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Gas-Phase Acylium Ion Transfer Reactions Mediated by a Proton Shuttle Mechanism

FILETI, Eudes E.; OLIVEIRA, Anselmo E. De; MORGON, Nelson H.; RIVEROS, Jose M.
Fonte: WILEY-BLACKWELL Publicador: WILEY-BLACKWELL
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.02%
The mechanism and the energy profile of the gas-phase reaction that mimics esterification under acidic conditions have been investigated at different levels of theory. These reactions are known to proceed with rate constants close to the collision limit in the gas-phase and questions have been raised as to whether the typical addition-elimination mechanism via a tetrahedral intermediate can explain the ease of these processes. Because these reactions are common to many organic and biochemical processes it is important to understand the intrinsic reactivity of these systems. Our calculations at different levels of theory reveal that a stepwise mechanism via a tetrahedral species is characterized by energy barriers that are inconsistent with the experimental results. For the thermoneutral exchange between protonated acetic acid and water and the exothermic reaction of protonated acetic acid and methanol our calculations show that these reactions proceed initially by a proton shuttle between the carbonyl oxygen and the hydroxy oxygen of acetic acid mediated by water, or methanol, followed by displacement at the acylium ion center. These findings suggest that the reactions in the gas-phase should be viewed as an acylium ion transfer reaction. (C) 2010 Wiley Periodicals...

The Incremental Shuttle Walk Test in Older Brazilian Adults

Juergensen, Soraia Pilon; de Oliveira Antunes, Leticia Claudia; Tanni, Suzana Erico; Banov, Marcos Carraro; Lucheta, Paulo Adolfo; Bucceroni, Alessandra Freire; Godoy, Irma; Dourado, Victor Zuniga
Fonte: Karger Publicador: Karger
Tipo: Artigo de Revista Científica Formato: 223-228
ENG
Relevância na Pesquisa
36.02%
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Processo FAPESP: 07/08673-3; Background: Despite widespread use of the incremental shuttle walk distance (ISWD), there are no reference equations for predicting it. Objectives: We aimed to evaluate ISWD in healthy subjects and to establish a reference equation for its prediction. Methods: 131 Brazilian individuals (61 males; 59 +/- 10 years) performed 2 walk tests in a 10-m long corridor. We assessed height, weight, body mass index, forced expiratory volume in 1 s, forced vital capacity and self-reported physical activity. Results: Mean ISWD was greater in males than in females (606 +/- 167 vs. 443 +/- 117 m; p < 0.001). ISWD correlated significantly (p < 0.05) with age (r = -0.51), height (r = 0.54) and weight (r = 0.20). A predictive model including age, height, weight and gender explained 50.3% of the ISWD variance. In an additional group of 20 subjects prospectively studied, the difference between measured and predicted ISWD was not statistically significant (534 +/- 84 vs. 552 +/- 87 m, respectively), representing 97 +/- 12% of the predicted value calculated with our reference equation for ISWD. Conclusions: This reference equation including demographic and anthropomorphic attributes could be useful for interpreting the walking performance of patients with chronic diseases that affect exercise capacity. Copyright (C) 2010 S. Karger AG...

Involvement of hippocampal NMDA receptors in retention of shuttle avoidance conditioning in rats

Roesler,R.; Kuyven,C.R.; Kruel,A.V.S.; Quevedo,J.; Ferreira,M.B.C.
Fonte: Associação Brasileira de Divulgação Científica Publicador: Associação Brasileira de Divulgação Científica
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/1998 EN
Relevância na Pesquisa
36.14%
The purpose of this research was to evaluate the role of hippocampal N-methyl-D-aspartate (NMDA) receptors in acquisition and consolidation of memory during shuttle avoidance conditioning in rats. Adult male Wistar rats were surgically implanted with cannulae aimed at the CA1 area of the dorsal hippocampus. After recovery from surgery, animals were trained and tested in a shuttle avoidance apparatus (30 trials, 0.5-mA footshock, 24-h training-test interval). Immediately before or immediately after training, animals received a bilateral intrahippocampal 0.5-µl infusion containing 5.0 µg of the NMDA competitive receptor antagonist aminophosphonopentanoic acid (AP5) or vehicle (phosphate-buffered saline, pH 7.4). Infusion duration was 2 min per side. Pre-training infusion of AP5 impaired retention test performance (mean ± SEM number of conditioned responses (CRs) during retention test session was 16.47 ± 1.78 in the vehicle group and 9.93 ± 1.59 in the AP5 group; P<0.05). Post-training infusion of AP5 did not affect retention (mean ± SEM number of conditioned responses during retention test session was 18.46 ± 1.94 in the vehicle group and 20.42 ± 2.38 in the AP5 group; P>0.10). This impairment could not be attributed to an effect on acquisition...

