Catálogo de publicaciones - revistas

<jats:p> The 4,639,221–base pair sequence of <jats:italic>Escherichia coli</jats:italic> K-12 is presented. Of 4288 protein-coding genes annotated, 38 percent have no attributed function. Comparison with five other sequenced microbes reveals ubiquitous as well as narrowly distributed gene families; many families of similar genes within <jats:italic>E. coli</jats:italic> are also evident. The largest family of paralogous proteins contains 80 ABC transporters. The genome as a whole is strikingly organized with respect to the local direction of replication; guanines, oligonucleotides possibly related to replication and recombination, and most genes are so oriented. The genome also contains insertion sequence (IS) elements, phage remnants, and many other patches of unusual composition indicating genome plasticity through horizontal transfer. </jats:p>

<jats:p>Protostars emit more x-rays, hard and soft, than young sunlike stars in more advanced stages of formation. The x-ray emission becomes harder and stronger during flares. The excess x-rays may arise as a result of the time-dependent interaction of an accretion disk with the magnetosphere of the central star. Flares produced by such fluctuations have important implications for the x-wind model of protostellar jets, for the flash-heating of the chondrules found in chondritic meteorites, and for the production of short-lived radioactivities through the bombardment of primitive rocks by solar cosmic rays.</jats:p>

<jats:p> Time-resolved two-photon photoemission in combination with the coherent excitation of several quantum states was used to study the ultrafast electron dynamics of image-potential states on metal surfaces. For a (100) surface of copper, the spectroscopy of quantum beats made previously unresolved high-order states (quantum number <jats:italic>n</jats:italic> ≥ 4) experimentally accessible. By exciting electrons close to the vacuum level, electron wave packets could be created and detected that described the quasi-classical periodic motion of weakly bound electrons. They traveled more than 200 Å away from the surface and oscillated back and forth with a period of 800 femtoseconds. </jats:p>

<jats:p> A highly motile chemoautotrophic strain of hydrogen sulfide <jats:bold>–</jats:bold> oxidizing bacteria from coastal seawater produces solid sulfur filaments of dimensions 0.5 to 2.0 micrometers by 20 to 500 micrometers. Filamentous sulfur is rapidly produced by direct excretion by a vibrioid organism, and the newly produced filaments are thickened by the deposition of sulfur by other members of the population. Microscopic observations of the flocculent discharge material collected from diffuse-flow hydrothermal vents (9°N, East Pacific Rise) revealed that the material from this source is composed largely of filamentous sulfur of morphology nearly identical to that obtained in the model laboratory system. </jats:p>

<jats:p> Optical spectra of the terrestrial nightglow in the 520- to 900-nanometer region, as measured by the W. M. Keck telescope on Mauna Kea in Hawaii and the associated high-resolution echelle spectrograph, showed many bands belonging to the important O <jats:sub>2</jats:sub> ( <jats:italic>b-X</jats:italic> ) Atmospheric Band emission system. Previous ground-based measurements have shown only a single band, from the lowest vibrational level of the emitting state. Of particular interest is the fact that at the 762-nanometer position of the <jats:italic>b-X</jats:italic> 0-0 band, where earlier studies have shown only absorption features, these results showed both absorption at the <jats:sup>16</jats:sup> O <jats:sup>16</jats:sup> O line positions and well-resolved emission at the positions of many of the <jats:sup>18</jats:sup> O <jats:sup>16</jats:sup> O and <jats:sup>17</jats:sup> O <jats:sup>16</jats:sup> O lines. These findings show that substantial advances can be made in understanding atmospheric emission phenomena by the use of astronomical tools. </jats:p>

<jats:p>An image of comet Hale-Bopp (C/1995 O1) in soft x-rays reveals a central emission offset from the nucleus, as well as an extended emission feature that does not correlate with the dust jets seen at optical wavelengths. Neon was found to be depleted in the cometary ice by more than a factor of 25 relative to solar abundance, which suggests that ices in Hale-Bopp formed at (or later experienced) temperatures higher than 25 kelvin. A helium line emission at a wavelength of 584 angstroms was detected and may be attributable to charge transfer of solar wind α particles in the cometary coma. Ionized oxygen and another helium line contribute to an emission observed at 538 angstroms.</jats:p>

<jats:p>Hubble Space Telescope images of asteroid 4 Vesta obtained during the favorable 1996 apparition show an impact crater 460 kilometers in diameter near the south pole. Color measurements within the 13-kilometer-deep crater are consistent with excavation deep into a high-calcium pyroxene-rich crust or olivine upper mantle. About 1 percent of Vesta was excavated by the crater formation event, a volume sufficient to account for the family of small Vesta-like asteroids that extends to dynamical source regions for meteorites. This crater may be the site of origin for the howardite, eucrite, and diogenite classes of basaltic achondrite meteorites.</jats:p>

<jats:p> Arrest of the cell cycle at the G <jats:sub>2</jats:sub> checkpoint, induced by DNA damage, requires inhibitory phosphorylation of the kinase Cdc2 in both fission yeast and human cells. The kinase Wee1 and the phosphatase Cdc25, which regulate Cdc2 phosphorylation, were evaluated as targets of Chk1, a kinase essential for the checkpoint. Fission yeast <jats:italic>cdc2-3w Δcdc25</jats:italic> cells, which express activated Cdc2 and lack Cdc25, were responsive to Wee1 but insensitive to Chk1 and irradiation. Expression of large amounts of Chk1 produced the same phenotype as did loss of the <jats:italic>cdc25</jats:italic> gene in <jats:italic>cdc2-3w</jats:italic> cells. Cdc25 associated with Chk1 in vivo and was phosphorylated when copurified in Chk1 complexes. These findings identify Cdc25, but not Wee1, as a target of the DNA damage checkpoint. </jats:p>

<jats:p> In response to DNA damage, mammalian cells prevent cell cycle progression through the control of critical cell cycle regulators. A human gene was identified that encodes the protein Chk1, a homolog of the <jats:italic>Schizosaccharomyces pombe</jats:italic> Chk1 protein kinase, which is required for the DNA damage checkpoint. Human Chk1 protein was modified in response to DNA damage. In vitro Chk1 bound to and phosphorylated the dual-specificity protein phosphatases Cdc25A, Cdc25B, and Cdc25C, which control cell cycle transitions by dephosphorylating cyclin-dependent kinases. Chk1 phosphorylates Cdc25C on serine-216. As shown in an accompanying paper by Peng <jats:italic>et al</jats:italic> . in this issue, serine-216 phosphorylation creates a binding site for 14-3-3 protein and inhibits function of the phosphatase. These results suggest a model whereby in response to DNA damage, Chk1 phosphorylates and inhibits Cdc25C, thus preventing activation of the Cdc2–cyclin B complex and mitotic entry. </jats:p>