This Week in Science

Science  22 Nov 2002:
Vol. 298, Issue 5598, pp. 1511
  1. In Brevia

    A study of a perennial grass found around hot springs by Redman et al. (p. 1581) revealed that it harbors a fungus that confers heat resistance, and that neither organism can live in the hot soils without the other.

  2. Laser Accelerators

    The probing of grand unification theories and fundamental interactions within atoms will require the ability to go beyond the energies of particle accelerators available today. However, with the maximum attainable energies of conventional accelerators being limited by the breakdown fields of the materials used in construction, other routes are being developed, especially the use of laser-plasma interactions. Malka et al. (p. 1596), using intense femtosecond laser pulses interacting with a jet of helium gas, show that a new acceleration mechanism opens up the forced laser wake field regime, where emitted electrons can attain energies greater than 200 million electron volts.

  3. Describing the Densest Matter

    Nucleon-nucleon interactions make it difficult to compress ordinary matter here on Earth beyond an approximate saturation density of about 2.7 × 1014 grams per cubic centimeter. However, this limit is exceeded by objects in space such as neutron stars (densities about nine times the saturation density) and supernovae (densities about four times the saturation density). Danielewicz et al. (p. 1592) have developed a new theoretical equation-of-state model, which relates densities to pressure and temperature conditions, for supersaturated matter based on recent high-density nuclear collision experiments. They limit the range of possible pressure and temperature conditions that may exist in neutron stars and supernovae, which not only constrains the concentrations of neutrons versus protons in these objects but also their dynamical behavior.

  4. Bacteria and Groundwater Arsenic

    In an attempt to prevent cholera and other water-borne diseases, Bangladesh encouraged the use of groundwater, which had the unintended effect after several years of exposing a large part of the population to arsenic present as a contaminant. The uncertain origin of the arsenic has hampered remediation efforts. In a study in southern Bangladesh, Harvey et al. (p. 1602; see the news story by Stokstad) rule out several suggested mechanisms and show that young organic carbon is being carried into aquifers by the extraction of groundwater for agriculture. Dissolved carbon is likely feeding bacteria, which liberate arsenic bound in solids in the aquifer back into the groundwater.

  5. Digging into Dog Domestication

    The origins of how humans and dogs developed their association, and when and where this occurred, is the subject of three reports (see the news story by Pennisi). Dogs and wolves (and some primates) are social animals that must comprehend and respond to their own species. Hare et al. (p. 1634) compared the performance of wolves and dogs in interpreting human signals (such as looking, pointing, and tapping) that relate to food location. Unlike wolves, dogs exhibit this skill that helped them interpret human communication. Some studies argue that the domestication of dogs from wolves occurred as a single event, and others favor multiple events and distinct Old and New World origins. The fossil record offers evidence that domestication occurred about 13,000 years ago in the Near East, whereas molecular clock data imply an earlier date. Leonard et al. (p. 1613) analyzed mitochondrial DNA (mtDNA) from ancient aboriginal North and South American dog remains and argue for an single Asian origin for dogs. Dogs migrated to the Americas with their human companions via the Bering Strait about 12,000 to 14,000 years ago, but these indigenous breeds appear to have been replaced by larger European breeds during the colonial period. Savolainen et al. (p. 1610; see the cover) collected mtDNA sequences from 38 Eurasian wolves and 654 domestic dogs sampled across Asia, Europe, Africa, and the Arctic America to assess the number and location of dog domestications. The higher genetic diversity in dogs from East Asia compared to Southwest Asia and Europe suggests that dogs originated in East Asia. This phylogenetic analysis, when interpreted in light of archaeological data, suggests that domestic dogs originated around 15,000 years ago, with several origins from wolves.

  6. Primitive Primates

    The origins and evolution of the euprimates—those primates that share many modern primate characteristics such as larger brains, grasping hands, convergence of the eyes for improved binocular vision, and fingernails—has been obscure in part because of limited earlier fossil material. Euprimates appear about 55 million years ago and evolved eventually to include the anthropoids and hominoids. Bloch and Boyer (p. 1606; see the Perspective by Sargis) now describe an earlier fossil plesiadapiform based on a nearly complete skull and large part of its skeleton. The fossil exhibits traits that in some ways are more primitive than the euprimates, but also has several derived features, including clear nails and the ability to grasp. These results that suggest that these groups shared a common origin and that grasping evolved before orbital convergence.

  7. Pulled Sideways

    Bacteria lack rigid cell walls and must respond rapidly to salt- induced gradients in osmotic pressure. In Escherichia coli, the mechanosensitive channel proteins MscL and MscS open in response to pressure applied perpendicularly to the membrane, which has the effect of pulling laterally on the membrane-embedded proteins. Bass et al. (p. 1582; see the Perspective by Bezanilla and Perozo) provide the crystal structure of MscS and describe how the transmembrane helices might reorient themselves as a result of being pulled sideways. The authors also propose how changes in transmembrane electrical potential might trigger movement of positively charged arginines and voltage-gate the osmolyte flux.

