This Week in Science

Science  21 Nov 2003:
Vol. 302, Issue 5649, pp. 1289
  1. Quantum Cascade Uplighter

    The emission wavelength of quantum cascade lasers can be tuned in the mid-infrared regime by careful design of the quantum well structure, but the light is emitted only in-plane. Such lasers would be more useful for integrated optoelectronics and chemical sensing applications if the light was emitted through the surface layer. Colombelli et al. (p. 1374; see the Perspective by Tredicucci) combine the design capabilities of quantum cascade lasers with the tunable optical properties of photonic crystals to engineer an electrically pumped microcavity laser that emits light through the surface of the device.

  2. Making Connections in Molecular Electronics

    Potential device applications for silicon nanowires and carbon nanotubes will require improvements in manipulating and connecting individual components, as illustrated by two reports (see the news story by Service). One type of assembly is the crossbar array, in which inputs along one set of wires affect the output state along a connected set of wires in the normal direction. This approach requires methods to modify the interaction between wires so that each horizontal wire affects only one vertical wire. Zhong et al. (p. 1377) show that by chemically treating the junction points of an array of silicon nanowires with tetramethyl ammonium chloride, they can lower the threshold voltage of the depletion mode from 5 to 1.5 volts. They show selective addressing of wires in a small 2-by-2 array. Keren et al. (p. 1380) present results of a DNA-assisted templating method to fabricate a carbon-nanotube field-effect transistor that operates at room temperature. The pseudo-self-assembly technique utilizes the molecular recognition between biological substrates and the biologically tagged nanotubes to localize their position. The nanotube is then wired up with interconnects using a solution-based metal plating process.

  3. Bake, Then Shake

    Phase transformations can occur on picosecond time scales, and the mechanisms by which individual atoms rearrange are only now beginning to be probed. Siwick et al. (p. 1382; see the cover and the Perspective by von der Linde) studied the real time melting of aluminum with ultrafast laser pulses. After the energy was absorbed by the sample, the transformation proceeded through thermally sampled configurations—the superheated sample shook itself apart. Within 3.5 picoseconds, the x-ray diffraction changed to one characteristic of liquid aluminum.

  4. From Electrical Activity to Dendritic Spine Morphology

    The activity-dependent remodeling of synapses that involves new gene expression is believed to be important for long-lasting forms of synaptic modification. Synaptic remodeling after plasticity induction involves the dismantling and reorganization of key cytoskeletal and scaffolding protein complexes. Pak and Sheng (p. 1368; see the Perspective by Meyer and Brose) investigated an important element of the signaling cascade that couples electrical activity to changes in dendritic spine morphology. An activity-inducible kinase, SNK, moves to synaptic spines and causes the phosphorylation and subsequent degradation of SPAR, a key protein in the postsynaptic density. This loss of SPAR then leads to the dismantling of dendritic spines.

  5. Making New Protein Folds

    A major goal in protein design is to specifically create proteins with predictable three-dimensional structures, including protein folds not found in nature. Kuhlman et al. (p. 1364; see the Perspective by Jones) take a large step in this direction. Using a fully automated design procedure that iterates between sequence design and structure prediction, they designed new folds and expressed the 93 residue a/b protein that they call Top7. The crystal structure, determined at 2.5 angstrom resolution, is strikingly similar to the design model.

  6. Looking at Herpes

    Studies on regular macromolecular complexes have made remarkable progress, but detailed analysis of irregular pleiomorphic complexes has remained difficult. Herpes simplex virus represents a truly huge complex (more than 500 megadaltons in mass and containing more than 7000 protein molecules, as well as DNA and lipids) that has not been amenable to standard structural analysis. Grünewald et al. (p. 1396) looked at the structure of the virus using cryoelectron tomography. Their images reveal the detailed organization of virion substructures—the nucleocapsid, tegument, and envelope—with a resolution of 5.5 to 6.3 nanometers.

  7. Digging into Extinctions

    Two reports present minerological and geochemical evidence that may lead to a better understanding of past extinction events (see the related news story by Kerr). Although there is abundant evidence for asteroid impacts in the past, meteorites have only been discovered in a few sedimentary rocks, including one chondrite from deep-sea sediments marking the Cretaceous-Tertiary boundary. Basu et al. (p. 1388) have now recovered dozens of meteoritic grains up to 100 mm across from two rocks samples from the purported Permian-Triassic (P-T) boundary in Antarctica. The grains contain remarkably unaltered magnesium-rich olivine and pyroxene and iron-rich metal. This association and the composition of the minerals are inconsistent with any terrestrial provenance, but are consistent with derivation from a chondritic meteorite. Whether these grains might be related to the dramatic and sudden extinction that is marked by the boundary remains uncertain, but is an interesting coincidence. Flood basalts, the igneous provinces formed by immense outpourings of lava, can reach volumes of more than 2 million cubic kilometers (greater than half that of the Mediterranean Sea). The emplacement of these structures likely injected large amounts of CO2 into the atmosphere that should have affected climate. However, the uncertainties associated with radiometric dating of these provinces have created an ambiguous chronology. Ravizza and Peucker-Ehrenbrink (p. 1392) present an osmium isotope record of the Deccan Trap flood basalts that associates Deccan volcanism with the warming event that predated the Cretaceous-Tertiary boundary by several hundred thousand years.

