Nucleus-encoded precursors to thylakoid lumen proteins of Euglena gracilis possess tripartite presequences. (25/2359)

The complete presequences of the nucleus-encoded precursors to two proteins, cytochrome c6 and the 30-kDa protein of the oxygen-evolving complex, that reside in the thylakoid lumen of the chloroplasts of Euglena gracilis are presented. Sorting of these proteins involves translocation across four membranes, the three-membraned chloroplast envelope and the thylakoid membrane. The tripartite presequences show the structure: signal sequence transit sequence signal sequence. Three hydrophobic domains become apparent: two of them correspond to signal sequences for translocation across the endoplasmic reticulum (ER) membrane and the thylakoid membrane, respectively, whereas the third constitutes the stop-transfer signal contained in the long stroma-targeting part of the tripartite presequence.  (+info)

Electronic spectra of PS I mutants: the peripheral subunits do not bind red chlorophylls in Synechocystis sp. PCC 6803. (26/2359)

Steady-state fluorescence and absorption spectra have been obtained in the Qy spectral region (690-780 nm and 600-750 nm, respectively) for several subunit-deficient photosystem I mutants from the cyanobacterium Synechocystis sp. PCC 6803. The 77 K fluorescence spectra of the wild-type and subunit-deficient mutant photosystem I particles are all very similar, peaking at approximately 720 nm with essentially the same excitation spectrum. Because emission from far-red chlorophylls absorbing near 708 nm dominates low-temperature fluorescence in Synechocystis sp., these pigments are not coordinated to any the subunits PsaF, Psa I, PsaJ, PsaK, PsaL, or psaM. The room temperature (wild-type-mutant) absorption difference spectra for trimeric mutants lacking the PsaF/J, PsaK, and PsaM subunits suggest that these mutants are deficient in core antenna chlorophylls (Chls) absorbing near 685, 670, 675, and 700 nm, respectively. The absorption difference spectrum for the PsaF/J/I/L-deficient photosystem I complexes at 5 K reveals considerably more structure than the room-temperature spectrum. The integrated absorbance difference spectra (when normalized to the total PS I Qy spectral area) are comparable to the fractions of Chls bound by the respective (groups of) subunits, according to the 4-A density map of PS I from Synechococcus elongatus. The spectrum of the monomeric PsaL-deficient mutant suggests that this subunit may bind pigments absorbing near 700 nm.  (+info)

An extraretinally expressed insect cryptochrome with similarity to the blue light photoreceptors of mammals and plants. (27/2359)

Photic entrainment of insect circadian rhythms can occur through either extraretinal (brain) or retinal photoreceptors, which mediate sensitivity to blue light or longer wavelengths, respectively. Although visual transduction processes are well understood in the insect retina, almost nothing is known about the extraretinal blue light photoreceptor of insects. We now have identified and characterized a candidate blue light photoreceptor gene in Drosophila (DCry) that is homologous to the cryptochrome (Cry) genes of mammals and plants. The DCry gene is located in region 91F of the third chromosome, an interval that does not contain other genes required for circadian rhythmicity. The protein encoded by DCry is approximately 50% identical to the CRY1 and CRY2 proteins recently discovered in mammalian species. As expected for an extraretinal photoreceptor mediating circadian entrainment, DCry mRNA is expressed within the adult brain and can be detected within body tissues. Indeed, tissue in situ hybridization demonstrates prominent expression in cells of the lateral brain, which are close to or coincident with the Drosophila clock neurons. Interestingly, DCry mRNA abundance oscillates in a circadian manner in Drosophila head RNA extracts, and the temporal phasing of the rhythm is similar to that documented for the mouse Cry1 mRNA, which is expressed in clock tissues. Finally, we show that changes in DCry gene dosage are associated predictably with alterations of the blue light resetting response for the circadian rhythm of adult locomotor activity.  (+info)

Genetic engineering of the unsaturation of fatty acids in membrane lipids alters the tolerance of Synechocystis to salt stress. (28/2359)

The role of unsaturated fatty acids in membrane lipids in the tolerance of the photosynthetic machinery to salt stress was studied by comparing the desA-/desD- mutant of Synechocystis sp. PCC 6803, which contained monounsaturated fatty acids, with the wild-type strain, which contained a full complement of polyunsaturated fatty acids. In darkness, the loss of oxygen-evolving photosystem II activity in the presence of 0.5 M NaCl or 0.5 M LiCl was much more rapid in desA-/desD- cells than in wild-type cells. Oxygen-evolving activity that had been lost during incubation with 0.5 M NaCl in darkness returned when cells were transferred to conditions that allowed photosynthesis or respiration. Recovery was much greater in wild-type than in desA-/desD- cells, and it was prevented by lincomycin. Thus, the unsaturation of fatty acids is important in the tolerance of the photosynthetic machinery to salt stress. It appears also that the activity and synthesis of the Na+/H+ antiporter system might be suppressed under high-salt conditions and that this effect can be reversed, in part, by the unsaturation of fatty acids in membrane lipids.  (+info)

Roseateles depolymerans gen. nov., sp. nov., a new bacteriochlorophyll a-containing obligate aerobe belonging to the beta-subclass of the Proteobacteria. (29/2359)

