Regulation of photosynthetic electron transport

AbstractThe photosynthetic electron transport chain consists of photosystem II, the cytochrome b6f 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"

The Dynamics of the Photosynthetic Apparatus in Algae

Plants and algae are subjected to changes in light quality and quantity and in nutrient availability in their natural habitat. To adapt to these changing environmental conditions, these organisms have developed efficient means to adjust their photosynthetic apparatus so as to preserve photosynthetic efficiency and appropriate photoprotection. Under limiting light, this system optimizes light capture and photosynthetic yield through a reorganization of its light-harvesting system. In contrast, under high light, when the absorption capacity of the system is exceeded, the excess absorbed light energy is dissipated as heat to prevent oxidative damage. One of the key photosynthetic complexes, photosystem II, is prone to photodamage but is efficiently repaired. The photosynthetic machinery is also able to adjust when specific micronutrients such as copper, iron or sulfur become limiting by remodeling some of the photosynthetic complexes and metabolic pathways. While some of these responses occur in the short term, others occur in the long term and involve an intricate signaling system within chloroplasts and between the chloroplast and the nucleus accompanied with changes in gene expression. These signals involve the tetrapyrrole pathway, plastid protein synthesis, the redox state of the photosynthetic electron transport chain, reactive oxygen species and several metabolites

State transitions at the crossroad of thylakoid signalling pathways

In order to maintain optimal photosynthetic activity under a changing light environment, plants and algae need to balance the absorbed light excitation energy between photosystem I and photosystem II through processes called state transitions. Variable light conditions lead to changes in the redox state of the plastoquinone pool which are sensed by a protein kinase closely associated with the cytochrome b 6 f complex. Preferential excitation of photosystem II leads to the activation of the kinase which phosphorylates the light-harvesting system (LHCII), a process which is subsequently followed by the release of LHCII from photosystem II and its migration to photosystem I. The process is reversible as dephosphorylation of LHCII on preferential excitation of photosystem I is followed by the return of LHCII to photosystem II. State transitions involve a considerable remodelling of the thylakoid membranes, and in the case of Chlamydomonas, they allow the cells to switch between linear and cyclic electron flow. In this alga, a major function of state transitions is to adjust the ATP level to cellular demands. Recent studies have identified the thylakoid protein kinase Stt7/STN7 as a key component of the signalling pathways of state transitions and long-term acclimation of the photosynthetic apparatus. In this article, we present a review on recent developments in the area of state transition

Symmetries, Currents and Conservation Laws of Self-Dual Gravity

We describe an infinite-dimensional algebra of hidden symmetries for the self-dual gravity equations. Besides the known diffeomorphism-type symmetries (affine extension of w(infinity) algebra), this algebra contains new hidden symmetries, which are an affine extension of the Lorentz rotations. The full symmetry algebra has both Kac-Moody and Virasoro-like generators, whose exponentiation maps solutions of the field equations to other solutions. Relations to problems of string theories are briefly discussed.Comment: 14 pages, LaTeX, the paper was reformatte

Expression and RNA binding properties of the chloroplast ribosomal protein S1 from Chlamydomonas reinhardtii

The gene encoding the chloroplast ribosomal protein S1 from Chlamydomonas reinhardtii, CreS1, was cloned and the RNA binding properties and the expression patterns were studied. Gel-shift analysis revealed that CreS1 binds AU-rich 5′-untranslated regions (5′-UTR) of chloroplast mRNAs with higher affinity than the corresponding sequence of a GC-rich nuclear transcript. The binding affinity of CreS1 for a mutant form of the psbD 5′-UTR with a deletion of a U-rich stretch that is required for translation decreases 4-fold as compared to the wild-type 5′-UTR. Our results suggest that CreS1 protein interacts with U-rich sequences. Most of CreS1 is bound to high-molecular-weight complexes which co-migrate with the 30S small ribosomal subunit, and only a small fraction of CreS1 exists in its free form. CreS1 is localized mainly to the chloroplast stroma albeit a significant fraction is associated with chloroplast membranes. The results suggest that most of CreS1 is associated with the 30S ribosomal subunit throughout the translation process. Upon a shift of cells from the dark to the light, the mRNA levels of CreS1 and Psrp-7, both components of the 30S ribosomal subunit, increase transiently and return to the dark levels after 8h. However, during this dark-to-light transition the levels of CreS1 and of other components of the 30S subunit remain the same suggesting that either protein synthesis or degradation is regulated. The possible implications of these findings are discusse

Perfectly Secure Steganography: Capacity, Error Exponents, and Code Constructions

