This Week in Science

Science  17 Dec 2004:
Vol. 306, Issue 5704, pp. 1997
  1. Toward Smaller Silicon Switches


    One important measure of the size of transistors is that of the “gate”—the region in the device that actually blocks or allows the flow of current in response to changes in applied potential. Gate lengths are now at about 50 nanometers, but smaller devices cannot be made simply by scaling down the present architectures because of potential problems with leakage currents (an inability to turn the switch off) and capacitive losses. Ieong et al. (p. 2057) present an overview of strategies for creating transistors on chips with gate lengths below 10 nanometers, including the use of multiple gates and ways to speed up the flow of charge carriers in the gate region.

  2. Cheating Heisenberg with Optical Combs

    The Heisenberg Uncertainty Principle leads to tradeoffs when choosing between frequency domain and time domain techniques for spectroscopy. Frequency-resolved spectra measure energy levels with high precision, but the pulses are too long to probe dynamics directly. Ultrashort pulses can probe coherent behavior in state transitions but are too broad to measure state energies. Marian et al. (p. 2063, published online 18 November 2004) have exploited one of the properties of ultrashort pulses, which is that they are actually composed of many discrete frequency lines. The authors apply pulse-to-pulse phase stabilization, using the optical combs previously developed for frequency standardization, to spectroscopy. In a study of Rb atoms, they combine the frequency resolution of the narrow comb lines (for state energies) with the time resolution of the pulse envelope (for coherent dynamics). In addition, they measure and correct for the momentum imparted to the atoms by the light field.

  3. DNA and RNA Swap Roles

    Two reports focus on the use of nucleic acids in creating complex material shapes and patterns and in directing molecular assembly (see the Perspective by Yan). Fragments of DNA can be designed that assemble into large-scale patterns and then be further functionalized or coated with metal particles. Chworos et al. (p. 2068) have now built a large library of shapes and patterns out of RNA, despite RNA's greater chemical lability. The authors start by constructing small- and large-sized tectoids, which are square in shape and that are designed with a variety of sticky tails at the corners. Three-dimensional periodic and aperiodic patterns can be formed from mixtures of the small and large shapes. The ribosome is an RNA and protein machine that strings amino acids into peptides specified by messenger RNA sequences. Liao and Seeman (p. 2072) have made a DNA machine that mimics some of the translational capabilities of the ribosome in that it can hook together sequences of DNA based on the way the machine has been set. The functional part of the device can assume two structural states, and is primed by short DNA segments that are not related to the sequence that the device assembles.

  4. Ironing Out Sedimentary Origins

    Some of the oldest rocks on Earth, dating to about 3.8 billion years ago, are found in southwestern Greenland, the Isua greenstone belt, and the related banded rocks on Akilia Island. Carbon isotopic data suggested that microorganisms helped to form some of these rocks in a sedimentary environment and thus represent some of the earliest evidence for life on Earth. Others argue that the rocks are not of sedimentary origin. Dauphas et al. (p. 2077) provide iron isotopic data which suggest that the banded quartz-pyroxene rocks on Akilia Island are of sedimentary origin and that it is likely that the iron was transported, oxidized, and precipitated from hydrothermal vents. The oxidation and subsequent isotopic fractionation could be produced by anoxygenic photoautotrophic bacteria, which would link these sediments with the earliest known life.

  5. Love Thy Neighbor--or Thyself

    In many plants, a particular gene system ensures that pollen from one plant is only capable of pollinating non-self plants, thus ensuring outcrossing. However Arabidopsis thaliana can self-pollinate. The genes that would normally enforce self-incompatibility, and thus outcrossing, still exist in Arabidopsis, but only as nonfunctional pseudogenes. Shimizu et al. (p. 2081) show that the sequence diversity found in these alleles through populations of Arabidopsis is considerably lower than found in active, self-incompatibility gene systems. In fact, the sequence diversity is so limited as to suggest the action of positive selection on these pseudogenes. Fixation of this transition to self-pollination has occurred recently, in evolutionary terms, perhaps when Arabidopsis ranges expanded after the Pleistocene. Self-fertility may prove useful to a species when it is expanding its habitat ranges.

  6. The Beginnings of an RNA Virus Replication Complex


    Many plant RNA viruses have a transfer RNA-like structure at the 3' terminus of the viral RNA genome that is required for recruitment of the replicase. An exception is alfalfa mosaic virus, where the 3' terminus comprises repeating hairpins separated by tetranucleotide repeats. The repeats bind to the viral coat protein (CP), and this interaction is required for replication. Guogas et al. (p. 2108) have determined the structure of a 39-nucleotide RNA segment bound to the N-terminal RNA binding domain of CP. Two CP peptides bind to sequential repeats in the RNA segment and the peptides and RNA co-fold into a defined structure. Such structural organization of the 3' terminus may present a conformation that is recognized by replicase enzymes.

