7,706 research outputs found

    CydDC-mediated reductant export in Escherichia coli controls the transcriptional wiring of energy metabolism and combats nitrosative stress

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    The glutathione/cysteine exporter CydDC maintains redox balance in Escherichia coli. A cydD mutant strain was used to probe the influence of CydDC upon reduced thiol export, gene expression, metabolic perturbations, intracellular pH homeostasis, and tolerance to nitric oxide (NO). Loss of CydDC was found to decrease extracytoplasmic thiol levels, whereas overexpression diminished the cytoplasmic thiol content. Transcriptomic analysis revealed a dramatic up-regulation of protein chaperones, protein degradation (via phenylpropionate/phenylacetate catabolism), ?-oxidation of fatty acids, and genes involved in nitrate/nitrite reduction. 1H NMR metabolomics revealed elevated methionine and betaine and diminished acetate and NAD+ in cydD cells, which was consistent with the transcriptomics-based metabolic model. The growth rate and ?pH, however, were unaffected, although the cydD strain did exhibit sensitivity to the NO-releasing compound NOC-12. These observations are consistent with the hypothesis that the loss of CydDC-mediated reductant export promotes protein misfolding, adaptations to energy metabolism, and sensitivity to NO. The addition of both glutathione and cysteine to the medium was found to complement the loss of bd -type cytochrome synthesis in a cydD strain (a key component of the pleiotropic cydDC phenotype), providing the first direct evidence that CydDC substrates are able to restore the correct assembly of this respiratory oxidase. These data provide an insight into the metabolic flexibility of E. coli , highlight the importance of bacterial redox homeostasis during nitrosative stress, and report for the first time the ability of periplasmic low molecular weight thiols to restore haem incorporation into a cytochrome complex

    Synthesis and Characterization of Three-Coordinate Ni(III)-Imide Complexes

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    A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane

    Implementation of a three-quantum-bit search algorithm

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    We report the experimental implementation of Grover's quantum search algorithm on a quantum computer with three quantum bits. The computer consists of molecules of 13^{13}C-labeled CHFBr2_2, in which the three weakly coupled spin-1/2 nuclei behave as the bits and are initialized, manipulated, and read out using magnetic resonance techniques. This quantum computation is made possible by the introduction of two techniques which significantly reduce the complexity of the experiment and by the surprising degree of cancellation of systematic errors which have previously limited the total possible number of quantum gates.Comment: Published in Applied Physics Letters, vol. 76, no. 5, 31 January 2000, p.646-648, after minor revisions. (revtex, mypsfig2.sty, 3 figures

    Scent of the familiar: An fMRI study of canine brain responses to familiar and unfamiliar human and dog odors

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    Understanding dogs’ perceptual experience of both conspecifics and humans is important to understand how dogs evolved and the nature of their relationships with humans and other dogs. Olfaction is believed to be dogs’ most powerful and perhaps important sense and an obvious place to begin for the study of social cognition of conspecifics and humans. We used fMRI in a cohort of dogs (N = 12) that had been trained to remain motionless while unsedated and unrestrained in the MRI. By presenting scents from humans and conspecifics, we aimed to identify the dimensions of dogs’ responses to salient biological odors – whether they are based on species (dog or human), familiarity, or a specific combination of factors. We focused our analysis on the dog\u27s caudate nucleus because of its well-known association with positive expectations and because of its clearly defined anatomical location. We hypothesized that if dogs’ primary association to reward, whether it is based on food or social bonds, is to humans, then the human scents would activate the caudate more than the conspecific scents. Conversely, if the smell of conspecifics activated the caudate more than the smell of humans, dogs’ association to reward would be stronger to their fellow canines. Five scents were presented (self, familiar human, strange human, familiar dog, strange dog). While the olfactory bulb/peduncle was activated to a similar degree by all the scents, the caudate was activated maximally to the familiar human. Importantly, the scent of the familiar human was not the handler, meaning that the caudate response differentiated the scent in the absence of the person being present. The caudate activation suggested that not only did the dogs discriminate that scent from the others, they had a positive association with it. This speaks to the power of the dog\u27s sense of smell, and it provides important clues about the importance of humans in dogs’ lives

    Essays on development finance

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Economics, 2008.Includes bibliographical references.This thesis consists of three essays that examine investment choices in less developed countries. Chapter 1 examines how the structure of existing microfinance contracts may discourage risky but high-expected return investments. I develop a theory that unifies models of investment choice, informal insurance, and formal financial contracts and test the predictions using a series of experiments with Indian microfinance clients. The experiments confirm that borrowers free-ride on their partners, making risky investments without compensating partners for this risk, and that the addition of peer-monitoring overcompensates, leading to sharp reductions in risk-taking and profitability. However, the theoretical prediction that group lending will crowd out informal insurance is not borne out by experimental evidence. While observed levels of informal insurance fall well short of the constrained Pareto frontier under both individual and joint liability, joint liability increases observed insurance transfers. Equity-like financing overcomes both of these inefficiencies and merits further testing in the field. Chapter 2 investigates the relationship between inflation uncertainty and the investment decisions of small, microfinance-funded firms in the Dominican Republic. Using loanlevel panel data from microfinance borrowers in the Dominican Republic, I find that periods of increased inflation uncertainty were associated with substantially lower investments in fixed assets and reduced business growth. This finding is robust to specifications controlling for other forms of systemic risk and aggregate economic activity, suggesting inflation uncertainty creates potentially large distortions to the investment decisions of poor entrepreneurs.(cont.) Chapter 3, co-authored with my advisor, Esther Duflo, turns to investment behavior for public goods. This paper proposes and implements a test of local government efficiency by using a policy in India that set aside leadership positions in local governments to members of disadvantaged minority groups. If local governments are efficient, even if they discriminate against minority groups by supplying fewer public goods, they should still supply the public goods that minority groups value most. We find that when leadership positions are reserved for disadvantaged minorities, hamlets in which these minorities live receive a greater allocation of public goods. Moreover, we find suggestive evidence that this increase in public goods in minority hamlets is not proportional to the distribution of goods when the leadership position is unreserved, suggesting that in the absence of reservation, local governments do not efficiently respond to the minority group's preferences.by Gregory M. Fischer.Ph.D
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