BBA - Bioenergetics (v.1807, #8)

Because life on earth is governed by the second law of thermodynamics, it is subject to increasing entropy. Oxygenic photosynthesis, the earth's major producer of both oxygen and organic matter, is a principal player in the development and maintenance of life, and thus results in increased order. The primary steps of oxygenic photosynthesis are driven by four multi-subunit membrane protein complexes: photosystem I, photosystem II, cytochrome b 6 f complex, and F-ATPase. Photosystem II generates the most positive redox potential found in nature and thus capable of extracting electrons from water. Photosystem I generates the most negative redox potential found in nature; thus, it largely determines the global amount of enthalpy in living systems. The recent structural determination of PSII and PSI complexes from cyanobacteria and plants sheds light on the evolutionary forces that shaped oxygenic photosynthesis. This newly available structural information complements knowledge gained from genomic and proteomic data, allowing for a more precise description of the scenario in which the evolution of life systems took place. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Life on earth is governed by the second law of thermodynamics and it is subject to increasing entropy. ► Oxygenic photosynthesis, the earth's major producer of both oxygen and organic matter, is a principal player in the development and maintenance of life, and thus results in increased order. ► The primary steps of oxygenic photosynthesis are driven by four multi-subunit membrane protein complexes: photosystem I, photosystem II, cytochrome b 6 f complex, and F-ATPase. ► The recent structural determination of PSII and PSI complexes from cyanobacteria and plants sheds light on the evolutionary forces that shaped oxygenic photosynthesis.
Keywords: Photosynthesis; Photosystem; Structure; Evolution; Oxygen; Earth;

The structure and function of eukaryotic photosystem I by Andreas Busch; Michael Hippler (864-877).
Eukaryotic photosystem I consists of two functional moieties: the photosystem I core, harboring the components for the light-driven charge separation and the subsequent electron transfer, and the peripheral light-harvesting complex (LHCI). While the photosystem I-core remained highly conserved throughout the evolution, with the exception of the oxidizing side of photosystem I, the LHCI complex shows a high degree of variability in size, subunits composition and bound pigments, which is due to the large variety of different habitats photosynthetic organisms dwell in. Besides summarizing the most current knowledge on the photosystem I-core structure, we will discuss the composition and structure of the LHCI complex from different eukaryotic organisms, both from the red and the green clade. Furthermore, mechanistic insights into electron transfer between the donor and acceptor side of photosystem I and its soluble electron transfer carrier proteins will be given. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.►Structure and function of Photosystem I ►Structure and function of light-harvesting complexes ►Electron transfer from plastocyanin to PSI ►Electron transfer from PSI to ferredoxin
Keywords: Photosystem I; Light harvesting; LHCI; Red algae; Green algae; Plastocyanin; Ferredoxin; Electron transfer;

The photosynthetic electron transport chain consists of photosystem II, the cytochrome b 6 f complex, photosystem I, and the free electron carriers plastoquinone and plastocyanin. Light-driven charge separation events occur at the level of photosystem II and photosystem I, which are associated at one end of the chain with the oxidation of water followed by electron flow along the electron transport chain and concomitant pumping of protons into the thylakoid lumen, which is used by the ATP synthase to generate ATP. At the other end of the chain reducing power is generated, which together with ATP is used for CO2 assimilation. A remarkable feature of the photosynthetic apparatus is its ability to adapt to changes in environmental conditions by sensing light quality and quantity, CO2 levels, temperature, and nutrient availability. These acclimation responses involve a complex signaling network in the chloroplasts comprising the thylakoid protein kinases Stt7/STN7 and Stl1/STN7 and the phosphatase PPH1/TAP38, which play important roles in state transitions and in the regulation of electron flow as well as in thylakoid membrane folding. The activity of some of these enzymes is closely connected to the redox state of the plastoquinone pool, and they appear to be involved both in short-term and long-term acclimation. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Linear and cyclic electron flow are required for efficient photosynthetic activity. ► The photosynthetic apparatus constantly adapts to changes in light conditions. ► Besides providing energy the photosynthetic apparatus acts as a light sensor. ► Excess absorbed light energy is dissipated through non-photochemical quenching. ► State transitions equilibrate light excitation energy between the two photosystems.
Keywords: Electron transport; Linear electron flow; Cyclic electron flow; Photosystem II; Photosystem I; Light-harvesting system; Cytochrome b 6 f complex; Thylakoid protein phosphorylation; State transitions;