Regions in Bacteroides plasmids pBFTM10 and pB8-51 that allow Escherichia coli-Bacteroides shuttle vectors to be mobilized by IncP plasmids and by a conjugative Bacteroides tetracycline resistance element.

Shoemaker, N B; Getty, C; Guthrie, E P; Salyers, A A
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /06/1986 EN
Relevância na Pesquisa
36.09%
Bacteroides-Escherichia coli shuttle vectors containing a nonmobilizable pBR322 derivative and either pBFTM10 (pDP1, pCG30) or pB8-51 (pEG920) were mobilized by IncP plasmid R751 or pRK231 (an ampicillin-sensitive derivative of RK2) between E. coli strains and from E. coli to Bacteroides recipients. IncI alpha R64 drd-ll transferred these vectors 1,000 times less efficiently than did the IncP plasmids. pDP1, pCG30, and pEG920 could be mobilized from B. uniformis donors to both E. coli and Bacteroides recipients by a conjugative Bacteroides Tcr (Tcr ERL) element which was originally found in a clinical Bacteroides fragilis strain (B. fragilis ERL). However, the shuttle vector pE5-2, which contains pB8-51 cloned in a restriction site that prevents its mobilization by IncP or IncI alpha plasmids, also was not mobilized at detectable frequencies from Bacteroides donors by the Tcr ERL element. The mobilization frequencies of pCG30, pDP1, and pEG920 by the Tcr ERL element in B. uniformis donors to E. coli recipients was about the same as those to isogenic B. uniformis recipients. Transfer of the shuttle vectors from B. uniformis donors to E. coli occurred at the same frequencies when the matings were done aerobically or anaerobically. Growth of the B. uniformis donors in tetracycline (1 microgram/ml) prior to conjugation increased the mobilization frequencies of the vectors to both E. coli and Bacteroides recipients 50 to 100 times.

Shuttle plasmids for Escherichia coli and Clostridium perfringens.

Squires, C H; Heefner, D L; Evans, R J; Kopp, B J; Yarus, M J
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /08/1984 EN
Relevância na Pesquisa
36.12%
Small plasmids which replicate in both Escherichia coli and Clostridium perfringens were made by recombining E. coli plasmid pBR322 with three different small (less than 4 kilobases) plasmids native to C. perfringens. Subsequently, two homologous, though distinct, tetracycline resistance determinants (tet) from other C. perfringens plasmids were cloned into them. Both tet systems made E. coli resistant to at least 5 micrograms of tetracycline per ml when resident on the shuttle plasmids. The shuttle vectors have been used to transform L-phase variants and autoplasts of C. perfringens. In the latter case, the intact transforming plasmid could be isolated from walled cells after cell wall regeneration. Reciprocal transformation experiments in which plasmid DNAs derived from E. coli or C. perfringens were used suggest that restriction barriers exist between these two organisms. The plasmids contain restriction enzyme recognition sites in locations which are useful for cloning experiments.

Analysis of mutation in human cells by using an Epstein-Barr virus shuttle system.

DuBridge, R B; Tang, P; Hsia, H C; Leong, P M; Miller, J H; Calos, M P
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /01/1987 EN
Relevância na Pesquisa
36.07%
We developed highly sensitive shuttle vector systems for detection of mutations formed in human cells using autonomously replicating derivatives of Epstein-Barr virus (EBV). EBV vectors carrying the bacterial lacI gene as the target for mutation were established in human cells and later returned to Escherichia coli for rapid detection and analysis of lacI mutations. The majority of the clonal cell lines created by establishment of the lacI-EBV vector show spontaneous LacI- frequencies of less than 10(-5) and are suitable for studies of induced mutation. The ability to isolate clonal lines represents a major advantage of the EBV vectors over transiently replicating shuttle vectors (such as those derived from simian virus 40) for the study of mutation. The DNA sequence changes were determined for 61 lacI mutations induced by exposure of one of the cell lines to N-nitroso-N-methylurea. A total of 33 of 34 lacI nonsense mutations and 26 of 27 missense mutations involve G X C to A X T transitions. These data provide support for the mutational theory of cancer.