  8. Pass the Photosynthesis Genes

    Photosynthesis completely transformed Earth by introducing oxygen into the atmosphere. Raymond et al. (p. 1616; see the news story by Pennisi) made a comparative sequence analysis of genomes from the five branches of photosynthetic bacteria known today to address the origin and evolution of photosynthesis and found that it entailed lateral gene transfers. This work draws attention to an emerging caveat, namely, many core genes for specific metabolic processes are uncharacterized, yet they appear to be of paramount importance to the evolution of certain cellular innovations.

  9. Controlling Chromatin Spreading

    Dosage compensation involves the propagation in cis of altered chromatin architecture and gene activation states over much of the X chromosome. The male-specific lethal (MSL) complex, which includes the roX1 and roX2 noncoding RNAs, is responsible for such spreading on the X chromosome in the fruit fly Drosophila, but the mechanism by which the spreading occurs is not fully understood. Park et al. (p. 1620) show that a limiting concentration of the roX RNAs relative to the MSL complex is critical for effective spreading to occur. This condition apparently ensures that MSL complexes are completed by the addition of nascent roX RNA while the RNA is still tethered to the DNA template, thereby limiting spreading and dosage compensation to the X chromosome.

  10. RNA Polymerase I in Action

    The kinetics of the RNA polymerase I RNA (pol I), the enzyme that transcribes ribosomal RNA, have been revealed by combining photobleaching microscopy of living cells with computational modeling. Dundr et al. (p. 1623; see the news story by Couzin) followed the entry of its subunits into the nucleolus, complex assembly, and elongation. The pol I subunits rapidly exchanged between the nucleoplasm and transcription site in the nucleolus, and the polymerase complex bound to the promoter as distinct subunits rather than as a preassembled unit. The assembly of the polymerase is a relatively inefficient process that occurs via metastable intermediates.

  11. Seeing and Touching an Object

    When observers combine different sources of information perceptually, do they preserve or discard the original sources? Hillis et al. (p. 1627) show that cues are combined within the visual modality in such a way that certain information is lost, and differences that should be obvious are simply not seen. However, this loss does not occur when cues from different modalities, vision and touch, are combined.

  12. B Cell Immunodeficiency

    Several cytokine receptors utilize the common cytokine receptor γ chain (γc), and mutations in γc result in the condition known as X-linked severe combined immunodeficiency (XSCID). Lack of signaling through the interleukin-7 (IL-7) and IL-15 receptors leads to a loss of natural killer and T cells in XSCID patients, although the cause of the B cell defect is not established. Ozaki et al. (p. 1630) observed that B cells from mice lacking IL-21 receptor had altered antibody production. When combined with a loss of IL-4 signaling, a severely impaired immunoglobulin G response was observed that is strikingly similar to that seen in XSCID individuals.

  13. Visualizing Enzyme Activity

    The activity of natural or engineered transmembrane pores can be used to detect enzyme activity, usually by measuring changes in ion current. Das et al. (p. 1600, see also Perspective by Hampp) have adapted this approach and now visually detect enzyme activity. Synthetic pores were introduced into vesicles containing a fluorescent dye. The pores are blocked by certain enzyme substrates (or products). Leakage of the dye from the vesicles—and, hence, fluorescence intensity—depends on how much substrate (or product) is present, and thus provides a measure of the activity of the enzyme in question.

  14. Triggering Death Through DNA

    A cell can commit suicide by initiating a death program that ultimately results in fragmentation of its DNA. A proapoptotic factor called apoptosis-inducing factor (AIF) is released from mitochondria into the cytosol and nucleus in response to certain stimuli, but it has been unclear how this evolutionarily conserved protein actually functions. Wang et al. (p. 1587) have determined that in Caenorhabditis elegans, released AIF associates with an endonuclease called EndoG that is also released from the mitochondria. Together, the complex promotes DNA degradation and cell killing and defines a conserved DNA degradation pathway initiated from the mitchondria.

  15. Are 100,000 "SNPs" Useless?

    Bailey et al. (Reports, 9 August 2002, p. 1003), in a study of segmental duplications within the human genome, noted that unrecognized duplications can lead to paralogous sequence variants being falsely identified as single nucleotide polymorphisms (SNPs)—and estimated, based on their analysis, that “about 100,000 paralogous sequence variants currently contaminate dbSNP,” the public SNP database. Hurles comments that nonallelic gene conversion among duplicated segments has been shown to be “capable of generating allelic diversity,” and that such variations, far from being mere artifacts of genome assembly, could be etiologically significant even though they might escape detection by “haplotype-based whole genome association studies of complex disease.” Bailey and Eichler, in a response, agree that gene conversion is a likely source for “some of the ‘SNP’ abundance” in the public database, but argue that artifacts in the public assemblies provide “the most prosaic explanation” for the observed enrichment of SNPs in the database. The full text of these comments can be seen at