Strains 61AT (T = type strain) and 61B2, the first bacteriochlorophyll (BChl) a-containing obligate aerobes to be classified in the beta-subclass of the Proteobacteria, were isolated from river water. The strains were originally isolated as degraders of poly(hexamethylene carbonate) (PHC). The organisms can utilize PHC and some other biodegradable plastics. The strains grow only under aerobic conditions. Good production of BChl a and caroterioid pigments is achieved on PHC agar plates and an equivalent production is observed under oligotrophic conditions on agar medium. Spectrometric results suggest that BChl a is present in light-harvesting complex I and the photochemical reaction centre. The main carotenoids are spirilloxanthin and its precursors. Analysis of the 16S rRNA gene sequence indicated that the phylogenetic positions of the two strains are similar to each other and that their closest relatives are the genera Rubrivivax, ideonella and Leptothrix with similarities of 96.3, 96.2 and 96.1%, respectively. The cells are motile, straight rods and contain poly-beta-hydroxybutyrate granules. Ubiquinone-8 is the predominant quinone. Vitamins are not required for growth. The G + C content of genomic DNA is 66.2-66.3 mol%. Genetic and phenotypic features suggest that the strains represent a new genus in the beta-subclass which is evenly distant from known genera. Consequently, the name Roseateles depolymerans gen. nov., sp. nov. is proposed for the strains; the type strain of Roseateles depolymerans is strain 61AT (= DSM 11813T).  (+info)

Rhodovulum iodosum sp. nov. and Rhodovulum robiginosum sp. nov., two new marine phototrophic ferrous-iron-oxidizing purple bacteria. (30/2359)

Two new strains of marine purple bacteria, N1T and N2T, were isolated from coastal sediment of the North Sea (Germany) with ferrous iron as the only electron donor for anoxygenic photosynthesis. The isolates are the first salt-dependent, ferrous-iron-oxidizing purple bacteria characterized so far. Analysis of 16S rRNA gene sequences revealed an affiliation with the genus Rhodovulum, which until now comprises only marine species. The sequence similarity of both strains was 95.2%, and their closest relative was Rhodovulum adriaticum. Like all known Rhodovulum species, the new strains had ovoid to rod-shaped cells, contained bacteriochlorophyll a and carotenoids of the spheroidene series, and were able to oxidize sulfide and thiosulfate. Like Rhodovulum adriaticum, both strains were unable to assimilate sulfate; for growth they needed a reduced sulfur source, e.g. thiosulfate. In contrast to the new strains, none of the known Rhodovulum species tested was able to oxidize ferrous iron or iron sulfide. In growth experiments, strains N1T and N2T oxidized 65 and 95%, respectively, of the ferrous iron supplied. Electron diffraction analysis revealed ferrihydrite as the main product of ferrous iron oxidation. In addition, traces of magnetite were formed. Strains N1T (= DSM 12328T) and N2T (= DSM 12329T) are described as Rhodovulum iodosum sp. nov. and Rhodovulum robiginosum sp. nov., respectively.  (+info)

Isolated transmembrane helices arranged across a membrane: computational studies. (31/2359)

A computational procedure for predicting the arrangement of an isolated helical fragment across a membrane was developed. The procedure places the transmembrane helical segment into a model triple-phase system 'water-octanol-water'; pulls the segment through the membrane, varying its 'global' position as a rigid body; optimizes the intrahelical and solvation energies in each global position by 'local' coordinates (dihedral angles of side chains); and selects the lowest energy global position for the segment. The procedure was applied to 45 transmembrane helices from the photosynthetic reaction center from Rhodopseudomonas viridis, cytochrome c oxidase from Paracoccus denitrificans and bacteriorhodopsin. In two thirds of the helical fragments considered, the procedure has predicted the vertical shifts of the fragments across the membrane with an accuracy of -0.15 +/- 3.12 residues compared with the experimental data. The accuracy for the remaining 15 fragments was 2.17 +/- 3.07 residues, which is about half of a helix turn. The procedure predicts the actual membrane boundaries of transmembrane helical fragments with greater accuracy than existing statistical methods. At the same time, the procedure overestimates the tilt values for the helical fragments.  (+info)

Alterations in carboxylate ligation at the active site of photosystem II. (32/2359)

Photosystem II (PSII) is the photosynthetic enzyme catalyzing the oxidation of water and reduction of plastoquinone (Q). This reaction occurs at a catalytic site containing four manganese atoms and cycling among five oxidation states, the Sn states, where n refers to the number of oxidizing equivalents stored. Biochemical and spectroscopic techniques have been used previously to conclude that aspartate 170 in the D1 subunit influences the structure and function of the PSII active site (Boerner, R. J., Nguyen, A. P., Barry, B. A., and Debus, R. J. (1992) Biochemistry 31, 6660-6672). Substitution of glutamate for aspartate 170 resulted in an assembled manganese cluster, which was capable of enzymatic turnover, but at lower steady-state oxygen evolution rates. Here, we obtained the difference (light-minus-dark) Fourier transform IR spectrum associated with the S2Q--minus-S1Q transition by illumination of oxygen-evolving wild-type and DE170D1 PSII preparations at 200 K. These spectra are known to be dominated by contributions from carboxylic acid and carboxylate residues that are close to or ligating the manganese cluster. Substitution of glutamate for aspartate 170 results in alterations in the S2Q--minus-S1Q spectrum; the alterations are consistent with a change in carboxylate coordination to manganese or calcium. In particular, the spectra are consistent with a shift from bridging/bidentate carboxylates in wild-type PSII to unidentate carboxylate ligation in DE170D1 PSII.  (+info)