Dynamics of reversible protein phosphorylation in thylakoids of flowering plants: The roles of STN7, STN8 and TAP38 by Paolo Pesaresi; Mathias Pribil; Tobias Wunder; Dario Leister (887-896).
Phosphorylation is the most common post-translational modification found in thylakoid membrane proteins of flowering plants, targeting more than two dozen subunits of all multiprotein complexes, including some light-harvesting proteins. Recent progress in mass spectrometry-based technologies has led to the detection of novel low-abundance thylakoid phosphoproteins and localised their phosphorylation sites. Three of the enzymes involved in phosphorylation/dephosphorylation cycles in thylakoids, the protein kinases STN7 and STN8 and the phosphatase TAP38/PPH1, have been characterised in the model species Arabidopsis thaliana. Differential protein phosphorylation is associated with changes in illumination and various other environmental parameters, and has been implicated in several acclimation responses, the molecular mechanisms of which are only partly understood. The phenomenon of State Transitions, which enables rapid adaptation to short-term changes in illumination, has recently been shown to depend on reversible phosphorylation of LHCII by STN7-TAP38/PPH1. STN7 is also necessary for long-term acclimation responses that counteract imbalances in energy distribution between PSII and PSI by changing the rates of accumulation of their reaction-centre and light-harvesting proteins. Another aspect of photosynthetic acclimation, the modulation of thylakoid ultrastructure, depends on phosphorylation of PSII core proteins, mainly executed by STN8. Here we review recent advances in the characterisation of STN7, STN8 and TAP38/PPH1, and discuss their physiological significance within the overall network of thylakoid protein phosphorylation. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.►Mass spectrometry and genetic approaches have advanced the field of thylakoid protein phosphorylation research. ►28 thylakoid proteins have been identified so far to be subject to phosphorylation. ►Two protein kinases (STN7 and STN8) and one phosphatase (TAP38/PPH1) involved in thylakoid protein phosphorylation have been identified. ►Reversible LHCII phosphorylation is the best understood instance of thylakoid protein phosphorylation. ►Only for very phosphoproteins the physiological significance of reversible phosphorylation has been elucidated.
Keywords: Acclimation; Photosynthesis; Regulation; State transitions;

In oxygen-evolving photosynthesis, the two photosystems—photosystem I and photosystem II—function in parallel, and their excitation levels must be balanced to maintain an optimal photosynthetic rate under natural light conditions. State transitions in photosynthetic organisms balance the absorbed light energy between the two photosystems in a short time by relocating light-harvesting complex II proteins. For over a decade, the understanding of the physiological consequences, the molecular mechanism, and its regulation has increased considerably. After providing an overview of the general understanding of state transitions, this review focuses on the recent advances of the molecular aspects of state transitions with a particular emphasis on the studies using the green alga Chlamydomonas reinhardtii. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► In oxygen-evolving photosynthesis PSI and PSII function in parallel. ► Their excitation levels must be balanced to maintain an optimal photosynthesis. ► State transitions balance the absorbed light energy between the two photosystems. ► Knowledge on its molecular mechanisms and regulation has increased considerably.
Keywords: Acclimation; Electron transfer; Light-harvesting complex; Non-photochemical quenching; Phosphorylation;

Having long been debated, it is only in the last few years that a concensus has emerged that the cyclic flow of electrons around Photosystem I plays an important and general role in the photosynthesis of higher plants. Two major pathways of cyclic flow have been identified, involving either a complex termed NDH or mediated via a pathway involving a protein PGR5 and two functions have been described—to generate ATP and to provide a pH gradient inducing non-photochemical quenching. The best evidence for the occurrence of the two pathways comes from measurements under stress conditions—high light, drought and extreme temperatures. In this review, the possible relative functions and importance of the two pathways is discussed as well as evidence as to how the flow through these pathways is regulated. Our growing knowledge of the proteins involved in cyclic electron flow will, in the future, enable us to understand better the occurrence and diversity of cyclic electron transport pathways. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Cyclic electron flow around Photosystem I has been shown to play an essential role in photosynthesis. ► Cyclic flow makes ATP and protects plants from stress by triggering non photochemical quenching. ► Two distinct pathways of cyclic flow exist, the PGR5 and the NDH pathways. ► Regulation of cyclic flow probably occurs though competition for oxidising ferredoxin.
Keywords: Photosynthesis; Stress; High light; Drought; Heat; Chilling; Redox regulation;

