Manganese-histidine cluster as the functional center of the water oxidation complex in photosynthesis

The recent model of Kambara and Govindjee for water oxidation [Kambara T. and Govindjee (1985) Proc. Natl. Acad. Sci. U.S.A., 82:6119–6123] has been extended in this paper by examining all the data in order to identify the most likely candidate for the redox-active ligand (RAL), suggested to operate between the water oxidizing complex (WOC) and Z, the electron donor to the reaction center P680. We have concluded that a very suitable candidate for RAL is the imidazole moiety of a histidine residue. The electrochemical data available on imidazole derivatives play heavily in this identification of RAL. Thus, we suggest that histidine might play the role of an electron mediator between the WOC and Z. A model of S-states in terms of their plausible chemical identity is presented here.Abbreviations J electronic spin of ion - P680 reaction center chlorophyll - RAL Redox active ligand - S_n state of the oxygen-evolving system - WOC water oxidation complex - Z electron donor to P680 Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement     

Mechanism of proton-pumping in the cytochrome b/f complex

Several models have been proposed to interpret the mechanism of proton-pumping associated with the electron transfer reactions in the cytochrome b/f complex. Energetics considerations suggest that the proton pump is coupled to the oxidation of cytochrome b by plastoquinone. Experiments performed in living cells under anaerobic conditions suggest that proton-pumping can occur through two independent mechanisms. When the two b cytochromes are reduced prior to a flash illumination i.e. after a long dark anaerobic incubation (>10 minutes), proton-pumping is very likely associated with the reduction of a semiquinone by cyt b which occurs at a site close to the inner face of the membrane. The electrogenic phase is associated with the tranfer of protons via a transmembrane channel. This process is not inhibited by 2-n-nonyl-4-hydroxyquinoline N-oxide (NQNO). Under repetitive-flash or under aerobic conditions, proton-pumping occurs according to a modified Q-cycle mechanism, which is inhibited by NQNO.Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement     

Membrane turnover in crab photoreceptors studied by high-resolution scanning electron microscopy and by a new technique of thick-section transmission electron microscopy

Summary Crab photoreceptors were examined after treatment by the osmium-DMSO-osmium method for high-resolution scanning electron microscopy. This technique of specimen preparation was also adapted for transmission electron microscopy, enabling sections up to 1 urn thick to be viewed in a conventional microscope at 75 kV. With appropriate pretreatment, some cytoskeletal elements can be visualised by both techniques. The methods were then used to investigate some of the daily changes known to occur in photoreceptor cell structure. Striking differences were found in the structure of Golgi bodies present in retinula cells during the synthesis and breakdown phases of the daily cycle of photoreceptor membrane turnover. Cyclic changes were also noticed in the mitochondria of retinula cells, and additional evidence was found for a previously proposed model of rhabdomeral microvillus formation.     

Photosynthetic characteristics of detached barley leaves during greening in the presence of SAN 9785

The effects of the pyridazinone compound SAN 9785 on the photosynthetic competence of leaves, on the photochemical activity of isolated thylakoids and on the formation and spectral properties of chlorophyll-protein complexes were studied during a 72-h greening period of detached etiolated leaves of barley (Hordeum vulgare L. cv. Horpácsi kétsoros). It was established that i) the photosynthetic capacity of the leaves decreased considerably (by 80 and 90%, as determined by¹⁴CO₂ fixation and fast fluorescence induction measurements, respectively); ii) the photochemical activity of isolated thylakoids from water to potassium ferricyanide and from dichlorophenol indophenol/ascorbate to methylviologen exhibited only slight reductions when expressed on a chlorophyll basis compared with the control; iii) the slow fluorescence induction curves of the treated leaves demonstrated the presence of a peculiar fluorescence component interrupting the quenching of fluorescence at around 1 min illumination; iv) a shortage of the chlorophyll-protein complex of photosystem I (CPI) occurred with a higher content of the monomer of the light harvesting complex in the thylakoids of treated leaves; and v) the fluorescence spectrum of the CPI band present in treated leaves indicates the destruction of the structural integrity of this complex during isolation from the membrane.Abbreviations Chl chlorophyll - CPI, CPII chlorophyll-protein complexes of the reaction centres of PSI and PSII - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DPIP 2,6-dichlorophenol indophenol - DPIPH₂ chemically reduced form of DPIP - F _o fluorescence of constant yield - F _v fluorescence of variable yield - F _i ,F _m mitial and maximum yield of fluorescence - LHCP³ monomer of the light-harvesting complex - LHCP² and LHCP¹ oligomers of the light-harvesting complex LHCP³ - PSI, PSII photosystems I, II - SAN 9785 4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazinone, also known as BASF 13-338 - SDS-PAGE sodium dodecyl sulphate-polyacrylamide gel electrophoresis     

Determination of the shape of the operculum by the bithorax complex in Drosophila melanogaster

