The composition of photosystem II (PSII) in the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017 was investigated to enhance the general understanding of the energetics of the PSII reaction center. We first purified photochemically active complexes consisting of a 47-kDa Chl protein (CP47), CP43 (PcbC), D1, D2, cytochrome b 559, PsbI, and a small polypeptide. The pigment composition per two pheophytin (Phe) a molecules was 55 ؎ 7 Chl d, 3.0 ؎ 0.4 Chl a, 17 ؎ 3 ␣-carotene, and 1.4 ؎ 0.2 plastoquinone-9. The special pair was detected by a reversible absorption change at 713 nm (P713) together with a cation radical band at 842 nm. FTIR difference spectra of the specific bands of a 3-formyl group allowed assignment of the special pair. The combined results indicate that the special pair comprises a Chl d homodimer. The primary electron acceptor was shown by photoaccumulation to be Phe a, and its potential was shifted to a higher value than that in the Chl a/Phe a system. The overall energetics of PSII in the Chl d system are adjusted to changes in the redox potentials, with P713 as the special pair using a lower light energy at 713 nm. Taking into account the reported downward shift in the potential of the special pair of photosystem I (P740) in A. marina, our findings lend support to the idea that changes in photosynthetic pigments combine with a modification of the redox potentials of electron transfer components to give rise to an energetic adjustment of the total reaction system. Acaryochloris marina ͉ FTIR ͉ reaction center ͉ photosynthesis ͉ electron transfer C hlorophyll (Chl) a has a ubiquitous role as an electron donor in the photochemical reactions of oxygenic photosynthesis, in which two kinds of photosystem, namely photosystem I (PSI) and photosystem II (PSII), cooperatively drive photosynthetic electron flow from water to NADP ϩ . The reduced cofactor, NADPH, is then used for CO 2 fixation. Chl a is a key pigment that serves as an electron donor called the special pair in PSI and PSII. Acaryochloris spp. are unique cyanobacteria that differ from the majority of photosynthetic organisms by having Chl d (3-desvinyl-3-formyl Chl a) (1-4) as the major pigment (Ͼ95%); Chl a is a minor component but is never absent (5, 6). In photosynthetic organisms, changes in pigment composition affect both the function of pigments and their reaction environments, including the modified proteins that accommodate them. Photosynthetic pigments function in two roles: as lightharvesting components and as electron transfer components. Light harvesting is mainly governed by the orientation and energy levels of pigments, and, in this context, a particular excitation energy level is not an absolute precondition for function, but a relative one. On the other hand, electron transfer reactions are governed by an absolute redox potential, because the photosynthetic oxidation of water requires a very high potential, whereas reduction of NADP ϩ requires a very low potential. For this reason, it is of particul...

Throughout the history of oxygen evolution, two types of photosystem reaction centres (PSI and PSII) have worked in a coordinated manner. The oxygen evolving centre is an integral part of PSII, and extracts an electron from water. PSI accepts the electron, and accumulates reducing power. Traditionally, PSI and PSII are thought to be spatially dispersed. Here, we show that about half of PSIIs are physically connected to PSIs in Arabidopsis thaliana. In the PSI-PSII complex, excitation energy is transferred efficiently between the two closely interacting reaction centres. PSII diverts excitation energy to PSI when PSII becomes closed-state in the PSI-PSII complex. The formation of PSI-PSII complexes is regulated by light conditions. Quenching of excess energy by PSI might be one of the physiological functions of PSI-PSII complexes.