Regulation of electron transport in microalgae by Pierre Cardol; Giorgio Forti; Giovanni Finazzi (912-918).
Unicellular algae are characterized by an extreme flexibility with respect to their responses to environmental constraints. This flexibility probably explains why microalgae show a very high biomass yield, constitute one of the major contributors to primary productivity in the oceans and are considered a promising choice for biotechnological applications. Flexibility results from a combination of several factors including fast changes in the light-harvesting apparatus and a strong interaction between different metabolic processes (e.g. respiration and photosynthesis), which all take place within the same cell. Microalgae are also capable of modifying their photosynthetic electron flow capacity, by changing its maximum rate and/or by diverting photogenerated electrons towards different sinks depending on their growth status. In this review, we will focus on the occurrence and regulation of alternative electron flows in unicellular algae and compare data obtained in these systems with those available in vascular plants. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.►The light-induced electron flow aliment both ATP and NADPH production in the chloroplast. ►In most cases, ATP and ADPH are produced in a ratio that is not sufficient for CO2 assimilation. ►Alternative electron flow processes may adjust the ATP and reducing power stoichiometries. ►These processes must be tightly regulated to avoid an excessive reduction of the quantum yield of photosynthesis.
Keywords: Microalgae; Photosynthesis; Linear electron flow; Cyclic electron flow; Water–water cycle; Mitochondria–chloroplast metabolic interaction;

Oxygenic photosynthesis uses light as energy source to generate an oxidant powerful enough to oxidize water into oxygen, electrons and protons. Upon linear electron transport, electrons extracted from water are used to reduce NADP+ to NADPH. The oxygen molecule has been integrated into the cellular metabolism, both as the most efficient electron acceptor during respiratory electron transport and as oxidant and/or “substrate” in a number of biosynthetic pathways. Though photosynthesis of higher plants, algae and cyanobacteria produces oxygen, there are conditions under which this type of photosynthesis operates under hypoxic or anaerobic conditions. In the unicellular green alga Chlamydomonas reinhardtii, this condition is induced by sulfur deficiency, and it results in the production of molecular hydrogen. Research on this biotechnologically relevant phenomenon has contributed largely to new insights into additional pathways of photosynthetic electron transport, which extend the former concept of linear electron flow by far. This review summarizes the recent knowledge about various electron sources and sinks of oxygenic photosynthesis besides water and NADP+ in the context of their contribution to hydrogen photoproduction by C. reinhardtii. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Research on photosynthetic hydrogen production by Chlamydomonas reinhardtii provided new insights into photosynthetic electron transport pathways. ► The two pathways of cyclic electron flow proposed to operate in C. reinhardtii have different functions during hydrogen photoproduction. ► The recently discovered monomeric plastidic NADPH-dehydrogenase Nda2 of Chlamydomonas is involved in non-photochemical PQ-reduction during photosynthetic hydrogen evolution. ► Starch is the major electron source for hydrogen production by sulfur deprived wild type Chlamydomonas cells, while in starchless mutants, photosystem 2 provides most of the electrons. ► The major ferredoxin isoform PetF is the natural electron donor of the FeFe hydrogenase HydA1 of C. reinhardtii.
Keywords: Anaerobiosis; Chlamydomonas; Ferredoxin; Hydrogen; Photosynthesis;