Reduction of Nitrate via a Dicarboxylate Shuttle in a Reconstituted System of Supernatant and Mitochondria from Spinach Leaves 1

Woo, K. C.; Jokinen, Mark; Canvin, David T.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /03/1980 EN
Relevância na Pesquisa
36.09%
Substantial rates of nitrate reduction could be achieved with a reconstituted system from spinach leaves containing supernatant, mitochondria, NAD+, oxaloacetate (OAA), and an oxidizable substrate. Appropriate substrates were glycine, pyruvate, citrate, isocitrate, fumarate, or glutamate. The reduction of NO3− with any of the substrates could be inhibited by n-butyl malonate, showing that the transfer of reducing power from the mitochondria to the supernatant involved the malate exchange carrier. The addition of ADP to the reconstituted system decreased NO3− reduction and this decrease could be reversed by the addition of rotenone or antimycin A. The operation of the OAA/malate shuttle was achieved most quickly in the system when low concentrations (≤0.1 millimolar) of OAA were added. A corresponding increase in the lag time for the operation of the OAA/malate shuttle was observed when the OAA concentration was increased. Concentrations for half-maximal activity of OAA, glycine, NAD+, and NO3− in the reconstituted system were 42 micromolar, 0.5 millimolar, 0.25 millimolar, and 26 micromolar, respectively. The transfer of reducing power from the mitochondria to the soluble phase via the OAA/malate shuttle can not only provide NADH for cytoplasmic reduction but can also sustain oxidation of tricarboxylic cycle acids and the generation of α-ketoglutarate independently of the respiratory electron transport chain.

Properties of the branched-chain 2-hydroxy acid/2-oxo acid shuttle in mouse spermatozoa.

Coronel, C E; Gallina, F G; Gerez de Burgos, N M; Burgos, C; Blanco, A
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 01/05/1986 EN
Relevância na Pesquisa
36.09%
Operation of the branched-chain 2-hydroxy acid/2-oxo acid shuttle for the transfer of reducing equivalents in mitochondria of mouse spermatozoa was studied in vitro in reconstituted systems. Results show that the branched-chain 2-oxo acids within the mitochondria are offered several metabolic pathways. (a) Decarboxylation: mouse sperm mitochondria possess high branched-chain 2-oxo acid decarboxylase activity. (b) Recycling to the cytosol by using a transport system which can be inhibited by alpha-cyano-3-hydroxycinnamate and pH 6.8. (c) Transamination to the corresponding amino acids: experiments presented indicate that leucine formed from 4-methyl-2-oxopentanoate may pass to the external phase, re-initiating the cycle. These two last possibilities would allow autocatalytic operation of the shuttle. The branched-chain 2-hydroxy acids apparently do not utilize the monocarboxylate carrier to penetrate the mitochondria.

Involvement of a Glycerol-3-Phosphate Dehydrogenase in Modulating the NADH/NAD+ Ratio Provides Evidence of a Mitochondrial Glycerol-3-Phosphate Shuttle in ArabidopsisW⃞ 111111111111111111111111 100000000000000000000001 100001111000000001000001 100010000100000010100001 100100000010000010100001 101000000001000100010001 101000000001000100010001 101000000001001111111001 101000000001001000001001 100100000010001000001001 100010000100010000000101 100001111000010000000101 100000000000000000000001 111111111111111111111111

