidence supporting such a structural role is largely lacking. The successful development of transformation technologies for Chlamydomonas (6) and tobacco chloroplasts (7) has paved the way to functional characterizations of plastid genome-encoded genes by reverse genetics. Linked to a selectable marker gene, mutant alleles can be introduced into plastids by chloroplast transformation, where they replace the endogenous wildtype allele by homologous recombination. During the past decade, reverse genetics has become a powerful tool in plastid functional genomics (reviewed in Ref. 8).Here, we have taken a reverse genetics approach to def...

Section: Photosystem II (Psii)mentioning

confidence: 99%

Lack of the Small Plastid-encoded PsbJ Polypeptide Results in a Defective Water-splitting Apparatus of Photosystem II, Reduced Photosystem I Levels, and Hypersensitivity to Light

Hager

Hermann

Biehler

et al. 2002

“…Application of improved SDS-PAGE systems that can resolve small polypeptides together with microsequencing of the Nterminal amino acids has allowed identification of a number of small (Ͻ10 kDa) polypeptides in various PSII preparations (2,3). Based on their deduced amino acid sequences, most of these small polypeptides contain a single transmembrane helix.…”

mentioning

confidence: 99%

Evidence That the PsbK Polypeptide Is Associated with the Photosystem II Core Antenna Complex CP43

Sugimoto

Takahashi

2003

Self Cite

PsbK is encoded by the chloroplast psbK gene and is one of the small polypeptides of photosystem II (PSII). This polypeptide is required for accumulation of the PSII complex. In the present study, we generated an antibody against recombinant mature PsbK of Chlamydomonas and used it in Western blots to localize PsbK in the PSII core complex. PsbK was found in the thylakoid membranes, and purification of the PSII core complex from detergent-solubilized thylakoid membranes showed that PsbK is tightly associated with the PSII core complex. We used potassium thiocyanate to separate PSII into subcore complexes, including the D1/D2/cytochrome b559 reaction center complex, CP47, and CP43, and we found that PsbK co-purifies with one of the core antenna complexes, CP43, during ion exchange chromatography. Subsequent gel filtration chromatography of the purified CP43 confirmed that PsbK is tightly associated with CP43. Steady-state levels of PsbK were also determined in Chlamydomonas mutants expressing various levels of PSII. Quantitative Western blotting revealed that the levels of PsbK in these mutants are approximately equal to those of CP43, suggesting that PsbK is stable only when associated with CP43 in the chloroplast. Together, our results indicate that PsbK is an integral part of the PSII complex and may participate in the assembly and stability of the PSII complex.

“…The core antenna complexes, CP47 and CP43, of which the apoproteins are encoded by the chloroplast psbB and psbC genes, respectively, and cytochrome b 559 coded for by the chloroplast psbE and psbF genes are closely associated with the D1/D2 heterodimer (7). In addition to these polypeptides containing cofactors, the PSII complex contains the lumenal extrinsic polypeptides, OEE1, OEE2, and OEE3, encoded by the nuclear psbO, psbP, and psbQ genes, respectively (8), and approximately 10 hydrophobic small polypeptides (9). The three extrinsic polypeptides are known to be involved in O 2 evolution, while the small polypeptides might be required for assembly, stability, and/or optimal function of PSII.…”

mentioning

confidence: 99%

PsbT Polypeptide Is Required for Efficient Repair of Photodamaged Photosystem II Reaction Center

Ohnishi¹,

Takahashi²

2001

Self Cite

PsbT is a small chloroplast-encoded hydrophobic polypeptide associated with the photosystem II (PSII) core complex. A psbT-deficient mutant (⌬psbT) of the green alga Chlamydomonas reinhardtii grows photoautotrophically, whereas its growth is significantly impaired in strong light. To understand the photosensitivity of ⌬psbT, we have studied the effect of strong illumination on PSII activity and proteins. It is shown that the level of PSII activity and proteins is reduced in the ⌬psbT more significantly than in wild type under strong light. When recovery of the photodamaged PSII is inhibited by a chloroplast protein synthesis inhibitor, the light-induced inactivation and degradation of PSII occur similarly in wild-type and mutant cells. On the contrary, the recovery of PSII activity after partial photoinactivation is remarkably delayed in the ⌬psbT cells, suggesting that PsbT is required for efficient recovery of the photodamaged PSII complex. These results therefore present the first evidence for involvement of this small PSII polypeptide in the recovery process. Partial disintegration of the purified PSII core complex and localization of PSII proteins in the resulting PSII subcore complexes have revealed that PsbT is associated with D1/D2 heterodimer. A possible role of PsbT in the recovery process is discussed.