Ferredoxin-NADP+ oxidoreductase (FNR) is a ubiquitous flavin adenine dinucleotide (FAD)-binding enzyme encoded by a small nuclear gene family in higher plants. The chloroplast targeted FNR isoforms are known to be responsible for the final step of linear electron flow transferring electrons from ferredoxin to NADP+, while the putative role of FNR in cyclic electron transfer has been under discussion for decades. FNR has been found from three distinct chloroplast compartments (i) at the thylakoid membrane, (ii) in the soluble stroma, and (iii) at chloroplast inner envelope. Recent in vivo studies have indicated that besides the membrane-bound FNR, also the soluble FNR is photosynthetically active. Two chloroplast proteins, Tic62 and TROL, were recently identified and shown to form high molecular weight protein complexes with FNR at the thylakoid membrane, and thus seem to act as the long-sought molecular anchors of FNR to the thylakoid membrane. Tic62–FNR complexes are not directly involved in photosynthetic reactions, but Tic62 protects FNR from inactivation during the dark periods. TROL–FNR complexes, however, have an impact on the photosynthetic performance of the plants. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► FNR is found in three chloroplast compartments: thylakoids, stroma and envelope. ► Soluble FNR is photosynthetically active. ► FNR forms thylakoid complexes together with Tic62 and Trol. ► Thylakoid complex formation is redox dependent.
Keywords: Arabidopsis; Chloroplast; Cyclic electron transfer; Ferredoxin-NADP+ oxidoreductase (FNR); Isoenzyme; Tic62; TROL;

Cyanobacterial NDH-1 complexes: Novel insights and remaining puzzles by Natalia Battchikova; Marion Eisenhut; Eva-Mari Aro (935-944).
Cyanobacterial NDH-1 complexes belong to a family of energy converting NAD(P)H:Quinone oxidoreductases that includes bacterial type-I NADH dehydrogenase and mitochondrial Complex I. Several distinct NDH-1 complexes may coexist in cyanobacterial cells and thus be responsible for a variety of functions including respiration, cyclic electron flow around PSI and CO2 uptake. The present review is focused on specific features that allow to regard the cyanobacterial NDH-1 complexes, together with NDH complexes from chloroplasts, as a separate sub-class of the Complex I family of enzymes. Here, we summarize our current knowledge about structure of functionally different NDH-1 complexes in cyanobacteria and consider implications for a functional mechanism. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Cyanobacterial NDH-1 complexes belong to a family of energy converting NAD(P)H:Quinone oxidoreductases. ► Structure of functionally different NDH-1 complexes in cyanobacteria is discussed. ► Certain aspects of reaction mechanisms are considered.
Keywords: Cyanobacteria; NDH-1 complex; Complex I; Carbon uptake; Cyclic electron flow; Photosynthesis;

Structure and biogenesis of the chloroplast NAD(P)H dehydrogenase complex by Lianwei Peng; Hiroshi Yamamoto; Toshiharu Shikanai (945-953).
Eleven genes (ndhA-ndhK) encoding proteins homologous to the subunits of bacterial and mitochondrial NADH dehydrogenase (complex I) were found in the plastid genome of most land plants. These genes encode subunits of the chloroplast NAD(P)H dehydrogenase (NDH) complex involved in photosystem I (PSI) cyclic electron transport and chlororespiration. Although the chloroplast NDH is believed to be closely and functionally related to the cyanobacterial NDH-1L complex, extensive proteomic, genetic and bioinformatic studies have discovered many novel subunits that are specific to higher plants. On the basis of extensive mutant characterization, the chloroplast NDH complex is divided into four parts, the A, B, membrane and lumen subcomplexes, of which subunits in the B and lumen subcomplexes are specific to higher plants. These results suggest that the structure of NDH has been drastically altered during the evolution of land plants. Furthermore, chloroplast NDH interacts with multiple copies of PSI to form the unique NDH–PSI supercomplex. Two minor light-harvesting-complex I (LHCI) proteins, Lhca5 and Lhca6, are required for the specific interaction between NDH and PSI. The evolution of chloroplast NDH in land plants may be required for development of the function of NDH to alleviate oxidative stress in chloroplasts. In this review, we summarize recent progress on the subunit composition and structure of the chloroplast NDH complex, as well as the information on some factors involved in its assembly. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Dozens of proteins have been shown to be subunits of the chloroplast NDH complex. ► Chloroplast NDH complex consists of A, B, membrane and lumen subcomplexes. ► Chloroplast NDH interacts with at least two copies of PSI to form the unique NDH–PSI supercomplex via Lhca5 and Lhca6. ► Several factors are required for assembly of the chloroplast NDH complex. ► Drastic alteration occurred not only in the structure but also in the biogenesis of NDH during the evolution from cyanobacteria to higher plants.
Keywords: Assembly; Chloroplast; Cyclic electron transport; NDH; NDH–PSI supercomplex; Photosystem I;