Shen, Wenyun; Wei, Yangdou; Dauk, Melanie; Tan, Yifang; Taylor, David C.; Selvaraj, Gopalan; Zou, Jitao
Fonte: American Society of Plant Biologists Publicador: American Society of Plant Biologists
Tipo: Artigo de Revista Científica
Publicado em /02/2006 EN
Relevância na Pesquisa
36.14%
A mitochondrial glycerol-3-phosphate (G-3-P) shuttle that channels cytosolic reducing equivalent to mitochondria for respiration through oxidoreduction of G-3-P has been extensively studied in yeast and animal systems. Here, we report evidence for the operation of such a shuttle in Arabidopsis thaliana. We studied Arabidopsis mutants defective in a cytosolic G-3-P dehydrogenase, GPDHc1, which, based on models described for other systems, functions as the cytosolic component of a G-3-P shuttle. We found that the gpdhc1 T-DNA insertional mutants exhibited increased NADH/NAD+ ratios compared with wild-type plants under standard growth conditions, as well as impaired adjustment of NADH/NAD+ ratios under stress simulated by abscisic acid treatment. The altered redox state of the NAD(H) pool was correlated with shifts in the profiles of metabolites concerning intracellular redox exchange. The impairment in maintaining cellular redox homeostasis was manifest by a higher steady state level of reactive oxygen species under standard growth conditions and by a significantly augmented hydrogen peroxide production under stress. Loss of GPDHc1 affected mitochondrial respiration, particularly through a diminished capacity of the alternative oxidase respiration pathway. We propose a model that outlines potential involvements of a mitochondrial G-3-P shuttle in plant cells for redox homeostasis.

A progressive shuttle run test to estimate maximal oxygen uptake.

Ramsbottom, R; Brewer, J; Williams, C
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /12/1988 EN
Relevância na Pesquisa
36.2%
The purpose of the present study was to examine the validity of using a 20 m progressive shuttle run test to estimate maximal oxygen uptake. Running ability was described as the final level attained on the shuttle run test and as time on a 5 km run. Maximal oxygen uptake (VO2 max) was determined directly for seventy-four volunteers (36 men, 38 women) who also completed the shuttle run test. Maximal oxygen uptake values were 58.5 +/- 7.0 and 47.4 +/- 6.1 ml.kg-1.min-1 for the men and women respectively (mean +/- SD, P less than 0.01). The levels attained on the shuttle run test were 12.6 +/- 1.5 (men) and 9.6 +/- 1.8 (women; P less than 0.01). The correlation between VO2 max and shuttle level was 0.92. The correlation between VO2 max and the 5 km run was -0.94 and the correlation between both field tests was -0.96. The results of this study suggest that a progressive shuttle run test provides a valid estimate of VO2 max and indicates 5 km running potential in active men and women.

Incremental shuttle walk test in the assessment of patients for heart transplantation

Lewis, M; Newall, C; Townend, J; Hill, S; Bonser, R
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /08/2001 EN
Relevância na Pesquisa
36.09%
OBJECTIVE—To compare the incremental shuttle walk test (ISWT) with treadmill exercise testing (TT) derived measurement of peak oxygen consumption (peak VO2) in patients undergoing assessment for cardiac transplantation.
DESIGN—Prospective comparison. All investigations occurred during a single period of admission for transplant assessment.
SETTING—Single UK cardiothoracic transplantation unit.
PATIENTS—25 patients recruited (21 men). Mean age was 53 years.
INTERVENTIONS—Patients underwent two TT of peak VO2 using the modified Naughton protocol and three (one practice) ISWT. Investigations were performed on consecutive days.
MAIN OUTCOME MEASURES—Main outcome measures were repeatability of TT and ISWT assessments; relation between peak VO2 and distance walked in the ISWT; and receiver operating characteristic (ROC) analysis to establish a distance walked in the ISWT that predicted which patients would have a peak VO2 greater than 14 ml/min/kg.
RESULTS—Both the ISWT and the TT were highly reproducible. Following the first practice walk, mean (SD) ISWT distances were 400.0 (146) m (ISWT2) and 401.3 (129) m (ISWT3), r = 0.90, p < 0.0001. Mean peak VO2 by TT was 15.2 (4.4) ml/kg/min (TT1) and 15.0 (4.4) ml/kg/min (TT2)...

