Inorganic Cousins of Fullerenes

The synthesis and characterization of two types of inorganic clusters that exhibit the high connectivity and symmetry of carbon-based fullerenes such as C60 is the subject of two reports (see the Perspective by Muller). Moses et al. (p. 778) have prepared the [As@Ni12@As20]3- anion through a simple solution route. In this structure, an arsenic atom resides at the center of a Ni icosahedron, which in turn resides at the center of an arsensic dodecahedron. An inorganic compound that exhibits the same patterns of alternating five- and six-membered rings as seen in C60 has been synthesized by Bai et al. (p. 781). They reacted an Fe(h5-P5) complex with CuCl to create a compound in which cyclo-P5 rings are surrounded by P4Cu2 rings to form to two hemispheres bridged by [Cu2Cl3]- and [Cu(CH3CN)2]+ units. The overall structure, which has 90 core atoms, is about three times larger than C60.

Carbon Nanotubes as Infrared Emitters

Recombination of charge carriers (electrons and "holes") has been used in many materials systems to create light emission. Misewich et al. (p. 783) show that semiconducting single-wall nanotubes held in a field-effect transistor can be biased in such a way that part of the nanotube will conduct electrons and the other part will conduct holes. Charge recombination leads to polarized infrared emission from an extremely small source region.

Sticky Suds

Anionic surfactants like sodium dodecyl sulfate (SDS) can be used to solubilize carbon nanotubes. Richard et al. (p. 775) used transmission electron microscopy to examine the morphology adopted by SDS and other surfactants on multiwalled carbon nanotubes. They find that "half-cylinders" form on the nanotube surfaces that extend around the tube. Variations in the surfactant chemistry were explored to establish criteria for functionalizing the nanotubes in a permanent but noncovalent fashion.
CREDIT: RICHARD ET AL.

Lower Mantle Lid

The composition and structure of the lowermost mantle and its interaction with the fluid outer core are difficult to assess. Badro et al. (p. 789; see the cover) measured the electrical properties of magnesiowustite, a lower mantle mineral, at high pressures and found a transition from the low- to high-spin state at 60 to 70 gigapascals. This transition between magnesiowustite and perovskite, the other dominant lower mantle mineral, causes most of the iron present to partition into the magnesiowustite in the lowermost mantle. Thus, the lowermost mantle may be composed of two layers near the core, and the lower layer could put an electrical and thermal insulating "lid" on core-mantle interactions.

When Every Second Counts

When an earthquake occurs, P waves are detected by seismic instruments just before the arrival of the S waves and surface waves that typically produce the strongest ground motions and cause the most damage. Allen and Kanamori (p. 786) have developed an early-warning system for southern California that uses the frequency content of the P-wave arrivals to determine the earthquake magnitude and thus the strength of the impending surface waves about 3 to 10 seconds before the strong ground motion begins. Such early warnings could be used to automatically halt transportation systems or critical industrial operations.
CREDIT: ALLEN AND KANAMORI

Digging Up Old DNA

Reconstruction of past ecological communities has usually required finding the fossil remains of animals, plants, and pollen. Willerslev et al. (p. 791) describe a technique that allows DNA retrieval from sediments without the need for macrofossils. Their results, from sediments in Siberia and New Zealand, reveal elements of the flora and vertebrate fauna from Pleistocene communities as old as 400,000 years. From these findings, they can set an upper limit on DNA survival at around 2 million years.

The Sequence (and More) of Chromosome 7

Sequence information is most useful when it can be tied to biological or medical information. Scherer et al. (p. 767) present the sequence of human chromosome 7 within a database that links the sequence information and annotation to a storehouse of information on genetic aberrations associated with disease, as well as comparative genomics information. Analysis provided new candidate genes that may be associated with disorders in human development, including autism.

Group Activities

Integrins comprise a globular ligand binding head and two flexible rod-like stalks that contain the transmembrane and cytoplasmic regions of the integrin a and b chains. Although the details of the integrin activation mechanism are controversial, several proposals agree that the stalks splay apart on activation and that integrin activation is accompanied by clustering of the receptors. Li et al. (p. 795; see the Perspective by Hynes) found two mutations in the transmembrane helix of the b subunit that enhanced the tendency of the b subunit to form homotrimers and that also induced clustering and constitutive phosphorylation of focal adhesion kinase. Homo-oligomerization may drive activation by stabilizing the activated state while simultaneously inducing the formation of clusters.

Switching in Molecular Motors

Kinesin motors in cells use the energy from ATP hydrolysis to move cargo along microtubules, but the molecular mechanism that links hydrolysis to motility has been unclear. Naber et al. (p. 798) use electron paramagnetic resonance spectroscopy to show that kinesin binding to the microtubule triggers a conformational change in the switch 1 region that closes the nucleotide binding pocket. This conformation would likely facilitate hydrolysis and prime the motor for a conformational change in switch 2 that is required for directed motility. Similar switch 1 movement has been identified in myosin and G proteins. Thus, kinesins, myosins, and G proteins use similar strategies for switching conformation to control activity.

Molecular Motors and ALS

Although mutations in the SOD1 gene have been identified in rare familial cases of the fatal motor neuron disease ALS (amyotrophic lateral sclerosis), the molecular basis of motor neuron degeneration is still unclear. Using two mouse models of motor neuron disease, Hafezparast et al. (p. 808) show that the progressive degeneration of spinal cord motor neurons in these mice is due to mutations in dynein, a crucial molecular motor involved in neuronal retrograde transport.
CREDIT: HAFEZPARAST ET AL.

Keeping the Line Moving Ahead

As an RNA polymerase molecule "reads" DNA templates, the enzyme often encounters blocks that affect the rate of transcription. These blocks are generated, for example, by specific DNA sequences elements and DNA binding proteins. These elongation blocks are more effective in vitro than in vivo, and Epshtein and Nudler (p. 801) characterize a mechanism that helps to account for the faster rate in vivo. When multiple RNA polymerases are engaged in elongation on the same DNA molecule, as is the case in vivo, a trailing polymerase can suppress pauses and arrests by a leading polymerase by preventing the front molecule from stalling or backtracking. The trailing polymerase effectively pushes the leading polymerase forward.

Distributed Processing
The recycling of messenger RNA (mRNA) progresses in the following stages: (i) nibbling away its 3’ poly(A) tail; (ii) removal of the 5’ methylated G; and (iii) wholesale degradation of the remains. Some of the enzymes catalyzing these steps have been localized to cytoplasmic bodies, yet it has been unclear whether these were storage depots or recycling centers. Sheth and Parker (p. 805; see the Perspective by Wickens and Goldstrohm) show that, in yeast, these foci, which they name P bodies, are the recycling centers. Blocking processing at an early stage leads to atrophy of P bodies, whereas blocking at a late stage multiplies their size and abundance.

More Responsive to Stimuli When Inhibited
The decline of cortical functions with age occurs not only in humans, but also in monkeys. Degradation of intracortical inhibition may underlie these declines. Leventhal et al. (p. 812; see the news story by Miller) tested this hypothesis by applying either GABA or GABA receptor agonists directly upon neurons in the primary visual cortex of old macaque monkeys. This treatment improved orientation and direction selectivity to visual stimuli in neurons of aged monkeys, but not in those of young ones.

Copyright (c) 2003 by the American Association for the Advancement of Science.