This Week in Science

Science  12 Jan 1996:
Vol. 271, Issue 5246, pp. 125
  1. The shape of Mars

    Why does the surface of Mars appear to be different in the northern hemisphere where the terrain is younger, less heavily cratered, and overall has a lower elevation than the southern hemisphere? Possible explanations include a large northern hemisphere impact that produced a partial magma ocean or internal mantle processes. Smith and Zuber (p. 184; see cover) reanalyzed occultation data and combined these results with more recent Earth-based radar measurements to conclude that the observed, broad, hemispheric topography variation is due to a 3-kilometer offset between the center of mass and center of figure of Mars.

  2. Designed to second order

    A number of inorganic and organic optical materials can be used to double the frequency of incident light. In these materials, the crystal or molecular structure is asymmetric, and thus electrons set up polarization waves, in response to the incident light, that oscillate at higher harmonics. Rosencher et al. (p. 168) review another approach to creating asymmetric potentials. Heterojunction quantum wells can be constructed by semiconductor epitaxy techniques in such a way that the wells are asymmetric. These structures can create large nonlinearities and may allow the creation of new optical devices.

  3. Pushed into place

    Molecules have been arranged into patterns on a metal surface with a scanning tunneling microscope at room temperature. Jung et al. (p. 181) attached bulky hydrocarbon groups to a flat aromatic core that stabilizes the molecule against thermal diffusion. The interactions are still weak enough that the tip can push the molecule and translate it in a controlled fashion.

  4. Sustaining a height

    Like a ship or iceberg floating by displacing water, many of Earth's mountain ranges seem to be supported by a root of crustal material that extends into the mantle. Wernicke et al. (p. 190) report recent geophysical and geochemical data that imply that the southern Sierra Nevada of California lack such a root of thick crust. The data indicate that the high elevations are supported by density variations in the mantle and suggest that the range may have been subsiding over the last several million years.

  5. Ancient waterways

    A key step in the evolution of terrestrial plants and angiosperms was the development of vessels, which are perforated cells that facilitate conduction of water through the plant. Li et al. (p. 188) report the discovery of fossil vessels dating from the Late Permian, about 260 million years ago, predating the origin of angiosperms by many millions of years. The fossil stems are similar to those in vines, and may be from Gigantopteridales, a group of Permian plants with large leaves.

  6. Thymic ligand

    T cell precursors originate in the bone marrow before moving to the thymus. There, as thymocytes, they divide, mature, and undergo selection. The exiting thymocytes are “single-positive” (that is, either CD4+ or CD8+) T cells. A critical-but poorly understood-early intrathymic event is the differentiation of “double-negative” (CD4-CD8-) to “double-positive” (CD4+CD8+) thymocytes. Boismenu et al. (p. 198) show that CD81, a molecule expressed in the outer cortex of fetal thymus, is required for the generation of double-positive thymocytes. Similarities between the developmental effects promoted by CD81 and the pre-T cell receptor (pre-TCR) hint that CD81 is the ligand for the pre-TCR.

  7. Feast or famine

    When yeast are starved of phosphate, the PHO4 transcription factor induces transcription of PHO5, a secreted acid phosphatase. The transcription of PHO5 is repressed when yeast are grown in phosphate-rich medium. O'Neill et al. (p. 209) examined the regulation of PHO4 activity and found that the cellular localization of PHO4 is regulated. PHO4 is nuclear when yeast are phosphate-starved and cytoplasmic when yeast are grown in phosphate-rich medium. PHO4 localization depends on phosphorylation by the PHO80/PHO85 cyclin-CDK kinase complex.

  8. Sleep circuitry

    Falling asleep seems so easy at times and so hard at others, yet the underlying neuronal mechanisms have not been elucidated, nor is the neuronal circuitry well described. Sherin et al. (p. 216) identified a group of cells located in the ventrolateral preoptic (VLPO) region of the rat hypothalamus. These cells were active in proportion to the amount of sleep enjoyed, and this relation could be dissociated from the circadian light-dark cycle. Furthermore, these cells project to a region of the posterior hypothalamus that is known to be involved in wakefulness, suggesting that the VLPO cells, when activated, may inhibit arousal-promoting neurons. These findings explain early work in which lesions of the VLPO region produced insomnia and also helped to delineate the circuitry responsible for sleep.

  9. Malaria detox

    For part of its life cycle, the malaria parasite exists within red blood cells. To survive, it must detoxify heme, which it does by polymerizing it to form hemozoin, an insoluble crystal. Once begun, polymerization proceeds spontaneously, but what initiates the process? One possibility, raised by Sullivan et al. (p. 219), is that histidine-rich proteins (HRPs) are responsible. Purified and recombinant HRPII and recombinant HRPIII possess heme polymerase activity; furthermore, HRPII and IV have been localized to the digestive vacuole. Although HRPs may be attractive new candidates for therapeutic intervention, the parasite may have additional detoxification strategies, as essential functions are notoriously redundant.

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