Role of the Malate-Aspartate Shuttle on the Metabolic Response to Myocardial Ischemia

Lu, Ming; Zhou, Lufang; Stanley, William C.; Cabrera, Marco E.; Saidel, Gerald M.; Yu, Xin
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
36.26%
The malate-aspartate (M-A) shuttle provides an important mechanism to regulate glycolysis and lactate metabolism in the heart by transferring reducing equivalents from cytosol into mitochondria. However, experimental characterization of the M-A shuttle has been incomplete because of the limitations in quantifying cytosolic and mitochondrial metabolites. In this study, we developed a multi-compartment model of cardiac metabolism with detailed presentation of the M-A shuttle to quantitatively predict non-observable fluxes and metabolite concentrations under normal and ischemic conditions in vivo. Model simulations predicted that the M-A shuttle is functionally localized to a subdomain that spans the mitochondrial and cytosolic spaces. With the onset of ischemia, the M-A shuttle flux rapidly decreased to a new steady-state in proportion to the reduction in blood flow. Simulation results suggest that the reduced M-A shuttle flux during ischemia was not due to changes in shuttle-associated enzymes and transporters. However, there was a redistribution of shuttle-associated metabolites in both cytosol and mitochondria. Therefore, the dramatic acceleration in glycolysis and the switch to lactate production that occur immediately after the onset of ischemia is mediated by reduced M-A shuttle flux through metabolite redistribution of shuttle associated-species across the mitochondrial membrane.

Inhibition of monocarboxylate transporter 2 in the retrotrapezoid nucleus in rats – a test of the astrocyte-neuron lactate-shuttle hypothesis

Erlichman, J.S.; Hewitt, Amy; Damon, Tracey L.; Hart, Michael; Kurascz, Jennifer; Li, A.; Leiter, J.C.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 07/05/2008 EN
Relevância na Pesquisa
36.14%
The astrocyte-neuronal lactate shuttle hypothesis (ANLSH) posits that lactate released from astrocytes into the extracellular space is metabolized by neurons. The lactate released should alter extracellular pH (pHe), and changes in pH in central chemosensory regions of the brainstem stimulate ventilation. Therefore, we assessed the impact of disrupting the lactate shuttle by administering 100 microM α-cyano-4-hydroxy-cinnamate (4-CIN), a dose that blocks the neuronal monocarboxylate transporter (MCT2), but not the astrocytic MCTs (MCT1 and MCT4). Administration of 4-CIN focally in the retrotrapezoid nucleus (RTN), a medullary central chemosensory nucleus, increased ventilation and decreased pHe in intact animals. In medullary brain slices, 4-CIN reduced astrocytic intracellular pH (pHi) slightly, but alkalinized neuronal pHi. Nonetheless, pHi fell significantly in both cell types when they were treated with exogenous lactate, although 100 microM 4-CIN significantly reduced the magnitude of the acidosis in neurons, but not astrocytes. Finally, 4-CIN treatment increased the uptake of a fluorescent 2-deoxy-d-glucose analogue in neurons, but did not alter the uptake rate of this 2-deoxy-d-glucose analogue in astrocytes. These data confirm the existence of an astrocyte to neuron lactate shuttle in intact animals in the RTN...

Development of Shuttle Vectors for Transformation of Diverse Rickettsia Species

Burkhardt, Nicole Y.; Baldridge, Gerald D.; Williamson, Phillip C.; Billingsley, Peggy M.; Heu, Chan C.; Felsheim, Roderick F.; Kurtti, Timothy J.; Munderloh, Ulrike G.
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Publicado em 21/12/2011 EN
Relevância na Pesquisa
36.14%
Plasmids have been identified in most species of Rickettsia examined, with some species maintaining multiple different plasmids. Three distinct plasmids were demonstrated in Rickettsia amblyommii AaR/SC by Southern analysis using plasmid specific probes. Copy numbers of pRAM18, pRAM23 and pRAM32 per chromosome in AaR/SC were estimated by real-time PCR to be 2.0, 1.9 and 1.3 respectively. Cloning and sequencing of R. amblyommii AaR/SC plasmids provided an opportunity to develop shuttle vectors for transformation of rickettsiae. A selection cassette encoding rifampin resistance and a fluorescent marker was inserted into pRAM18 yielding a 27.6 kbp recombinant plasmid, pRAM18/Rif/GFPuv. Electroporation of Rickettsia parkeri and Rickettsia bellii with pRAM18/Rif/GFPuv yielded GFPuv-expressing rickettsiae within 2 weeks. Smaller vectors, pRAM18dRG, pRAM18dRGA and pRAM32dRGA each bearing the same selection cassette, were made by moving the parA and dnaA-like genes from pRAM18 or pRAM32 into a vector backbone. R. bellii maintained the highest numbers of pRAM18dRGA (13.3 – 28.1 copies), and R. parkeri, Rickettsia monacensis and Rickettsia montanensis contained 9.9, 5.5 and 7.5 copies respectively. The same species transformed with pRAM32dRGA maintained 2.6...

