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Functional and structural analysis of the photosynthetic apparatus of Rhodobacter veldkampii.
No full textStructural role od PufX in the dimerization of the photosynthetic core complex in Rhodobacter sphaeroides
No full textMonomeric and dimeric PufX-containing core complexes have been purified from membranes of wild-type Rhodobacter sphaeroides. Reconstitution of both samples by detergent removal in the presence of lipids leads to the formation of two-dimensional crystals constituted of dimeric core complexes. Two-dimensional crystals were further analyzed by cryoelectron microscopy and atomic force microscopy. A projection map at 26-Å resolution reveals that core complexes assemble in an "S"-shaped dimeric complex. Each core complex is composed of one reaction center, 12 light-harvesting 1 /-heterodimers, and one PufX protein. The light-harvesting 1 assemblies are open with a gap of density of 30-Å width and surround oriented reaction centers. A maximum density is found at the dimer junction. Based on the projection map, a model is proposed, in which the two PufX proteins are located at the dimer junction, consistent with the finding of dimerization of monomeric core complexes upon reconstitution. This localization of PufX in the core complex implies that PufX is the structural key for the dimer complex formation rather than a channel-forming protein for the exchange of ubiquinone/ubiquinol between the reaction center and the cytochrome bc1 complex
Structure of the core complex of Blastochloris viridis and of Rhodobacter sphaeroides
No full textIn purple photosynthetic bacteria, two light harvesting complexes
(LH), LH2 and LH1, ensure the collection of light. Then, the excitation
energy is funneled towards the reaction center (RC), where after
two photoreactions and proton captures, ubiquinol (QH2) formed
at the QB site of the RC dissociates into the membrane. The cytochrome
(cyt) bc1 complex utilizes QH2 and oxidized cytochrome c2
as reductant and oxidant, respectively. The net result is a cyclic
electron transfer that promotes the formation of a proton gradient
across the membrane, which is utilized for ATP synthesis by
F1F0ATPsynthase (for review see Hu 2002, #1264). The description
of the bacterial photosynthetic apparatus at atomic level is nearly
complete with the structures of two RC, two LH2, and the cytochrome
bc1 complex (see http://blanco.biomol.uci.edu/Membrane_
Proteins_xtal.html). The last component not yet solved is the core
complex formed by the LH1 and the RC, in which the transformation
of light energy into charge separation occurs. A central
question is the coupling between the RC and the cytochrome bc1
complex, or how the quinones produced by the RC are transferred
through the LH1 fence to reach the cytochrome bc1 complex.
Here we present the structural analysis of two different core complexes,
from Blastochloris (Blc.) viridis and from Rhodobacter (Rb)
sphaeroides
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Structural and functional analysis of the reaction center-light harvesting complex of Rhodobacter sphaeroides
No full textDimeric photosynthetic reaction center-light harvesting complex (RC-LH1) has been purified from the photosynthetic
bacterium Rhodobacter sphaeroides grown under semiaerobic conditions. Removal of the detergent in the presence of lipids
leads to the formation of two-dimensional crystals. Analysis by cryoelectron mycroscopy at a resolution of 26 A ° reveals an
‘‘S’’-shaped dimeric complex where the continuity of the LH1 ring that surrounds the RC is interrupted. The higher density of
the projection map at the junction between the two monomers of core complex is attributed to a dimer of the PufX peptide
(Scheuring et al., 2004, J. Biol. Chem. 279, 3620). These data confirm the structural role of PufX, a single transmembrane
protein required for the photosynthetic phenotype (Farchaus et al., 1992, EMBO J., 11, 2779), responsible for the dimerization
of the RC-LH1 complex (Francia et al., 1999, Biochemistry 38, 6834).The functionality of the isolated complex, purified from
photosynthetically and semiaerobically grown bacteria, was analyzed by time resolved spectroscopy.
Upon excitation of the sample with an actinic laser pulse, the kinetics of charge recombination from the state P +QAQB- to
the neutral state PQAQB exibhit a slow phase with an half time of approximately 4 s, at least four times larger than what
usually observed in RC complex deprived of the LH1. Stoichiometric determinations of the quinone (Q10) present in the RCLH1
indicate a Q10/RC-LH1 ratio >10. These quinones are functionally coupled to the RC-LH1 complex, as judged from the
extent of cytochrome c2 rapidly oxidized under continuous illumination. Charge recombination kinetics have been analyzed on
the basis of a model proposed by Shinkarev and Wraight (Shinkarev and Wraight, 1993, in ‘‘The photosynthetic reaction
center’’ Vol. 1, 193) that take into account the binding of quinone at the QB site when a quinone pool is present. The slowing
down of the recombination reaction experimentally detected cannot be simply explained by a quinone concentration effect. The
model predicts a lower limit of the charge recombination rate constant when the quinone concentration is raised to infinity that
is well above the one measured in the RC-LH1, indicating that, even in the presence of saturating quinone conditions the
recombination reaction cannot be so slow as experimentally observed. These data suggests that a stabilization of the charge
separated state P +QAQB-, leading to a slower recombination reaction, is induced by the LH1 antenna complex
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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