An analysis of steganographic systems subject to the following perfect undetectability condition is presented in this paper. Following embedding of the message into the covertext, the resulting stegotext is required to have exactly the same probability distribution as the covertext. Then no statistical test can reliably detect the presence of the hidden message. We refer to such steganographic schemes as perfectly secure. A few such schemes have been proposed in recent literature, but they have vanishing rate. We prove that communication performance can potentially be vastly improved; specifically, our basic setup assumes independently and identically distributed (i.i.d.) covertext, and we construct perfectly secure steganographic codes from public watermarking codes using binning methods and randomized permutations of the code. The permutation is a secret key shared between encoder and decoder. We derive (positive) capacity and random-coding exponents for perfectly-secure steganographic systems. The error exponents provide estimates of the code length required to achieve a target low error probability. We address the potential loss in communication performance due to the perfect-security requirement. This loss is the same as the loss obtained under a weaker order-1 steganographic requirement that would just require matching of first-order marginals of the covertext and stegotext distributions. Furthermore, no loss occurs if the covertext distribution is uniform and the distortion metric is cyclically symmetric; steganographic capacity is then achieved by randomized linear codes. Our framework may also be useful for developing computationally secure steganographic systems that have near-optimal communication performance.Comment: To appear in IEEE Trans. on Information Theory, June 2008; ignore Version 2 as the file was corrupte

LHC-like proteins involved in stress responses and biogenesis/repair of the photosynthetic apparatus

LHC (light-harvesting complex) proteins of plants and algae are known to be involved both in collecting light energy for driving the primary photochemical reactions of photosynthesis and in photoprotection when the absorbed light energy exceeds the capacity of the photosynthetic apparatus. These proteins usually contain three transmembrane (TM) helices which span the thylakoid membranes and bind several chlorophyll, carotenoid and lipid molecules. In addition, the LHC protein family includes LHC-like proteins containing one, two, three or even four TM domains. One-helix proteins are not only present in eukaryotic photosynthetic organisms but also in cyanobacteria where they have been named high light-inducible proteins. These small proteins are probably the ancestors of the members of the extant LHC protein family which arouse through gene duplications, deletions and fusions. During evolution, some of these proteins have diverged and acquired novel functions. In most cases, LHC-like proteins are induced in response to various stress conditions including high light, high salinity, elevated temperature and nutrient limitation. Many of these proteins play key roles in photoprotection, notably in non-photochemical quenching of absorbed light energy. Moreover, some of these proteins appear to be involved in the regulation of chlorophyll synthesis and in the assembly and repair of Photosystem II and also of Photosystem I possibly by mediating the insertion of newly synthesized pigments into the photosynthetic reaction centers

Characterization of Tbc2, a nucleus-encoded factor specifically required for translation of the chloroplast psbC mRNA in Chlamydomonas reinhardtii

Genetic analysis has revealed that the three nucleus-encoded factors Tbc1, Tbc2, and Tbc3 are involved in the translation of the chloroplast psbC mRNA of the eukaryotic green alga Chlamydomonas reinhardtii. In this study we report the isolation and phenotypic characterization of two new tbc2 mutant alleles and their use for cloning and characterizing the Tbc2 gene by genomic complementation. TBC2 encodes a protein of 1,115 residues containing nine copies of a novel degenerate 38–40 amino acid repeat with a quasiconserved PPPEW motif near its COOH-terminal end. The middle part of the Tbc2 protein displays partial amino acid sequence identity with Crp1, a protein from Zea mays that is implicated in the processing and translation of the chloroplast petA and petD RNAs. The Tbc2 protein is enriched in chloroplast stromal subfractions and is associated with a 400-kD protein complex that appears to play a role in the translation of specifically the psbC mRNA

A novel multifunctional factor involved in trans-splicing of chloroplast introns in Chlamydomonas

In the chloroplast of Chlamydomonas reinhardtii, psaA mRNA is spliced in trans from three separate precursors which assemble to form two group II introns. A fourth transcript, tscA, completes the structure of the first intron. Of the fourteen nucleus-encoded factors involved in psaA splicing, only two are required for splicing of both introns. We cloned and characterized the first of these more general factors, Raa1. Consistently with its role in psaA splicing, Raa1 is imported in the chloroplast where it is found in a membrane fraction and is part of a large ribonucleoprotein complex. One mutant, raa1-L137H, is defective for splicing of both introns, but another allelic mutant, raa1-314B, still expresses the 3′ part of the Raa1 gene and is deficient only in splicing of intron 2. This observation and a deletion analysis indicate the presence of two domains in Raa1. The C-terminal domain is necessary and sufficient for processing of tscA RNA and splicing of the first intron, while the central domain is essential for splicing of the second intron. The combination of these two functional domains in Raa1 suggests that this new factor may coordinate trans-splicing of the two introns to improve the efficiency of psaA maturatio