Utilization of Virus ϕCh1 Elements To Establish a Shuttle Vector System for Halo(alkali)philic Archaea via Transformation of Natrialba magadii

Mayrhofer-Iro, M.; Ladurner, A.; Meissner, C.; Derntl, C.; Reiter, M.; Haider, F.; Dimmel, K.; Rössler, N.; Klein, R.; Baranyi, U.; Scholz, H.; Witte, A.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /04/2013 EN
Relevância na Pesquisa
36.07%
In the study described here, we successfully developed a transformation system for halo(alkali)philic members of the Archaea. This transformation system comprises a series of Natrialba magadii/Escherichia coli shuttle vectors based on a modified method to transform halophilic members of the Archaea and genomic elements of the N. magadii virus ϕCh1. The shuttle vector pRo-5, based on the repH-containing region of ϕCh1, stably replicated in E. coli and N. magadii and in several halophilic and haloalkaliphilic members of the Archaea not transformable so far. The ϕCh1 operon ORF53/ORF54 (repH) was essential for pRo-5 replication and was thus identified as the minimal replication origin. The plasmid allowed homologous and heterologous gene expression, as exemplified by the expression of ϕCh1 ORF3452, which encodes a structural protein, and the reporter gene bgaH of Haloferax lucentense in N. magadii. The new transformation/vector system will facilitate genetic studies within N. magadii and other haloalkaliphilic archaea and will allow the detailed characterization of the gene functions of N. magadii virus ϕCh1 in their extreme environments.

A comparison of heavy lift launch vehicle options for the 1990's

Schreiber, Jonathan K.
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Tipo: Tese de Doutorado Formato: v, 70 p.;28 cm.
EN_US
Relevância na Pesquisa
56.18%
This thesis compares attributes of the heavy lift launch vehicle options that are available for the United States to use in support of the Space Exploration Initiative. The systems compared are the Advanced Launch System, Shuttle-C, Saturn V/F-1 and Energia. The F-1 development history is presented along with the proposed development of the ALS and Shuttle-C. Advantages and disadvantages of each of the systems are presented and possible trade-offs between them are discussed. The thesis shows that the option that is most cost effective and can be used to support the deployment of Space Station Freedom is that of developing a booster with a core engine like the F-1. In conclusion, a recommendation is given as to what the best option, with regard to both short and long term mission requirements, is for a new heavy lift launch vehicle for the United States.; US Navy (USN) author

The space shuttle : an attempt at low-cost, routine access to space

Wonch, Jeffrey D.
Fonte: Monterey, California: Naval Postgraduate School Publicador: Monterey, California: Naval Postgraduate School
Tipo: Tese de Doutorado Formato: viii, 105 p. ill.
EN_US
Relevância na Pesquisa
56.32%
Approved for public release; distribution unlimited.; This thesis examines the Space Shuttle's purpose of low-cost, routine access to space and how the lessons learned during the Space Shuttle program have affected the thinking on new heavy-fit launch systems. The thesis objective is to show the Space Shuttle was an attempt at developing a routine, low-cost access to space but, because of Shuttle-unique capabilities, cost-effective operations may never be realized with the Shuttle system. The Space Shuttle concept definition is addressed and the impact on design by DOD influence. The Space Shuttle developmental history is presented, and how budgetary constraints, coupled with NASA's desire to build a low-cost system resulted in over-runs in schedule and costs. The thesis looks at the operational period of the Space Shuttle, the use of Government subsidies to keep the price of a Shuttle launch artificially low, and the difficulties experienced by NASA in maintaining planned launch schedule. The Challenger accident resulted in restructure of U.S. space policy as well as how the Shuttle will be used in the future. In conclusion, lessons learned from the Space Shuttle program are presented that the next generation of space transportation systems can build upon.; Lieutenant...

Construction of a Helicobacter pylori-Escherichia coli shuttle vector for gene transfer in Helicobacter pylori.

