Catálogo de publicaciones - revistas

<jats:p>Doppler data generated with the Galileo spacecraft’s radio carrier wave during two Europa encounters on 19 December 1996 (E4) and 20 February 1997 (E6) were used to measure Europa’s external gravitational field. The measurements indicate that Europa has a predominantly water ice-liquid outer shell about 100 to 200 kilometers thick and a deep interior with a density in excess of about 4000 kilograms per cubic meter. The deep interior could be a mixture of metal and rock or it could consist of a metal core with a radius about 40 percent of Europa’s radius surrounded by a rock mantle with a density of 3000 to 3500 kilograms per cubic meter. The metallic core is favored if Europa has a magnetic field.</jats:p>

<jats:p>On 19 December 1996 as Galileo passed close to Jupiter’s moon, Europa, the magnetometer measured substantial departures from the slowly varying background field of Jupiter’s magnetosphere. Currents coupling Europa to Jupiter’s magnetospheric plasma could produce perturbations of the observed size. However, the trend of the field perturbations is here modeled as the signature of a Europa-centered dipole moment whose maximum surface magnitude is ∼240 nanotesla, giving a rough upper limit to the internal field. The dipole orientation is oblique to Europa’s spin axis. This orientation may not be probable for a field generated by a core dynamo, but higher order multipoles may be important as they are at Uranus and Neptune. Although the data can be modeled as contributions of an internal field of Europa, they do not confirm its existence. The dipole orientation is also oblique to the imposed field of Jupiter and thus not directly produced as a response to that field. Close to Europa, plasma currents appear to produce perturbations with scale sizes that are small compared with a Europa radius.</jats:p>

<jats:p>Synchrotron x-ray imaging and stress measurements of diamond-anvil cell gaskets revealed large elastic strains at the diamond tip at a pressure of 300 gigapascals. The diamond, generally considered a rigid body, bent 16 degrees over a distance of 300 micrometers without failure. To complement these measurements, a technique was developed that permits x-ray diffraction to be measured through a beryllium gasket. Measurements on tungsten and iron revealed the strain anisotropy, deviatoric stress, and texture and showed that the yield strengths of these materials increase by up to two orders of magnitude at confining pressures of 200 to 300 gigapascals. The results allow identification of the maximum amount of strain accommodated by the anvil tips before failure. Further development of ultrahigh pressure techniques requires relieving stress concentrations associated with this large elastic deformation.</jats:p>

<jats:p>Mass transport properties of silicate liquids exhibit complex behavior as a function of pressure, as the tetrahedral framework structure of the liquid shifts to a more compact arrangement of atoms. For highly polymerized aluminosilicate liquids, oxygen diffusivities pass through a maximum at pressures below 10 gigapascals, whereas up to 15 gigapascals diffusivities continue to increase for sodium tetrasilicate liquid. A diffusivity maximum indicates a change in the mechanism of formation of 5-coordinated silicon or aluminum in the liquid. In the case of aluminosilicate liquids, this mechanism is restricted to aluminum sites in the network, suggesting that not only degree of polymerization, but also the ratio of aluminum to aluminum plus silicon strongly influences the behavior of magmatic processes at depth.</jats:p>

<jats:p> Dormant seeds of a California chaparral annual were induced to germinate by smoke or vapors emitted from smoke-treated sand or paper. Nitrogen oxides induced 100 percent germination in a manner similar to smoke. Smoke-treated water samples inducing germination were comparable in acidity and concentration of nitrate and nitrite to nitrogen dioxide (NO <jats:sub>2</jats:sub> )–treated samples. Vapors from smoke-treated and NO <jats:sub>2</jats:sub> -treated filter paper had comparable NO <jats:sub>2</jats:sub> flux rates. Chaparral wildfires generate sufficient nitrogen oxides from combustion of organic matter or from postfire biogenic nitrification to trigger germination of <jats:italic>Emmenanthe penduliflora</jats:italic> . Nitrogen oxide–triggered germination is not the result of changes in imbibition, as is the case with heat-stimulated seeds. </jats:p>

