Assembly of the Photosynthetic Apparatus

Abstract

The primary reactions of photosynthesis are mediated by three protein complexes embedded in the thylakoid membranes of chloroplasts. These complexes are PSII, the cytochrome b 6 f complex (Cytb 6 f), and PSI, which are connected in series through the photosynthetic electron transport chain. Light energy captured by the light-harvesting systems of PSII (LHCII) and PSI (LHCI) is transferred to the reaction center chlorophylls to create a charge separation across the membrane. This leads to the formation of a strong oxidant on the donor side of PSII capable of splitting water into molecular oxygen, protons, and electrons. The electrons are transferred stepwise from PSII to the plastoquinone pool, Cytb 6 f, plastocyanin, and PSI where a second charge separation creates a strong reductant capable of reducing ferredoxin that subsequently reduces NADP + to NADPH. Besides this linear electron pathway there is a cyclic pathway in which electrons are cycled around PSI through the Cytb 6 f complex. The electron transfer reactions are coupled to proton pumping into the lumen space of the thylakoids and the resulting pH gradient drives the ATP synthase for ATP production. Ultimately NADPH and ATP are used for CO 2 assimilation by the Calvin-Benson cycle. Thylakoid membranes of land plants are organized in two domains, the grana stacks, comprising 80% of the membrane, and the stromal nonappressed membranes that connect different grana stacks. The photosynthetic complexes with exception of Cytb 6 f, are not distributed evenly through the thylakoid membrane system. Whereas PSII is mostly confined to the grana regions, PSI and ATP synthase are located in the stroma lamellae. Electron transfer between these complexes is mediated by the diffusible electron carriers plastoquinone and plastocyanin.A characteristic feature of the photosynthetic complexes is that they all consist of numerous chloroplast and nucleus-encoded subunits. In the case of PSII, PSI, and Cytb 6 f these complexes contain in addition pigments such as chlorophylls and xanthophylls as well as hemes, quinones, and iron-sulfur (Fe-S) centers that act as redox cofactors. Hence the biogenesis of the photosynthetic apparatus involves a concerted interplay between the chloroplast and nucleocytosolic genetic systems as well as a tight coordination between protein and pigment synthesis and insertion into the thylakoid membranes. Analysis of numerous mutants affected in photosynthetic activity in Chlamydomonas reinhardtii, Arabidopsis (Arabidopsis thaliana), and Zea mays has provided many new insights into the assembly of the photosynthetic complexes (Barkan and Goldschmidt-Clermont, 2000;Eberhard et al., 2008).A remarkable feature of photosynthetic organisms is their ability to adapt to changing light conditions, which is reflected in changes in the organization of the photosynthetic complexes. This Update reviews recent advances in our understanding of the assembly of these complexes and for some of them, their remodeling and/or reversible association into supercomplexes...

Structure of the plant photosystem I supercomplex at 2.6 Å resolution

“…Eukaryotes can modulate the distribution of excitation energy transfer between their two photosystems via a mechanism called state transitions (Lunde et al, 2000;Bellafiore et al, 2005;Rochaix, 2011;Rochaix et al, 2012). Under state II conditions, PSI associates with a mobile pool of the LHCII antennae, which increases its absorbance cross-section (Kargul et al, 2005;de Bianchi et al, 2010).…”

PsbN Is Required for Assembly of the Photosystem II Reaction Center in Nicotiana tabacum

“…Over the past years, predominantly forward and reverse genetic approaches combined with biochemical and functional analyses have identified various auxiliary proteins that assist in distinct steps of the rather complicated PSII assembly process (Mulo et al, 2008;Rochaix, 2011;Chi et al, 2012b;Nickelsen and Rengstl, 2013). For instance, HCF136 was shown to be required primarily for accumulation of PSII (Meurer et al, 1998).…”

Regulatory factors for the assembly of thylakoid membrane protein complexes

“…Each assembly step is likely to be regulated by a number of factors [6][7][8][9][10][11][12]. In the last few decades, analysis of photosynthetic mutants in oxygenic photosynthetic organisms such as Synechocystis, Chlamydomonas reinhardtii, Arabidopsis thaliana, maize and tobacco has identified a considerable number of regulatory proteins involved in these processes and has provided insights into the molecular mechanisms underlying the assembly of thylakoid membrane protein complexes.…”