Lee, W K; An, Y S; Kim, K H; Kim, S H; Song, J Y; Ryu, B D; Choi, Y J; Yoon, Y H; Baik, S C; Rhee, K H; Cho, M J
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /12/1997 EN
Relevância na Pesquisa
36.14%
In this study, a Helicobacter pylori-Escherichia coli shuttle vector was constructed for transferring DNA into H. pylori. The smallest cryptic plasmid (1.2 kb), pHP489, among those harbored by 77 H. pylori isolates was selected as a base replicon for constructing vectors. HindIII-digested pHP489 was ligated with a kanamycin resistance gene [aph(3')-III], which originated from Campylobacter jejuni, to produce the recombinant plasmid pHP489K. pHP489K was efficiently transformed into and stably maintained in H. pylori strains. The shuttle vector pBHP489K (3.6 kb) was constructed by the recombination of pHP489, ColE1, and aph(3')-III sequences. pBHP489K was reciprocally transformed into and maintained in both H. pylori and E. coli. Introduction of the shuttle vector clone DNA (pBHP489K/AB; 6.7 kb), containing the ureA and ureB genes of H. pylori, into urease-negative mutants of H. pylori led to the restoration of their urease activity. The transformants were confirmed to contain the incoming plasmid DNA. pBHP489K satisfied the requirements for an H. pylori-E. coli shuttle vector, implying that it might be a useful vector for investigating pathogenicity and restriction-modification systems of H. pylori.

Studies in vitro on shuttle systems of mouse spermatozoa.

Burgos, C; Coronel, C E; de Burgos, N M; Rovai, L E; Blanco, A
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 15/11/1982 EN
Relevância na Pesquisa
36.19%
Observations on systems reconstituted in vitro with different starting substrates (2-hydroxy-acids, 2-oxo-acids or leucine) indicate that a branched-chain 2-hydroxy-acid/2-oxo-acid shuttle for the transfer of reducing equivalents from cytosol to mitochondria may be operational in mouse sperm. Evidence is presented suggesting that the 2-oxo-acids produced by intramitochondrial oxidation of 2-hydroxy-acids ingressed from the cytosol can recycle back into the external phase. Observations in vitro demonstrate that, in addition to the branched-chain 2-hydroxy-acid/2-oxo-acid shuttle, the malate/aspartate system is also active in mouse sperm. On the contrary, the lactate/pyruvate redox couple does not appear to function as part of a shuttle system in mouse sperm mitochondria. The glycerol 3-phosphate shuttle probably is not functionally significant in mouse spermatozoa, since the activity of the 'soluble' glycerol 3-phosphate dehydrogenase is very low.

Cache-Oblivious Streaming B-Trees

Bender, Michael A.; Farach-Colton, Martin; Fineman, Jeremy T.; Fogel, Yonatan R.; Kuszmaul, Bradley C.; Nelson, Jelani
Fonte: Association for Computer Machinery Publicador: Association for Computer Machinery
Tipo: Monograph or Book
EN_US
Relevância na Pesquisa
36.09%
A streaming B-tree is a dictionary that efficiently implements insertions and range queries. We present two cache-oblivious streaming B-trees, the shuttle tree, and the cache-oblivious lookahead array (COLA). For block-transfer size B and on N elements, the shuttle tree implements searches in optimal \(O(log_{B+1}N)\) transfers, range queries of L successive elements in optimal \(O(log_{B+1}N +L/B)\) transfers, and insertions in \(O((log_{B+1}N)/B^{\Theta(1/(log log B)^2)}+(log^2N)/B)\) transfers, which is an asymptotic speedup over traditional B-trees if \(B ≥ (log N)^{1+c log log log^2 N}\) for any constant c >1. A COLA implements searches in O(log N) transfers, range queries in O(log N + L/B) transfers, and insertions in amortized O((log N)/B) transfers, matching the bounds for a (cache-aware) buffered repository tree. A partially deamortized COLA matches these bounds but reduces the worst-case insertion cost to O(log N) if memory size \(M = \Omega(log N)\). We also present a cache-aware version of the COLA, the lookahead array, which achieves the same bounds as Brodal and Fagerberg's (cache-aware) \(B^{\epsilon}\)-tree. We compare our COLA implementation to a traditional B-tree. Our COLA implementation runs 790 times faster for random insertions...