  8. Population Size and Genomic Complexity

    Adaptation has been considered as a primary force in evolution. Lynch and Conery (p. 1401) hypothesize that the evolution of genomic complexity may represent a more passive process that depended on the effective size of populations. Using nucleotide variation data from more than 40 phylogenetically diverse species, they observed a decline in effective population size moving from prokaryotes up to vertebrates. This observation led the authors to suggest that the expansion of the cell size and number in eukaryotes resulted in a reduction in population size, which in turn promoted types of genomic evolution that were previously impossible in microbial populations with enormous size. Examinations of mobile element expansions, intron size and number, and the half-lives of duplicate genes are consistent with this theory.

  9. Movement Without Motors

    Amoeboid cell locomotion requires protrusion of the leading edge and retraction of the trailing cell body. Analysis of motile systems has shown that protrusion is generated by polymerization and bundling of actin filaments. However, the lack of a suitable in vitro model system has made it difficult to study the mechanism of retraction. Miao et al. (p. 1405; see the Perspective by Mogilner and Oster) have reconstituted retraction in vitro using the motility apparatus of sperm from the nematode, Ascaris suum. This system, with a cytoskeleton based on major sperm protein (MSP), has previously been used to reconstitute protrusion that is analogous to the actin-based protrusion of crawling eukaryotic cells. Addition of Yersinia enterocolytica tyrosine phosphatase (YOP) to cell-free sperm extracts resulted in retraction of the MSP motility apparatus. The retraction was generated primarily by disassembly and rearrangement of the cytoskeleton. Thus, as for protrusion, cytoskeletal assembly and disassembly can generate retraction without the assistance of conventional molecular motors.

  10. Contradictory Calcium Entry

    Calcium ions enter muscle cells through voltage-gated channels (so-called L-type channels) and initiate muscle contraction. However, in an analysis of mice lacking a different channel (the αlH T-type calcium channel), Chen et al. (p. 1416) now show that calcium entering cells through T-type calcium channels can have the opposite effect and cause relaxation of smooth muscle and dilation of coronary arteries. The T-type channels, which may act by influencing large-conductance calcium-sensitive potassium channels, provide a potential new target for treatment of cardiovascular disease.

  11. Central Lynch Pin in Psychosis

    The causes of some mental illnesses such as schizophrenia have been very difficult to pin down, probably because numerous neurotransmitter systems go awry in the brains of such patients. This apparent complexity may be because each of these transmitters activates a common downstream pathway that causes the negative symptoms of the disease. Mice injected with drugs that activate various transmitter pathways (amphetamines, LSD, and PCP) all exhibit psychosis-like symptoms. Svenningsson et al. (p. 1412) show that phosphorylation of the protein DARPP-32 at specific sites is necessary for each of the drugs to have these effects. Mice that did not have the ability to phosphorylate DARPP-32 did not show psychosis-like behaviors in response to these three different drugs.

  12. Notes on the Underground

    Accurate global carbon balance estimates depend on many factors that are difficult to measure, one of which is the longevity of the fine roots of trees. Fine roots have been estimated to account for as much as two-thirds of the net primary productivity in forests, given the assumption of an average turnover time of about 1 year. However, this assumption is controversial, and important because calculations of forest productivity depend on that value. Matamala et al. (p. 1385; see the Perspective by Trumbore and Gaudinski) report estimates of fine-root turnover times made by measuring the carbon isoptopic composition of roots from trees grown with 13C-labeled CO2. The mean residence time can be far longer than 1 year, and fine roots of different species can have much different mean lifetimes. These findings indicate that past studies may have overestimated net primary production and potential soil carbon sequestration in forest ecosystems.

  13. Nucleolar Clustering of tRNA Gene Families

    The role of intranuclear organization on gene expression is understood for very few genes, the best example being the localization of the tandemly repeated ribosomal RNA (rRNA) genes in the nucleolus, where ribosome assembly is coordinated. Although early pre-transfer RNA (pre-tRNA) processing activities and some pre-tRNA intermediates are also concentrated at the nucleolus, tRNA genes are scattered throughout the genome. Thompson et al. (p. 1399) now show that five isoacceptor tRNA gene families are also physically clustered in close proximity to the nucleolus. The clustering is dependent on RNA polymerase III transcription, which suggests that the nucleolus may represent a pol III transcription factory.

  14. Keeping Neuronal Clocks in Time

    Hundreds of neurons in a region of the brain called the suprachiasmatic nucleus (SCN) comprise the mammalian central circadian clock that controls the rhythm of biological processes and behaviors across dark-light cycles. Each neuron expresses a functional molecular clock, yet the neurons operate collectively as a single pacemaker. Organization of cellular rhythms was directly observed by Yamaguchi et al. (p. 1408), who visualized the expression of a bioluminescent marker in the SCN of transgenic mice and observed local transcription of a core clock component. Gene expression was spatially organized across the SCN and appeared as a dorsal-to-ventral wave in real time. The intercellular communication that coordinates this synchronicity involves electrical signals to sustain this temporal and spatial order across the assemblage.