  7. Fundamentals of Iron Metabolism

    The regulation of iron metabolism is a key component in maintaining health (see the Perspective by Beutler). Nemeth et al. (p. 2090, published online 28 October 2004) show that hepcidin, a peptide hormone produced by the liver in response to iron loading and inflammation, binds directly to the iron exporter ferroportin. Internalization of ferroportin leads to its degradation and prevents the export of iron from the cells. Iron overload diseases can be caused by the absence of hepcidin, and anemias can arise from increased production of hepcidin. Cells tightly regulate their responses to iron levels by using two proteins—iron regulatory protein (IRP) 1 and 2. Mice lacking IRP2 are severely compromised, but mice lacking IRP1 appear normal. Meyron-Holtz et al. (p. 2087) find that at physiological O2 levels, cells lacking IRP2 misregulate iron metabolism, whereas in cells cultured in high levels of O2—as commonly used in tissue culture—IRP1 can substitute for IRP2.

  8. Every Breath You Take

    The mammalian carotid body in the neck is a chemoreceptor that senses O2 levels in the circulatory system and adjusts the respiratory rate accordingly. When O2 becomes scarce, large-conductance calcium-sensitive potassium (BK) channels become inhibited, which causes cell depolarization and a cascade of responses that ultimately increases ventilation. Williams et al. (p. 2093, published online 4 November 2004; see the Perspective by Hoshi and Lahiri) now find that hemoxygenase-2 (HO-2) acts as an O2 sensor to control BK channel activity. At normal O2 concentrations, HO-2 uses O2 as a substrate to generate carbon monoxide (CO), a critical channel activator. During hypoxia, when O2 becomes scarce, HO-2 activity and CO generation fall, which inhibits BK channels and results in carotid body excitation.

  9. Mitochondrial Maintenance Versus Induction

    This replication of mammalian mitochondrial (mt) DNA is initiated at a number of start sites, or origins. Fish et al. (p. 2098) have identified an origin for mtDNA replication that is preferentially used by the cell under steady-state maintenance circumstances. The cell uses the other, previously described, origins after mtDNA has been depleted or when there are physiological demands for new mitochondria.

  10. Back Door to Phosphorylation

    Protein phosphorylation typically occurs through the catalytic activity of a kinase that transfers the phosphate moiety from adenosine triphosphate to a substrate. Saiardi et al. (p. 2101; see the Perspective by York and Hunter) show that the inositol pyrophosphate IP7 can act as a phosphate donor to eukaryotic proteins. The nonenzymatic covalent protein modification was observed in cell extracts and in yeast cells. Because IP7 and many of its targets have been implicated in various biological processes, this type of phosphorylation may represent an intracellular signaling mechanism.

  11. Brain Repair Mechanism


    The transcription factors Olig1 and Olig2 are closely related in sequence, but affect their key targets, oligodendrocyte cells, in different ways. Oligodendrocytes are responsible for wrapping neurons of the central nervous system in an insulating myelin sheath. Olig2 is important for developmental specification of oligodendrocyte cells. Arnett et al. (p. 2111) now show that Olig1 does not play a role in brain development but in repair. Mice lacking Olig1 are deficient in their ability to repair demyelinated brain lesions, the kind of lesions that occur in multiple sclerosis.

  12. Nobly Bound Oxo Species

    In the noble metal surface catalysis used for fuel cells, controlled combustion, and other oxidation reactions, a terminal oxo, or single oxygen atom bound to the metal, is a commonly invoked intermediate. However, chemists have long doubted that certain late transition metals, such as nickel, palladium, and platinum, could support a terminal oxo ligand in a high oxidative state. The problem is repulsion between the oxygen electrons and the six or more electrons found in the metal d-orbitals. Anderson et al. (p. 2074, published online 25 November 2004) have succeeded in synthesizing a d6 Pt(IV) oxo compound, stabilized by two tungsten oxide cluster ligands that delocalize electron density from the platinum center. Extensive x-ray and neutron diffraction techniques verified that the oxygen atom was in fact terminal and did not bear a hydrogen atom.

  13. Fixing Broken DNA

    Repair of DNA double-strand breaks is critical for genome stability, and cells have developed mechanisms to repair these chromosomal defects. It has been proposed that modified histones at double-strand breaks may attract chromatin-remodeling complexes that allow access for the DNA repair machinery. Kusch et al. (p. 2084, published online 4 November 2004) characterize a chromatin-remodeling complex involved in histone exchange during genome repair. The protein complex from Drosophila includes proteins such as chromatin-remodeling enzymes and histone variants. This complex facilitates DNA repair by acetylating the phosphorylated histone and then exchanging the modified protein with an unmodified version to create an altered chromatin structure at the DNA lesion.

  14. Linking Age Control Mechanisms

    Four different factors have been linked independently to mammalian aging—nutrient availability, the Forkhead transcription factor Foxo3a, the nicotinamide adenine dinucleotide-dependent deacetylase SIRT1, and the tumor suppressor protein p53. Nemoto et al. (p. 2105) now report that these elements intersect in a manner that may modulate mammalian life-span. Under conditions of nutritional stress, Foxo3a stimulates SIRT1 expression in mammalian cells, which requires p53 and two p53-binding sites within the SIRT1 promoter. Moreover, Foxo3a and p53 interaction increases under nutrient-starved conditions. The signaling pathway may constitute a homeostatic regulatory network that responds to nutrient availability and, consequently, controls aging.

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