Summary We have studied the course of the operculum line in the larval hypoderm of several bithorax complex mutants of Drosophila melanogaster. The bifurcation of the line, a characteristic of the first abdominal segment in wild-type (A₁), can also appear in the metathoracic (T₃) and other abdominal segments (A₂, A₃) depending on mutations within the bithorax complex. Therefore, we concluded that the course of the operculum line and thus the shape of the operculum is not determined by a suprasegmental gradient of positional information but by the functional state of the genes of the bithorax complex in each metamere. The dorsal and ventral branches of the operculum line react differently, the dorsal branch being more sensitive to the effect of loss of function mutations (bxd, iab-2 ^k), the ventral branch more affected by gain of function mutations (Hab). In some cases the effects of the mutations on the operculum line differed from those in the adult, suggesting a difference in sensitivity of larval hypodermal cells and histoblast cells to the functional gene products of the bithorax complex.     

Partial purification,subunit structure and thermal stability of the photochemical reaction center of the thermophilic green bacterium Chloroflexus aurantiacus

Spectrally pure reaction center preparations from Chloroflexus aurantiacus have been obtained in a stable form; however, the product contained several contaminating polypeptides. The reaction center pigment molecules (probably three bacteriochlorophyll a and three bacteriopheophytin a molecules) are associated with two polypeptides (M_r = 30000 and 28000) in a reaction center complex of M_r = 52000. No carotenoid is present in the complex. These data together with previous spectral data suggest that the Chloroflexus reaction center represents a more primitive evolutionary form of the purple bacterial reaction center, and that it has little if any relationship to the green bacterial component. A reaction center preparation from Rhodopseudomonas sphaeroides R26 was fully denatured at 50°C while the Chloroflexus reaction center required higher temperatures (70–75°C) for complete denaturation. Thus, an intrinsic membrane protein of a photosynthetic thermophile has been demonstrated to have greater thermal stability than the equivalent component of a mesophile.     

Dephosphorylation and reactivation of phosphorylated purified ox-kidney branched-chain dehydrogenase complex by co-purified phosphatase

The branched-chain 2 oxoacid dehydrogenase complex has been purified from well-washed ox-kidney mitochondria together with branched-chain dehydrogenase kinase. The complex was inactivated and phosphorylated by ATP in about 5 min at 30 degrees C. After hydrolysis of ATP by a contaminating ATPase (5-10 min) the complex was dephosphorylated and reactivated. Dephosphorylation and reactivation were linearly correlated. Reactivation was dependent upon Mg2+ (K0.5 greater than 1 mM) and inhibited completely by 50 mM fluoride. Reactivation and dephosphorylation are attributed to a mitochondrial branched-chain dehydrogenase phosphatase.     

Conformation of antibiotic X-537A (lasalocid-A) and its calcium complex in acetonitrile solvent using circular dichroism spectroscopy

The calcium binding characteristics of antibiotic X-537A (lasalocid-A) in a lipophilic solvent, acetonitrile (CH3CN), have been studied using circular dichroism (CD) spectroscopy. The analysis of the data indicated that in this medium polar solvent, X-537A forms predominantly the charged complexes of stoichiometries 2:1 and 1:1, the relative amounts of the two being dependent on [Ca2+]. The conformations of the complexes, arrived at on the basis of the data, seem to indicate a rigid part encompassing Ca2+, liganded to 3 oxygens of the molecule, viz., the carbonyl, the substituted tetrahydrofuran ring and the substituted pyran ring oxygens (apart from, possibly, the liganding provided by nitrogen atoms of the solvent molecules), and a flexible part consisting of the salicylic acid group of the molecule.     

Titration of the binding sites for the oligomycin-sensitivity conferring protein in beef heart submitochondrial particles

The binding parameters of the oligomycin-sensitivity conferring protein (OSCP) in inside-out particles from beef heart mitochondria have been tested by means of two assays, the oligomycin-sensitive ATP-Pi exchange, and the oligomycin-sensitive ATP hydrolysis. The total number of OSCP binding sites in A particles was equal to 220 pmol/mg particle protein. Each mole of ATPase active site was able to bind 1.1 +/- 0.5 mol OSCP with Kd 1.7 nM.     

Enzymatic methylation of DNA and poly(dG-dC) X poly(dG-dC) modified by 4-acetoxyaminoquinoline-1-oxide, the ultimate carcinogen of 4-nitroquinoline-1-oxide

Both the initial velocity and the overall methylation of Ac-4HAQO modified DNA by a calf brain DNA (cytosine-5-)-methyltransferase are increased as compared to native DNA. The affinity of the modified DNA for the enzyme decreases as a function of the extent of the modification. Heat-denatured, single-stranded DNA shows exactly the opposite results: the more it is modified, the less it is methylated. The poly(dG-dC) X poly(dG-dC) modified by 4NQO is as well methylated as the non-modified one. The carcinogen may induce a tertiary structure favouring the 'walking' of the enzyme along the DNA. The hypermethylation caused by this carcinogen could have a significance in gene activity and cellular differentiation.