The vast majority of oxygenic photosynthetic organisms use monovinyl chlorophyll for their photosynthetic reactions. For the biosynthesis of this type of chlorophyll, the reduction of the 8-vinyl group that is located on the B-ring of the macrocycle is essential. Previously, we identified the gene encoding 8-vinyl reductase responsible for this reaction in higher plants and termed it DVR. Among the sequenced genomes of cyanobacteria, only several Synechococcus species contain DVR homologues. Therefore, it has been hypothesized that many other cyanobacteria producing monovinyl chlorophyll should contain a vinyl reductase that is unrelated to the higher plant DVR. To identify the cyanobacterial gene that is responsible for monovinyl chlorophyll synthesis, we developed a bioinformatics tool, correlation coefficient calculation tool, which calculates the correlation coefficient between the distributions of a certain phenotype and genes among a group of organisms. The program indicated that the distribution of a gene encoding a putative dehydrogenase protein is best correlated with the distribution of the DVR-less cyanobacteria. We subsequently knocked out the corresponding gene (Slr1923) in Synechocystis sp. PCC6803 and characterized the mutant. The knock-out mutant lost its ability to synthesize monovinyl chlorophyll and accumulated 3,8-divinyl chlorophyll instead. We concluded that Slr1923 encodes the vinyl reductase or a subunit essential for monovinyl chlorophyll synthesis. The function and evolution of 8-vinyl reductase genes are discussed.Chlorophyll is an essential molecule that is utilized in photosynthetic reactions (1). Various chlorophyll species exist among photosynthetic organisms. Anoxygenic photosynthetic bacteria contain bacteriochlorophyll a, b, c, d, e, and g (2), and oxygenic photosynthetic organisms such as cyanobacteria, algae, and land plants produce chlorophyll a, b, c, d and 3,8-divinyl chlorophyll a 3 and b ( Fig. 1) (3, 4). Among them, the biosynthesis of chlorophyll a, b, c, d and all bacteriochlorophyll species requires the reduction of the 8-vinyl group (2). All of these chlorophyll molecules have a common backbone structure. Modification of the side chains on the common backbone structure gives rise to the diversity of chlorophylls (5, 6). Among the variety of chlorophyll species, bacteriochlorophyll a and monovinyl chlorophyll a (Fig. 1) are most commonly used for the photochemistry in photosynthetic organisms. The exceptions to this are a group of marine cyanobacteria, Prochlorococcus species (7), and a cyanobacterial symbiont, Acaryochloris marina (8), which use 3,8-divinyl chlorophyll a (Fig. 1) and chlorophyll d for their photochemistry, respectively. It is not clear why the majority of photosynthetic organisms prefer monovinyl chlorophylls instead of divinyl chlorophyll species. Akimoto et al. (9) recently suggested that the replacement of monovinyl chlorophylls by 3,8-divinyl chlorophylls in the dvr mutant of Arabidopsis thaliana significantly changed the antenna system and t...

Photosystem I (PSI) functions to harvest light energy for conversion into chemical energy. The organisation of PSI is variable depending on the species of organism. Here we report the structure of a tetrameric PSI core isolated from a cyanobacterium, Anabaena sp. PCC 7120, analysed by single-particle cryo-electron microscopy (cryo-EM) at 3.3 Å resolution. The PSI tetramer has a C2 symmetry and is organised in a dimer of dimers form. The structure reveals interactions at the dimer-dimer interface and the existence of characteristic pigment orientations and inter-pigment distances within the dimer units that are important for unique excitation energy transfer. In particular, characteristic residues of PsaL are identified to be responsible for the formation of the tetramer. Time-resolved fluorescence analyses showed that the PSI tetramer has an enhanced excitation-energy quenching. These structural and spectroscopic findings provide insights into the physiological significance of the PSI tetramer and evolutionary changes of the PSI organisations.

The fucoxanthin chlorophyll (Chl) a/c-binding protein (FCP) is responsible for excellent light-harvesting strategies that enable survival in fluctuating light conditions. Here, we report the light-harvesting and quenching states of two FCP complexes, FCP-A and FCP-B/C, isolated from the diatom Chaetoceros gracilis. Pigment analysis revealed that FCP-A is enriched in Chl c, whereas FCP-B/C is enriched in diadinoxanthin, reflecting differences in low-temperature steady-state absorption and fluorescence spectra of each FCP complex. Time-resolved fluorescence spectra were measured at 77 K, and the characteristic lifetimes were determined using global fitting analysis of the spectra. Tens of picosecond (ps) components revealed energy transfer to low-energy Chl a from Chls a and c, whereas the other components showed only fluorescence decay components with no concomitant rise components. The normalized amplitudes of hundreds of picosecond components were relatively 30% in the total fluorescence, whereas those of longest-lived components were 60%. The hundreds of picosecond components were assigned as excitation energy quenching, whereas the longest-lived components were assigned as fluorescence from the final energy traps. These results suggest that 30% of FCP complex forming quenching state and the other 60% of FCP complex forming light-harvesting state exist heterogeneously in each FCP fraction under continuous low-light condition.