Catálogo de publicaciones - revistas

<jats:p> Circadian rhythmicity is universally associated with the ability to perceive light, and the oscillators (“clocks”) giving rise to these rhythms, which are feedback loops based on transcription and translation, are reset by light. Although such loops must contain elements of positive and negative regulation, the clock genes analyzed to date— <jats:italic>frq</jats:italic> in <jats:italic>Neurospora</jats:italic> and <jats:italic>per</jats:italic> and <jats:italic>tim</jats:italic> in <jats:italic>Drosophila</jats:italic> —are associated only with negative feedback and their biochemical functions are largely inferred. The <jats:italic>white collar–1</jats:italic> and <jats:italic>white collar–2</jats:italic> genes, both global regulators of photoresponses in <jats:italic>Neurospora</jats:italic> , encode DNA binding proteins that contain PAS domains and are believed to act as transcriptional activators. Data shown here suggest that <jats:italic>wc-1</jats:italic> is a clock-associated gene and <jats:italic>wc-2 is</jats:italic> a clock component; both play essential roles in the assembly or operation of the <jats:italic>Neurospora</jats:italic> circadian oscillator. Thus DNA binding and transcriptional activation can now be associated with a clock gene that may provide a positive element in the feedback loop. In addition, similarities between the PAS-domain regions of molecules involved in light perception and circadian rhythmicity in several organisms suggest an evolutionary link between ancient photoreceptor proteins and more modern proteins required for circadian oscillation. </jats:p>

<jats:p> Critical-current density ( <jats:italic>J</jats:italic> <jats:sub>c</jats:sub> ) is a parameter of primary importance for potential applications of high-temperature copper oxide superconductors. It is limited principally by the breakdown of zero-resistive current due to thermally activated flux flow at high temperatures and high magnetic fields. One promising method to overcome this limitation is to introduce efficient pinning centers into crystals that can suppress the flux flow. A marked increase in <jats:italic>J</jats:italic> <jats:sub>c</jats:sub> was observed in Bi <jats:sub>2</jats:sub> Sr <jats:sub>2</jats:sub> CaCu <jats:sub>2</jats:sub> O <jats:sub>8+δ</jats:sub> (Bi-2212) single crystals doped with a large amount of Pb. By electron microscopy, characteristic microstructures were revealed that probably underlie the observed enhancement in <jats:italic>J</jats:italic> <jats:sub>c</jats:sub> : thin (10 to 50 nanometers), platelike domains having a modulation-free structure appeared with spacings of 50 to 100 nanometers along the <jats:italic>b</jats:italic> axis. </jats:p>

<jats:p>A quantum-cascade long-wavelength infrared laser based on superlattice active regions has been demonstrated. In this source, electrons injected by tunneling emit photons corresponding to the energy gap (minigap) between two superlattice conduction bands (minibands). A distinctive design feature is the high oscillator strength of the optical transition. Pulsed operation at a wavelength of about 8 micrometers with peak powers ranging from ∼0.80 watt at 80 kelvin to 0.2 watt at 200 kelvin has been demonstrated in a superlattice with 1-nanometer-thick AlInAs barriers and 4.3-nanometer-thick GaInAs quantum wells grown by molecular beam epitaxy. These results demonstrate the potential of strongly coupled superlattices as infrared laser materials for high-power sources in which the wavelength can be tailored by design.</jats:p>

<jats:p> The formyl cation, HCO <jats:sup>+</jats:sup> , has long been believed to be an important intermediate in the chemistry of carbon monoxide (CO) in acidic environments, but its spectroscopic observation in solution has been elusive. This species was generated by the reaction of CO with the liquid superacid hydrofluoric acid–antimony pentafluoride (HF-SbF <jats:sub>5</jats:sub> ) under pressure and was observed by nuclear magnetic resonance and infrared spectroscopy. Equilibria between CO in the gas phase, CO dissolved in HF-SbF <jats:sub>5</jats:sub> , the SbF <jats:sub>5</jats:sub> adduct of formyl fluoride, and HCO <jats:sup>+</jats:sup> associated with several equilibrating anions of the type [Sb <jats:sub> <jats:italic>x</jats:italic> </jats:sub> F <jats:sub> 5 <jats:italic>x</jats:italic> +1 </jats:sub> ] <jats:sup>−</jats:sup> are proposed to describe the system. </jats:p>

