Electron Transfer in a Bio-Photoelectrode Based on Photosystem I Multilayer Immobilized on the Conducting Glass.

A film of ~40 layers of partially oriented photosystem I (PSI) complexes isolated from the red alga Cyanidioschyzon merolae formed on the conducting glass through electrodeposition was investigated by time-resolved absorption spectroscopy and chronoamperometry. The experiments were performed at a range of electric potentials applied to the film and at different compositions of electrolyte solution being in contact with the film. The amount of immobilized proteins supporting light-induced charge separation (active PSI) ranged from ~10%, in the absence of any reducing agents (redox compounds or low potential), to ~20% when ascorbate and 2,6-dichlorophenolindophenol were added, and to ~35% when the high negative potential was additionally applied. The origin of the large fraction of permanently inactive PSI (65-90%) was unclear. Both reducing agents increased the subpopulation of active PSI complexes, with the neutral P700 primary electron donor, by reducing significant fractions of the photo-oxidized P700 species. The efficiencies of light-induced charge separation in the PSI film (10-35%) did not translate into an equally effective generation of photocurrent, whose internal quantum efficiency reached the maximal value of 0.47% at the lowest potentials. This mismatch indicates that the vast majority of the charge-separated states in multilayered PSI complexes underwent charge recombination.

Competition between intra-protein charge recombination and electron transfer outside photosystem I complexes used for photovoltaic applications.

Photosystem I (PSI) complexes isolated from three different species were electrodeposited on FTO conducting glass, forming a photoactive multilayer of the photo-electrode, for investigation of intricate electron transfer (ET) properties in such green hybrid nanosystems. The internal quantum efficiency of photo-electrochemical cells (PEC) containing the PSI-based photo-electrodes did not exceed ~ 0.5%. To reveal the reason for such a low efficiency of photocurrent generation, the temporal evolution of the transient concentration of the photo-oxidized primary electron donor, P+, was studied in aqueous suspensions of the PSI complexes by time-resolved absorption spectroscopy. The results of these measurements provided the information on: (1) completeness of charge separation in PSI reaction centers (RCs), (2) dynamics of internal charge recombination, and (3) efficiency of electron transfer from PSI to the electrolyte, which is the reaction competing with the internal charge recombination in the PSI RC. The efficiency of the full charge separation in the PSI complexes used for functionalization of the electrodes was ~ 90%, indicating that incomplete charge separation was not the main reason for the small yield of photocurrents. For the PSI particles isolated from a green alga Chlamydomonas reinhardtii, the probability of ET outside PSI was ~ 30-40%, whereas for their counterparts isolated from a cyanobacterium Synechocystis sp. PCC 6803 and a red alga Cyanidioschyzon merolae, it represented a mere ~ 4%. We conclude from the transient absorption data for the PSI biocatalysts in solution that the observed small photocurrent efficiency of ~ 0.5% for all the PECs analyzed in this study is likely due to: (1) limited efficiency of ET outside PSI, particularly in the case of PECs based on PSI from Synechocystis and C. merolae, and (2) the electrolyte-mediated electric short-circuiting in PSI particles forming the photoactive layer, particularly in the case of the C. reinhardtii PEC.

NuA4 and H2A.Z control environmental responses and autotrophic growth in Arabidopsis.

Nucleosomal acetyltransferase of H4 (NuA4) is an essential transcriptional coactivator in eukaryotes, but remains poorly characterized in plants. Here, we describe Arabidopsis homologs of the NuA4 scaffold proteins Enhancer of Polycomb-Like 1 (AtEPL1) and Esa1-Associated Factor 1 (AtEAF1). Loss of AtEAF1 results in inhibition of growth and chloroplast development. These effects are stronger in the Atepl1 mutant and are further enhanced by loss of Golden2-Like (GLK) transcription factors, suggesting that NuA4 activates nuclear plastid genes alongside GLK. We demonstrate that AtEPL1 is necessary for nucleosomal acetylation of histones H4 and H2A.Z by NuA4 in vitro. These chromatin marks are diminished genome-wide in Atepl1, while another active chromatin mark, H3K9 acetylation (H3K9ac), is locally enhanced. Expression of many chloroplast-related genes depends on NuA4, as they are downregulated with loss of H4ac and H2A.Zac. Finally, we demonstrate that NuA4 promotes H2A.Z deposition and by doing so prevents spurious activation of stress response genes.

On the nature of uncoupled chlorophylls in the extremophilic photosystem I-light harvesting I supercomplex.

Photosystem I core-light-harvesting antenna supercomplexes (PSI-LHCI) were isolated from the extremophilic red alga Cyanidioschyzon merolae and studied by three fluorescence techniques in order to characterize chlorophylls (Chls) energetically uncoupled from the PSI reaction center (RC). Such Chls are observed in virtually all optical experiments of any PSI core and PSI-LHCI supercomplex preparations across various species and may influence the operation of PSI-based solar cells and other biohybrid systems. However, the nature of the uncoupled Chls (uChls) has never been explored deeply before. In this work, the amount of uChls was controlled by stirring the solution of C. merolae PSI-LHCI supercomplex samples at elevated temperature (~303 K) and was found to increase from <2% in control samples up to 47% in solutions stirred for 3.5 h. The fluorescence spectrum of uChls was found to be blue-shifted by ~20 nm (to ~680 nm) relative to the fluorescence band from Chls that are well coupled to PSI RC. This effect indicates that mechanical stirring leads to disappearance of some red Chls (emitting at above ~700 nm) that are present in the intact LHCI antenna associated with the PSI core. Comparative diffusion studies of control and stirred samples by fluorescence correlation spectroscopy together with biochemical analysis by SDS-PAGE and BN-PAGE indicate that energetically uncoupled Lhcr subunits are likely to be still physically attached to the PSI core, albeit with altered three-dimensional organization due to the mechanical stress.

