This Week in Science

Science  02 May 1997:
Vol. 276, Issue 5313, pp. 653
  1. Collaring circadian clocks

    Most organisms contain internal circadian clocks that keep track of the day-night cycle even in constant light or darkness. Several clock components are known–frequency in the fungus Neurospora and period and timeless in Drosophila. Crosthwaite et al. (p. 763; see the Perspective by Kay, p. 753) report that two genes from Neurospora, white collar-1 (wc-1) and wc-2, also encode key components of the clock. In addition to keeping time, wc-1 and wc-2, which are transcriptional activators with PAS domains, are required for Neurospora's responses to light, suggesting that circadian clocks evolved from early organisms' light-regulated pathways.

  2. Bright infrared lasers

    Infrared lasers that are bright and whose wavelength can be precisely tuned could have important applications in communications. Scamarcio et al. (p. 773) describe a long-wavelength (8 micrometers) infrared laser based on semiconductor superlattices formed by molecular beam epitaxy. Photons are emitted when electrons tunnel between two superlattice conduction bands; different wavelengths can be achieved by changing the energy gap between these minibands. The use of bands rather than discrete states allows the laser to operate at high powers.

  3. Microdelivered molecules

    The controlled delivery and immobilization of small quantities of reactants is a necessary step in many biological assays and combinatorial screening techniques. Delamarche et al. (p. 779) have designed microfluidity networks that allow controlled delivery and immobilization with submicrometer control. A network of capillaries was formed from an elastomeric mold; molecules such as immunoglobulins were introduced by flow and were immobilized onto the substrate. After removing the mold, the patterned areas showed high specificity and contrast.

    Figure
  4. Paleo plant patterns

    Paleovegetation changes in the tropics have been used to estimate that coolings by up to about 5°C occurred during the last glacial maximum (LGM), but other paleoclimate data and climate models have suggested that temperature changes were not so great. However, atmospheric CO2 levels were also much lower during the LGM, and how this effect changed vegetation has been unclear. Jolly and Haxeltine (p. 786), using a coupled climate-biosphere model, found that the much lower CO2 levels alone could account for the shift in vegetation.

  5. Calcium signaling and heart disease

    High blood pressure leads to cardiac hypertrophy and heart failure, which is a leading cause of death in developed countries. However, the underlying defects in excitation-contraction coupling mechanisms that control and coordinate contraction of individual heart cells have been unknown. Gómez et al. (p. 800; see the Perspective by Yue, p. 755) studied two forms of cardiac dysfunction in rats and found that much of the mechanism by which excitation (depolarization of membrane potential) leads to contraction was functioning normally. However, calcium ions flowing into the cell through channels in the plasma membrane were less effective in opening calcium-dependent channels on intracellular stores of calcium (which would ordinarily cause release of more calcium and promote contraction). At least in the models tested, the results define a potential site for therapeutic intervention that might reduce an unresponsiveness at the cellular level that can eventually lead to heart failure.

  6. Making DNA late

    When cells divide, some portions of the DNA are replicated earlier than others. In yeast, telomeres, the specialized structures at the ends of chromosomes, seem to be responsible for delaying the process origins of DNA replication near them. Raghuraman et al. (p. 806) show that this effect must be re-established anew with each cell cycle. A replication origin located near a telomere in one cell cycle will replicate late, but when removed from the vicinity of the telomere by inducible recombination, it replicates earlier in the very next cell cycle.

  7. Reaction centers in action

    In the bacterial photosynthetic reaction center (RC), light absorption causes charge to transfer from a primary donor group to the ubiquinone acceptor groups, QA and QB; the resulting charge separation thus converts photon energy into a chemical gradient. Stowell et al. (p. 812) obtained x-ray crystal structures of the RC grown in the dark and then activated by light. Light activation moves the terminal acceptor QB about 5 angstroms and rotates it roughly 180°, and a kinetic model based on the observed structural changes can account for the electron transfer rates.

  8. Mice, hedgehogs, and skin cancer

    Basal cell nevus syndrome (BCNS) is an inherited disease characterized by developmental defects and a predisposition to certain cancers, including basal cell carcinoma (BCC). Patients with BCNS have mutations in the patched gene, which codes for a transmembrane receptor that, in lower organisms, transmits critical developmental signals. Oro et al. (p. 817) have generated transgenic mice that overexpress in their skin a ligand for this receptor, called Sonic hedgehog (SHH). The mice developed many features of BCNS, including multiple BCCs and skeletal abnormalities. A putative mutation in the Shh gene was identified in a small number of human tumors.