Flexibility in photosynthetic electron transport: The physiological role of plastoquinol terminal oxidase (PTOX) by Allison E. McDonald; Alex G. Ivanov; Rainer Bode; Denis P. Maxwell; Steven R. Rodermel; Norman P.A. Hüner (954-967).
Oxygenic photosynthesis depends on a highly conserved electron transport system, which must be particularly dynamic in its response to environmental and physiological changes, in order to avoid an excess of excitation energy and subsequent oxidative damage. Apart from cyclic electron flow around PSII and around PSI, several alternative electron transport pathways exist including a plastoquinol terminal oxidase (PTOX) that mediates electron flow from plastoquinol to O2. The existence of PTOX was first hypothesized in 1982 and this was verified years later based on the discovery of a non-heme, di-iron carboxylate protein localized to thylakoid membranes that displayed sequence similarity to the mitochondrial alternative oxidase. The absence of this protein renders higher plants susceptible to excitation pressure dependant variegation combined with impaired carotenoid synthesis. Chloroplasts, as well as other plastids (i.e. etioplasts, amyloplasts and chromoplasts), fail to assemble organized internal membrane structures correctly, when exposed to high excitation pressure early in development. While the role of PTOX in plastid development is established, its physiological role under stress conditions remains equivocal and we postulate that it serves as an alternative electron sink under conditions where the acceptor side of PSI is limited. The aim of this review is to provide an overview of the past achievements in this field and to offer directions for future investigative efforts. Plastoquinol terminal oxidase (PTOX) is involved in an alternative electron transport pathway that mediates electron flow from plastoquinol to O2. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► PTOX is a non-heme, di-iron carboxylate protein localized to thylakoid membranes that displays sequence similarity to the mitochondrial alternative oxidase. ► PTOX plays a key role in plastid development and we hypothesize that it serves as an alternative electron sink under conditions where the acceptor side of PSI is limited. ► The aim of this review is to provide an overview of the past achievements in this field and to offer directions for future investigative efforts.
Keywords: Immutans; Chloroplast; Alternative electron transport; Environmental stress; Photosynthesis; Terminal oxidase;

Chlorophyll a and chlorophyll b are the major constituents of the photosynthetic apparatus in land plants and green algae. Chlorophyll a is essential in photochemistry, while chlorophyll b is apparently dispensable for their photosynthesis. Instead, chlorophyll b is necessary for stabilizing the major light-harvesting chlorophyll-binding proteins. Chlorophyll b is synthesized from chlorophyll a and is catabolized after it is reconverted to chlorophyll a. This interconversion system between chlorophyll a and chlorophyll b refers to the chlorophyll cycle. The chlorophyll b levels are determined by the activity of the three enzymes participating in the chlorophyll cycle, namely, chlorophyllide a oxygenase, chlorophyll b reductase, and 7-hydroxymethyl-chlorophyll reductase. This article reviews the recent progress on the analysis of the chlorophyll cycle and its enzymes. In particular, we emphasize the impact of genetic modification of chlorophyll cycle enzymes on the construction and destruction of the photosynthetic machinery. These studies reveal that plants regulate the construction and destruction of a specific subset of light-harvesting complexes through the chlorophyll cycle. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Chlorophyll b is essential in stabilizing light-harvesting complexes (LHC). ► Chlorophyll b breakdown is requisite for the degradation of LHC. ► Interconversion of chlorophyll a and chlorophyll b refers to the chlorophyll cycle. ► Two out of three enzymes involved in the chlorophyll cycle have been identified. ► The regulatory mechanism of the chlorophyll cycle is partly understood.
Keywords: Chlorophyll; Chlorophyllide a oxygenase; Tetrapyrrole;