<jats:p>During translation errors of aminoacylation are corrected in editing reactions which ensure that an amino acid is stably attached to its corresponding transfer RNA (tRNA). Previous studies have not shown whether the tRNA nucleotides needed for effecting translational editing are the same as or distinct from those required for aminoacylation, but several considerations have suggested that they are the same. Here, designed tRNAs that are highly active for aminoacylation but are not active in translational editing are presented. The editing reaction can be controlled by manipulation of nucleotides at the corner of the L-shaped tRNA. In contrast, these manipulations do not affect aminoacylation. These results demonstrate the segregation of nucleotide determinants for the editing and aminoacylation functions of tRNA.</jats:p>

<jats:p> The <jats:italic>Saccharomyces cerevisiae</jats:italic> gene <jats:italic>NDJ1</jats:italic> (nondisjunction) encodes a protein that accumulates at telomeres during meiotic prophase. Deletion of <jats:italic>NDJ1</jats:italic> ( <jats:italic>ndj1Δ</jats:italic> ) caused nondisjunction, impaired distributive segregation of linear chromosomes, and disordered the distribution of telomeric Rap1p, but it did not affect distributive segregation of circular plasmids. Induction of meiotic recombination and the extent of crossing-over were largely normal in <jats:italic>ndj1Δ</jats:italic> cells, but formation of axial elements and synapsis were delayed. Thus, Ndj1p may stabilize homologous DNA interactions at telomeres, and possibly at other sites, and it is required for a telomere activity in distributive segregation. </jats:p>

<jats:p> Intracellular vesicle targeting involves the interaction of vesicle proteins, termed v-SNAREs, with target membrane proteins, termed t-SNAREs. Assembly of v-SNARE–t-SNARE targeting complexes is modulated by members of the Sec1-Sly1 protein family, and by small guanosine triphosphatases termed Rabs. The interactions of these proteins during assembly of the endoplasmic reticulum–to-Golgi targeting complex in <jats:italic>Saccharomyces cerevisiae</jats:italic> were studied. The data suggest that the Rab protein Ypt1p transiently interacts with the t-SNARE Sed5p and results in displacement of the negative regulator Sly1p, allowing subsequent formation of the v-SNARE–t-SNARE targeting complex. </jats:p>

<jats:p> In transcription initiation, the DNA strands must be separated to expose the template to RNA polymerase. As the closed initiation complex is converted to an open one, specific protein-DNA interactions involving bases of the nontemplate strand form and stabilize the promoter complex in the region of unwinding. Specific interaction between RNA polymerase and the promoter in <jats:italic>Escherichia coli</jats:italic> was detected and quantified as the binding affinity of nontemplate oligonucleotide sequences. The RNA polymerase subunit sigma factor 70 contacted the bases of the nontemplate DNA strand through its conserved region 2; a mutation that affected promoter function altered the binding affinity of the oligonucleotide to the enzyme. </jats:p>

<jats:p> Ligand-gated membrane channels selectively facilitate the entry of iron into prokaryotic cells. The essential role of iron in metabolism makes its acquisition a determinant of bacterial pathogenesis and a target for therapeutic strategies. In Gram-negative bacteria, TonB-dependent outer membrane proteins form energized, gated pores that bind iron chelates (siderophores) and internalize them. The time-resolved operation of the <jats:italic>Escherichia coli</jats:italic> ferric enterobactin receptor FepA was observed in vivo with electron spin resonance spectroscopy by monitoring the mobility of covalently bound nitroxide spin labels. A ligand-binding surface loop of FepA, which normally closes its transmembrane channel, exhibited energy-dependent structural changes during iron and toxin (colicin) transport. These changes were not merely associated with ligand binding, but occurred during ligand uptake through the outer membrane bilayer. The results demonstrate by a physical method that gated-porin channels open and close during membrane transport in vivo. </jats:p>