UV optical difference spectrum associated with the reduction of electron acceptor A1in photosystem I of higher plants

The secondary electron acceptor of photosystem I inGloeobacter violaceusPCC 7421 is menaquinone-4 that is synthesized by a unique but unknown pathway

The secondary electron acceptor of photosystem (PS) I in the cyanobacterium Gloeobacter violaceus PCC 7421 was identified as menaquinone-4 (MQ-4) by comparing high performance liquid chromatograms and absorption spectra with an authentic compound. The MQ-4 content was estimated to be two molecules per one molecule of chlorophyll (Chl) a', a constituent of P700. Comparative genomic analyses showed that six of eight men genes, encoding phylloquinone/MQ biosynthetic enzymes, are missing from the G. violaceus genome. Since G. violaceus clearly synthesizes MQ-4, the combined results indicate that this cyanobacterium must have a novel pathway for the synthesis of 1,4-dihydroxy-2-naphthoic acid.

Deuteration effects on the in vivo EPR spectrum of the reduced secondary photosystem I electron acceptor A1 in cyanobacteria

The photoreduction of the secondary PSI electron acceptor A1 in vivo has recently been detected via X-band EPR spectroscopy in intact spinach chloroplasts and in marine cyanobacteria Synechococcus PCC 7002 [Klughammer, C., and Pace, R. J. (1997) Biochim. Biophys. Acta 1318, 133-144]. A further study of the A1- EPR spectrum of Synechococcus PCC 7002 at room temperature with higher-field resolution revealed partially resolved hyperfine structure which was dominated by 0.4 mT splittings of three equivalent protons. The hyperfine splitting was not significantly affected by incubation of the cyanobacteria in 2H2O medium for 20 h, but was absent in fully deuterated cyanobacteria that were grown in 2H2O medium. Anisotropic g-factors consistent with a phylloquinone radical were derived by spectra simulation. Biosynthetic protonation of quinones via the CH3 donor L-methionine in deuterated cells maintained hyperfine structure in the A1- spectrum, indicating the incorporation of CH3 groups in 60% of the deuterated, photoactive A1 molecules. Conversely, biosynthetic quinone deuteration via L-[methyl-d3]methionine in protonated cells led to the loss of the 0. 4 mT splittings in 54% of the A1 molecules. These observations confirm the conclusion of Heathcote et al. [(1996) Biochemistry 35, 6644-6650] of the identity of EPR-detected, photoreduced A1- in vivo with a phylloquinone (vitamin K1) radical in PSI. The partially resolved hyperfine structure of the A1- spectrum indicates an altered spin distribution in the bound vitamin K1- radical in vivo compared to that of unbound vitamin K1- in vitro.

Photoaccumulation in photosystem I does produce a phylloquinone (A1.-) radical

Previous work has challenged the assignment of a photoaccumulated EPR signal to the phylloquinone electron acceptor in photosystem I, A1.-. Biosynthetic deuteration of the phylloquinone in the cyanobacterium Anabaena variabilis has been shown to narrow this photoaccumulated signal, demonstrating that the signal arises from A1.-. The ESP signal attributed to P700.+A1.- is also narrowed by this deuteration, showing that the photoaccumulated EPR signal and the ESP signal are monitoring the same redox component. Confirmation that the photoaccumulated EPR signal comes from deuterated phylloquinone was obtained by exchanging the deuterated for protonated phylloquinone, which broadened the photoaccumulated EPR signal.

Dynamics of the history of photosynthesis research

A personal view of the history of progress in photosynthesis research beginning in the seventeenth century and ending in 1992 is presented in a chart form. The 350-year time span is divided arbitrarily into seven periods by the "development junctures", which are likened to bamboo joints. The tempo of progress is reflected in the duration of the periods, starting from over 200 years for Period I, which progressively shortens in subsequent periods. This brief introduction highlights some of the events to show the dynamic nature of the progress in photosynthesis research.

Photosystem I complex

Photosystem I is an integral component of the thylakoid membrane which catalyzes the photoreduction of ferredoxin using plastocyanin or cytochrome c as electron donor. In higher plants, the photosystem I complex is composed of eight protein subunits, chlorophyll a, carotenoids, phylloquinone and bound iron sulfur clusters. The molecular biology and biochemistry of the complex are discussed in relation to the structure and function of the individual components. The mechanisms involved in the assembly of the components into a functional complex are also discussed.

Molecular cloning and nucleotide sequence of the psaA and psaB genes of the cyanobacterium Synechococcus sp. PCC 7002

The psaA and psaB genes, which encode the P700 chlorophyll a apoproteins of the Photosystem I complex, have been cloned from the unicellular, transformable cyanobacterium Synechococcus sp. PCC 7002. The nucleotide sequence of these genes and of their flanking sequences have been determined by the chain termination method. As found in the chloroplast genomes of higher plants, the psaA gene lies 5' to the psaB gene; however, the cyanobacterial genes are separated by a greater distance (173 vs. 25-26 bp). The psaA gene is predicted to encode a polypeptide of 739 amino acid residues (81.7 kDa), and the psaB gene is predicted to encode a polypeptide of 733 residues (81.4 kDa). The cyanobacterial psa gene products are 76% to 81% identical to their higher plant homologues; moreover, because of conservative amino acid replacements, the cyanobacterial sequences are more than 95% homologous to those determined for higher plants. These results provide the basis for a genetic analysis of Photosystem I, and are discussed in relationship to structural and functional aspects of the Photosystem I complexes of both cyanobacteria and higher plants.

Is phylloquinone an obligate electron carrier in photosystem I?

Comparative quantitative analysis of phylloquinone content and photochemically competent P-700 has been performed on photosystem I particles subjected to photolysis with ultraviolet irradiation. Nonirradiated control particles exhibit a phylloquinone/P-700 stoichiometry of 1.9 +/- 0.2. Photolysis of the photosystem I particles induces a progressive depletion of phylloquinone, however, photochemistry as assayed at room temperature by the photooxidation of P-700 is unaffected. These data are not consistent with the assignment of phylloquinone as a functional intermediate at room temperature between P-700 and the iron-sulfur clusters, center A and center B.

Comparison of the electron spin polarized spectrum found in plant photosystem I and in iron-depleted bacterial reaction centers with time-resolved K-band EPR; evidence that the photosystem I acceptor A1 is a quinone

The suggestion that the electron acceptor A1 in plant photosystem I (PSI) is a quinone molecule is tested by comparisons with the bacterial photosystem. The electron spin polarized (ESP) EPR signal due to the oxidized donor and reduced quinone acceptor (P 870 (+) Q(-)) in iron-depleted bacterial reaction centers has similar spectral characteristics as the ESP EPR signal in PSI which is believed to be due to P 700 (+) A 1 (-) , the oxidized PSI donor and reduced A1. This is also true for better resolved spectra obtained at K-band (∼24 GHz). These same spectral characteristics can be simulated using a powder spectrum based on the known g-anisotropy of reduced quinones and with the same parameter set for Q(-) and A1 (-). The best resolution of the ESP EPR signal has been obtained for deuterated PSI particles at K-band. Simulation of the A1 (-) contribution based on g-anisotropy yields the same parameters as for bacterial Q(-) (except for an overall shift in the anisotropic g-factors, which have previously been determined for Q(-)). These results provide evidence that A1 is a quinone molecule. The electron spin polarized signal of P700 (+) is part of the better resolved spectrum from the deuterated PSI particles. The nature of the P700 (+) ESP is not clear; however, it appears that it does not exhibit the polarization pattern required by mechanisms which have been used so far to explain the ESP in PSI.