Transfer of Sulfur from IscS to IscU during Fe/S Cluster Assembly

The cysteine desulfurase enzymes NifS and IscS provide sulfur for the biosynthesis of Fe/S proteins. NifU and IscU have been proposed to serve as template or scaffold proteins in the initial Fe/S cluster assembly events, but the mechanism of sulfur transfer from NifS or IscS to NifU or IscU has not been elucidated. We have employed [35S]cysteine radiotracer studies to monitor sulfur transfer between IscS and IscU from Escherichia coli and have used direct binding measurements to investigate interactions between the proteins. IscS catalyzed transfer of 35S from [35S]cysteine to IscU in the absence of additional thiol reagents, suggesting that transfer can occur directly and without involvement of an intermediate carrier. Surface plasmon resonance studies and isothermal titration calorimetry measurements further revealed that IscU binds to IscS with high affinity (Kd ~2 µM) in support of a direct transfer mechanism. Transfer was inhibited by treatment of IscU with iodoacetamide, and 35S was released by reducing reagents, suggesting that transfer of persulfide sulfur occurs to cysteinyl groups of IscU. A deletion mutant of IscS lacking C-terminal residues 376-413 (IscSDelta 376-413) displayed cysteine desulfurase activity similar to the full-length protein but exhibited lower binding affinity for IscU, decreased ability to transfer 35S to IscU, and reduced activity in assays of Fe/S cluster assembly on IscU. The findings with IscSDelta 376-413 provide additional support for a mechanism of sulfur transfer involving a direct interaction between IscS and IscU and suggest that the C-terminal region of IscS may be important for binding IscU

El uso de las analogías en la enseñanza de la física

El propósito de ésta investigación fue indagar la relación analógica que poseen los estudiantes y que ésta pueda contribuir con la enseñanza de la Física, a través del uso de las analogías, permitiendo relacionar los hechos sociales cotidianos del comportamiento humano y los conceptos físicos. Esto permitirá un acercamiento de los estudiantes hacia el aprendizaje de la Física. La investigación de campo muestra la posibilidad de desarrollar estrategias didácticas con el uso de las analogías y la riqueza simbólica que proporcionan en las clases de Física para reforzar algunos hechos o situaciones de la temática desarrollada a objeto de lograr que los estudiantes entiendan los conceptos físicos y reforzar situaciones o descripciones complejas o abstractas

El uso de las analogías en la enseñanza de la física

El propósito de ésta investigación fue indagar la relación analógica que poseen los estudiantes y que ésta pueda contribuir con la enseñanza de la Física, a través del uso de las analogías, permitiendo relacionar los hechos sociales cotidianos del comportamiento humano y los conceptos físicos. Esto permitirá un acercamiento de los estudiantes hacia el aprendizaje de la Física. La investigación de campo muestra la posibilidad de desarrollar estrategias didácticas con el uso de las analogías y la riqueza simbólica que proporcionan en las clases de Física para reforzar algunos hechos o situaciones de la temática desarrollada a objeto de lograr que los estudiantes entiendan los conceptos físicos y reforzar situaciones o descripciones complejas o abstractas

Superconducting atomic contacts under microwave irradiation

We have measured the effect of microwave irradiation on the dc current-voltage characteristics of superconducting atomic contacts. The interaction of the external field with the ac supercurrents leads to replicas of the supercurrent peak, the well known Shapiro resonances. The observation of supplementary fractional resonances for contacts containing highly transmitting conduction channels reveals their non-sinusoidal current-phase relation. The resonances sit on a background current which is itself deeply modified, as a result of photon assisted multiple Andreev reflections. The results provide firm support for the full quantum theory of transport between two superconductors based on the concept of Andreev bound states

Doping and dimensionality effects on the core-level spectra of layered ruthenates

Core-level spectra of the Mn-doped Sr3Ru2O7 and Srn+1RunO3n+1 (n = 1, 2 and 3) crystals are investigated with X-ray photoelectron spectroscopy. Doping of Mn to Sr3Ru2O7 considerably affects the distribution of core-level spectral weight. The satellite of Ru 3d core levels exhibits a substantial change with doping, indicating an enhanced electron localization across the doping- induced metal-insulator transition. However, the Ru 3p core levels remain identical with Mn-doping, thus showing no sign of doping-induced multiple Ru valences. In the Srn+1RunO3n+1 (n = 1, 2 and 3), the Ru 3d core-level spectra are similar, indicating that the chemical bonding environment around Ru ions remains the same for different layered compounds. Meanwhile the Sr 3d shallow core levels shift to higher binding energy with increasing n, suggesting their participation in Sr-O bonding with structural evolution.Comment: 6 pages with 6 figures, to be published in PR

Semiclassical Construction of Random Wave Functions for Confined Systems

We develop a statistical description of chaotic wavefunctions in closed systems obeying arbitrary boundary conditions by combining a semiclassical expression for the spatial two-point correlation function with a treatment of eigenfunctions as Gaussian random fields. Thereby we generalize Berry's isotropic random wave model by incorporating confinement effects through classical paths reflected at the boundaries. Our approach allows to explicitly calculate highly non-trivial statistics, such as intensity distributions, in terms of usually few short orbits, depending on the energy window considered. We compare with numerical quantum results for the Africa billiard and derive non-isotropic random wave models for other prominent confinement geometries.Comment: To be submitted to Physical Review Letter

Evaluación de los niveles inmunitarios contra la fiebre aftosa en bovinos en Urabá, Antioquia.

Ganadería BovinaMaestría en CienciasMaestrí

Statistical Properties of Many Particle Eigenfunctions

Wavefunction correlations and density matrices for few or many particles are derived from the properties of semiclassical energy Green functions. Universal features of fixed energy (microcanonical) random wavefunction correlation functions appear which reflect the emergence of the canonical ensemble as the number of particles approaches infinity. This arises through a little known asymptotic limit of Bessel functions. Constraints due to symmetries, boundaries, and collisions between particles can be included.Comment: 13 pages, 4 figure

Reconstrucción paleogeográfica de autrigones, caristios y várdulos

Sin resume