<jats:p>Microfluidic networks (μFNs) were used to pattern biomolecules with high resolution on a variety of substrates (gold, glass, or polystyrene). Elastomeric μFNs localized chemical reactions between the biomolecules and the surface, requiring only microliters of reagent to cover square millimeter–sized areas. The networks were designed to ensure stability and filling of the μFN and allowed a homogeneous distribution and robust attachment of material to the substrate along the conduits in the μFN. Immunoglobulins patterned on substrates by means of μFNs remained strictly confined to areas enclosed by the network with submicron resolution and were viable for subsequent use in assays. The approach is simple and general enough to suggest a practical way to incorporate biological material on technological substrates.</jats:p>

<jats:p> Measurement of protactinium-231 ( <jats:sup>231</jats:sup> Pa) in carbonates by thermal ionization mass spectroscopy yields <jats:sup>231</jats:sup> Pa ages that are more than 10 times more precise than those determined by decay counting. Carbonates between 10 and 250,000 years old can now be dated with <jats:sup>231</jats:sup> Pa methods. Barbados corals that have identical <jats:sup>231</jats:sup> Pa and thorium-230 ( <jats:sup>230</jats:sup> Th) ages indicate that the timing of sea level change over parts of the last glacial cycle is consistent with the predictions of the Astronomical Theory. Two Devils Hole calcite subsamples record identical <jats:sup>231</jats:sup> Pa and <jats:sup>230</jats:sup> Th ages, suggesting that the chronology of this climate record is accurate. </jats:p>

<jats:p>Estimates of glacial-interglacial climate change in tropical Africa have varied widely. Results from a process-based vegetation model show how montane vegetation in East Africa shifts with changes in both carbon dioxide concentration and climate. For the last glacial maximum, the change in atmospheric carbon dioxide concentration alone could explain the observed replacement of tropical montane forest by a scrub biome. This result implies that estimates of the last glacial maximum tropical cooling based on tree- line shifts must be revised.</jats:p>

<jats:p>Field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern Tibetan plateau since about 4 million years ago. Instead, central eastern Tibet has been nearly stationary relative to southeastern China, southeastern Tibet has rotated clockwise without major crustal shortening, and the crust along portions of the eastern plateau margin has been extended. Modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle. This model also predicts east-west extension on the high plateau without convective removal of Tibetan lithosphere and without eastward movement of the crust east of the plateau.</jats:p>

<jats:p> The neurofibromatosis type 1 (NF1) tumor suppressor protein is thought to restrict cell proliferation by functioning as a Ras-specific guanosine triphosphatase–activating protein. However, <jats:italic>Drosophila</jats:italic> homozygous for null mutations of an <jats:italic>NF1</jats:italic> homolog showed no obvious signs of perturbed Ras1-mediated signaling. Loss of <jats:italic>NF1</jats:italic> resulted in a reduction in size of larvae, pupae, and adults. This size defect was not modified by manipulating Ras1 signaling but was restored by expression of activated adenosine 3′,5′-monophosphate–dependent protein kinase (PKA). Thus, NF1 and PKA appear to interact in a pathway that controls the overall growth of <jats:italic>Drosophila.</jats:italic> </jats:p>

<jats:p> The human neurofibromatosis type 1 (NF1) tumor suppressor protein functions as a Ras-specific guanosine triphosphatase–activating protein, but the identity of Ras- mediated pathways modulated by NF1 remains unknown. A study of <jats:italic>Drosophila NF1</jats:italic> mutants revealed that NF1 is essential for the cellular response to the neuropeptide PACAP38 (pituitary adenylyl cyclase–activating polypeptide) at the neuromuscular junction. The peptide induced a 100-fold enhancement of potassium currents by activating the Ras-Raf and adenylyl cyclase–adenosine 3’,5’-monophosphate (cAMP) pathways. This response was eliminated in <jats:italic>NF1</jats:italic> mutants. NF1 appears to regulate the <jats:italic>rutabaga</jats:italic> -encoded adenylyl cyclase rather than the Ras-Raf pathway. Moreover, the <jats:italic>NF1</jats:italic> defect was rescued by the exposure of cells to pharmacological treatment that increased concentrations of cAMP. </jats:p>