  9. Chemical routes to higher currents

    Many applications of superconducting materials require high current-carrying capability. When high currents are passed through a superconductor, however, the induced magnetic field lines begin moving and dissipating energy, which results in the superconductivity being destroyed; this threshold is the critical current. Chong et al. found that incorporating high lead concentrations into a single-crystal Bi2Sr2CaCu2O8+δ superconductor improved pinning and increased critical currents. The technique may point toward a simple chemical route to improved current-carrying performance in superconducting applications.

  10. Catching a cation

    Superacids, such as HF-SbF5, are stronger than anhydrous sulfuric acid and can protonate extremely weak bases; their ability to protonate alkanes has found use in the petrochemical industry. One important reaction performed in superacidic media is the formation of aromatic aldehydes by the reaction of aromatic compounds with CO. The formation of the proposed formyl cation intermediate, HCO+, has been difficult to observe in solution, but by using high pressures of CO (26 atmospheres), de Rege et al. observed HCO+ in HF-SbF5 by infrared and nuclear magnetic resonance spectroscopies. [See the Perspective by Prakash.]

  11. Calibrating Quaternary climate records

    A key argument for control of Earth's climate by changes in its orbit around the sun (Milhankovitch forcing) is the timing of high sea-level stands, which reflect the melting of the ice sheets. The ages determined for the previous interglacial high sea-level stands, primarily on corals in Barbados, have supported this theory, but recent dates on a calcite climate record from Devils Hole, Nevada, have suggested that the climate change was earlier than that supported by the Milhankovitch theory. Edwards et al. applied the recently developed protactinium dating method to both of these records and affirms that both records are accurate. One explanation is that the climate near Devils Hole may not show a simple response to solar insolation. [See the news story by Kerr.]

  12. Movement in Tibet

    Deciphering the complicated deformation attributed to the collision of the Indian plate with the Eurasian plate that has produced the Himalayan Mountains and the high Tibetan plateau will allow geologists to understand the rheology and dynamics of plates. Royden et al. used a three-dimensional, analytic solution to a basal deformation convergent plate model with an idealized upper crust whose viscosity varies with depth. Their model can explain the lack of shortening and east-west extension along the eastern margin of the Tibetan plateau, which they attribute to a decoupling of a weaker upper crust from a stronger lower crust.

  13. NF-1 signaling pathways

    Mutation of the human neurofibromatosis type 1 (NF1) protein leads to formation of benign tumors and diverse other symptoms. The tumor suppressor function of NF1 was thought to be mediated through inhibition of the guanosine triphosphatase Ras. The et al. and Guo et al. examined the role of NF1 in Drosophila and found that signaling through Ras appeared to be normal. Instead, abnormalities in signaling at neuromuscular junctions and in growth of NF1 mutant flies appeared to be caused by failure to activate the enzyme adenylate cyclase and consequently to activate the adenosine 3′,5′-monophosphate-dependent protein kinase.

  14. Not marginal effects

    River margins, known as riparian zones, are among the most biologically rich of environments. Most rivers in the world are perturbed by water regulation schemes whose environmental impact is poorly understood. Nilsson et al. collected data on plant species diversity and vegetation cover along the margins of 43 reservoirs and 45 run-of-river impoundments that range in age from 1 to 70 years. Reservoir development uniformly resulted in long-term impoverishment of vegetation; this was also true for most characteristics of run-of-river impoundments, although such schemes do generally allow more scope for recovery. These findings should have an impact on the management of boreal river systems, especially as extensive relicensing of dams is scheduled to take place over the next few years. [See the news story by William.]

  15. In and out of nucleosomes

    Inside the nucleus, the DNA is wrapped around histones. These nucleosomes, as well as a variety of other multimolecular complexes, make up the structure of chromatin. Changes in gene expression may require transcription factors to come and go into the chromatin, perhaps altering the organization of the surrounding chromatin in the process. Pazin et al. show that when a specific transcriptional repressor leaves the chromatin, the nucleosomes it had displaced upon first binding remain displaced until a process requiring adenosine triphosphate allows these nucleosomes to roll back in to the open space.

  16. Working memory

    One of the main divisions in visual processing lies between “what” and “where,” which are defined as the identity of an object (and its shape and color) and the location of the object within the field of view. “What” is processed through a ventral pathway via the inferior temporal cortex and “where” is processed through a dorsal pathway subserved by the posterior parietal cortex. Rao et al. found neurons in the monkey prefrontal cortex that appear to code for both types of information, suggesting that these pathways converge in a region of the brain that already is known to support working memory, which describes a buffer that holds information for immediate use.

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