Chlorophyll breakdown in higher plants by Stefan Hörtensteiner; Bernhard Kräutler (977-988).
Chlorophyll breakdown is an important catabolic process of leaf senescence and fruit ripening. Structure elucidation of colorless linear tetrapyrroles as (final) breakdown products of chlorophyll was crucial for the recent delineation of a chlorophyll breakdown pathway which is highly conserved in land plants. Pheophorbide a oxygenase is the key enzyme responsible for opening of the chlorin macrocycle of pheophorbide a characteristic to all further breakdown products. Degradation of chlorophyll was rationalized by the need of a senescing cell to detoxify the potentially phototoxic pigment, yet recent investigations in leaves and fruits indicate that chlorophyll catabolites could have physiological roles. This review updates structural information of chlorophyll catabolites and the biochemical reactions involved in their formation, and discusses the significance of chlorophyll breakdown. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Chlorophyll is broken down in a multi-step pathway called PAO pathway. ► The PAO pathway operates during leaf senescence and fruit ripening. ► The end products of the pathway are fluorescent and nonfluorescent catabolites. ► The pathway aims to detoxify chlorophyll, but may also have physiological roles.
Keywords: Chlorophyll breakdown; Chlorophyll catabolite; Fruit ripening; Senescence;

The biogenesis and physiological function of chloroplast superoxide dismutases by Marinus Pilon; Karl Ravet; Wiebke Tapken (989-998).
Iron-superoxide dismutase (FeSOD) and copper/zinc-superoxide dismutase (Cu/ZnSOD) are evolutionarily conserved proteins in higher plant chloroplasts. These enzymes are responsible for the efficient removal of the superoxide formed during photosynthetic electron transport and function in reactive oxygen species metabolism. The availability of copper is a major determinant of Cu/ZnSOD and FeSOD expression. Analysis of the phenotypes of plants that over-express superoxide dismutases in chloroplasts has given support for the proposed roles of these enzymes in reactive oxygen species scavenging. However, over-production of chloroplast superoxide dismutase gives only limited protection to environmental stress and does not result in greatly improved whole plant performance. Surprisingly, plant lines that lack the most abundant Cu/ZnSOD or FeSOD activities perform as well as the wild-type under most conditions tested, indicating that these superoxide dismutases are not limiting to photoprotection or the prevention of oxidative damage. In contrast, a strong defect in chloroplast gene expression and development was seen in plants that lack the two minor FeSOD isoforms, which are expressed predominantly in seedlings and that associate closely with the chloroplast genome. These findings implicate reactive oxygen species metabolism in signaling and emphasize the critical role of sub-cellular superoxide dismutase location. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Superoxide can be produced during photosynthetic electron transport. ► Superoxide dismutase catalyzes removal of superoxide. ► FeSOD and Cu/ZnSOD occur in chloroplasts, MnSOD is mitochondrial. ► Much of the chloroplast SOD activities can be missed without severe consequences. ► Superoxide dismutase may have a signaling role in the chloroplast.
Keywords: Superoxide dismutase; Chloroplast; Photosynthesis; Copper homeostasis; Reactive oxygen species;

The Clp protease system; a central component of the chloroplast protease network by Paul Dominic B. Olinares; Jitae Kim; Klaas J. van Wijk (999-1011).
Intra-plastid proteases play crucial and diverse roles in the development and maintenance of non-photosynthetic plastids and chloroplasts. Formation and maintenance of a functional thylakoid electron transport chain requires various protease activities, operating in parallel, as well as in series. This review first provides a short, referenced overview of all experimentally identified plastid proteases in Arabidopsis thaliana. We then focus on the Clp protease system which constitutes the most abundant and complex soluble protease system in the plastid, consisting of 15 nuclear-encoded members and one plastid-encoded member in Arabidopsis. Comparisons to the simpler Clp system in photosynthetic and non-photosynthetic bacteria will be made and the role of Clp proteases in the green algae Chlamydomonas reinhardtii will be briefly reviewed. Extensive molecular genetics has shown that the Clp system plays an essential role in Arabidopsis chloroplast development in the embryo as well as in leaves. Molecular characterization of the various Clp mutants has elucidated many of the consequences of loss of Clp activities. We summarize and discuss the structural and functional aspects of the Clp machinery, including progress on substrate identification and recognition. Finally, the Clp system will be evaluated in the context of the chloroplast protease network. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► In-depth review of the Clp protease system in plastids of higher plants. ► Update on experimentally identified proteases in higher plant plastids. ► Discussion of substrates of the Clp protease system in Arabidopsis.
Keywords: Clp proteases; Chloroplast; Protease network;