Remodeling of excitation energy transfer in extremophilic red algal PSI-LHCI complex during light adaptation.

Photosynthetic PSI-LHCI complexes from an extremophilic red alga C. merolae grown under varying light regimes are characterized by decreasing size of LHCI antenna with increasing illumination intensity [1]. In this study we applied time-resolved fluorescence spectroscopy to characterize the kinetics of energy transfer processes in three types of PSI-LHCI supercomplexes isolated from the low (LL), medium (ML) and extreme high light (EHL) conditions. We show that the average rate of fluorescence decay is not correlated with the size of LHCI antenna and is twice faster in complexes isolated from ML-grown cells (~25-30 ps) than from both LL- and EHL-exposed cells (~50-55 ps). The difference is mainly due to a contribution of a long ~100-ps decay component detected only for the latter two PSI samples. We propose that the lack of this phase in ML complexes is caused by perfect coupling of this antenna to PSI core and lack of low-energy chlorophylls in LHCI. On the other hand, the presence of the slow, ~100-ps, fluorescence decay component in LL and EHL complexes may be due to the weak coupling between PSI core and LHCI antenna complex, and due to the presence of particularly low-energy or red chlorophylls in LHCI. Our study has revealed the remarkable functional flexibility of light harvesting strategies that have evolved in the extremophilic red algae in response to harsh or limiting light conditions involving accumulation of low energy chlorophylls that exert two distinct functions: as energy traps or as far-red absorbing light harvesting antenna, respectively.

Molecular Mechanisms of Photoadaptation of Photosystem I Supercomplex from an Evolutionary Cyanobacterial/Algal Intermediate.

The monomeric photosystem I-light-harvesting antenna complex I (PSI-LHCI) supercomplex from the extremophilic red alga Cyanidioschyzon merolae represents an intermediate evolutionary link between the cyanobacterial PSI reaction center and its green algal/higher plant counterpart. We show that the C. merolae PSI-LHCI supercomplex is characterized by robustness in various extreme conditions. By a combination of biochemical, spectroscopic, mass spectrometry, and electron microscopy/single particle analyses, we dissected three molecular mechanisms underlying the inherent robustness of the C. merolae PSI-LHCI supercomplex: (1) the accumulation of photoprotective zeaxanthin in the LHCI antenna and the PSI reaction center; (2) structural remodeling of the LHCI antenna and adjustment of the effective absorption cross section; and (3) dynamic readjustment of the stoichiometry of the two PSI-LHCI isomers and changes in the oligomeric state of the PSI-LHCI supercomplex, accompanied by dissociation of the PsaK core subunit. We show that the largest low light-treated C. merolae PSI-LHCI supercomplex can bind up to eight Lhcr antenna subunits, which are organized as two rows on the PsaF/PsaJ side of the core complex. Under our experimental conditions, we found no evidence of functional coupling of the phycobilisomes with the PSI-LHCI supercomplex purified from various light conditions, suggesting that the putative association of this antenna with the PSI supercomplex is absent or may be lost during the purification procedure.

AtEAF1 is a potential platform protein for Arabidopsis NuA4 acetyltransferase complex.

BACKGROUND: Histone acetyltransferase complex NuA4 and histone variant exchanging complex SWR1 are two chromatin modifying complexes which act cooperatively in yeast and share some intriguing structural similarities. Protein subunits of NuA4 and SWR1-C are highly conserved across eukaryotes, but form different multiprotein arrangements. For example, the human TIP60-p400 complex consists of homologues of both yeast NuA4 and SWR1-C subunits, combining subunits necessary for histone acetylation and histone variant exchange. It is currently not known what protein complexes are formed by the plant homologues of NuA4 and SWR1-C subunits. RESULTS: We report on the identification and molecular characterization of AtEAF1, a new subunit of Arabidopsis NuA4 complex which shows many similarities to the platform protein of the yeast NuA4 complex. AtEAF1 copurifies with Arabidopsis homologues of NuA4 and SWR1-C subunits ARP4 and SWC4 and interacts physically with AtYAF9A and AtYAF9B, homologues of the YAF9 subunit. Plants carrying a T-DNA insertion in one of the genes encoding AtEAF1 showed decreased FLC expression and early flowering, similarly to Atyaf9 mutants. Chromatin immunoprecipitation analyses of the single mutant Ateaf1b-2 and artificial miRNA knock-down Ateaf1 lines showed decreased levels of H4K5 acetylation in the promoter regions of major flowering regulator genes, further supporting the role of AtEAF1 as a subunit of the plant NuA4 complex. CONCLUSIONS: Growing evidence suggests that the molecular functions of the NuA4 and SWR1 complexes are conserved in plants and contribute significantly to plant development and physiology. Our work provides evidence for the existence of a yeast-like EAF1 platform protein in A. thaliana, filling an important gap in the knowledge about the subunit organization of the plant NuA4 complex.