This Week in Science

Science  16 Jan 2004:
Vol. 303, Issue 5656, pp. 281
  1. Hiding Out in Hair Follicles

    Stem cells for mammalian skin are thought to reside in the hair follicles, where a stem cell niche harbors its residents until the time comes to initiate new hair growth or to regenerate skin. Tumbar et al. (p. 359) develop a strategy to highlight the slowly dividing skin cells likely to represent such epithelial stem cells. Further analysis into the transcriptional profile of these cells reveals suites of proteins dedicated to reading the environment in preparation for stem cell activation.

  2. Putting Pathogens in Their Evolutionary Place

    Until now there has been no attempt to develop a theoretical framework that integrates epidemiology and phylogenetics, although several studies that attempt to take an integrated approach have been initiated. Grenfell et al. (p. 327) review the epidemiological implications of observed RNA virus phylogenetic patterns, or “phylodynamics,” and use these observations to formulate the evolutionary infection profile, a term that describes the rate of accumulation of evolutionary change as a result of the effects of host immunity.

  3. Creating Communities

    Studies of community assembly rarely address historical processes over evolutionary time. Gillespie (p. 356; see the cover) uses the distinctive chronological arrangement of the Hawaiian Islands, coupled with an adaptive radiation of Tetragnatha spiders, to examine the evolutionary aspects of community assembly. The successive formation times of the islands provide snapshots of communities at different stages of development and allow the temporal patterns of species accumulation through adaptive radiation to be compared with those known from ecological studies of colonization. The patterns revealed are the same as those expected from colonization alone, with species number increasing to a maximum on an island of intermediate age, before declining to similar numbers on each of the older islands. Thus, the principles underlying community assembly may be universal.

  4. Finding an Acceptable Balance

    The precise measurement of small masses, such as those of ions, is made by determining their cyclotron frequency as they move in a magnetic field. However, much as sequential mass measurements of macroscopic objects with a balance is less precise than direct comparisons, the precision of comparative ion masses is limited to around 1 × 10−10. Rainville et al. (p. 334) present a technique in which two ions are confined in a Penning trap. The motion of the ions can be controlled simultaneously, which allows their relative mass to be determined with a precision of around 1 × 10−11. By reducing sources of error, the authors hope to improve the precision by at least another order of magnitude. The introduction of a measurement technique to determine mass with such ultrahigh precision should find immediate application in metrology, fundamental physics, and chemistry.

  5. The Top and Bottom of Mantle Flow

    Mantle plumes are thought to be upwelling thermal anomalies that may cause volcanism, topographic swells, and chemical anomalies at Earth's surface. The depth, extent, and even the existence of plumes are debated. Montelli et al. (p. 338; see the 5 December news story by Kerr) add to this discussion a finite-frequency global tomographic model of mantle velocity anomalies that are attributed to plumes. Six plumes extend throughout the depth of the mantle, but most others are shorter. The Hawaiian plume reaches to about 2800 kilometers, the Yellowstone plume cannot be seen in the model, and two new and unexpected plumes connect with the mid-ocean ridges in the Atlantic and Indian oceans. A model of radial anisotropy throughout the entire mantle by Panning and Romanowicz (p. 351) indicates that, near the core mantle boundary, most of the flow is horizontal. This finding supports the theory that the anomalous thin layer called D” at the base of the mantle is a mechanical boundary layer. There are two regions where the dominant flow is vertical, beneath the two superplumes under the Pacific Ocean and Africa.

  6. For Peat's Sake

    The concentration of methane in the atmosphere underwent a large and steep increase 12,000 years ago, at the end of the last glaciation. One proposed source is the formation at that time of the high-latitude Northern peatlands. Smith et al. (p. 353) present radiocarbon data from peatlands across western Siberia, the most extensive peatlands in the world, which chronicle the initiation of peat formation there for the past 13,000 years. Peatlands formed and expanded rapidly beginning ∼11,500 years ago, which corresponds well to the interhemispheric atmospheric gradient during the Holocene.

  7. Ape Populations and Ebola

    Ebola virus infections are not only of terrifying public health importance, but are also of major conservation importance. Responding to the discovery of gorilla, chimpanzee, and Duiker (antelope) corpses in the forests of Gabon and the Republic of Congo, Leroy et al. (p. 387; see the news story by Vogel tested tissue samples for the presence of Ebola virus—which they found in the majority of cases. The occurrence of animal corpses presaged human outbreaks, often with an index case in a hunter. Each outbreak was caused by a genetically distinct virus, and many highly localized epidemic chains could be distinguished. A large proportion of the gorilla and chimpanzee populations in this region have probably died as a result of multiple rounds of Ebola virus infection in the past 4 years, putting the apes under threat of extinction.

  8. Template-Assisted Mesoscopic Assembly

    By combining two very different polymers into a single chain or by linking hydrophobic and hydrophilic molecules together, it is possible to make diblock structures that self-assemble into vesicles, micelles, or bilayers in solution. Park et al. (p. 348) expand this to organic-inorganic assemblies, where one block consists of a polymer chain that is attached to a rod of gold. The driving force for assembly is governed by the polymer-solvent interactions and, unlike in most small molecule cases, is strongly dependent on assistance from the original template in which the materials were synthesized.

  9. Monkey See, Monkey Do?

    What are the relative roles of motor and premotor cortex in motor control? Schwartz et al. (p. 380; see Perspective by Gottlieb) trained human or monkey subjects to use a simulated vision display, in which they used their arm motion (unseen) to move a computer-generated cursor (seen). As the subjects made cycles of motion around an object, the relationship between arm movement and cursor movement was gradually distorted, so that, for example, an oval-shaped arm movement traced a circularly shaped object. Whereas motor cortex neuronal activity correlated with the actual motor commands, premotor nerve cell activity correlated with the action plan, i.e., what the monkey perceived to be the goal.

  10. Remodeling DNA by Histone Exchange

    Inside all eukaryotic cells, DNA is wrapped up with specific proteins to form chromatin. Gene expression requires opening of the protein wrapper, mainly in the form of nucleosomes that themselves consist of histones, so that the transcription machinery can gain access to the DNA. This process normally shuffles and slides nucleosomes around during chromatin remodeling. The proteins of the wrapper itself can also vary through incorporation of variant histones that have important effects on gene expression. Mizuguchi et al. (p. 343; see Perspective by Owen-Hughes) have studied how the variant histone H2AZ is incorporated into chromatin. A protein complex containing the Swr1 protein exchanged the standard histone H2A for variant H2AZ. Swr1 thus defines a new class of chromatin remodeling complexes that act as protein exchangers, rather than as nucleosome shufflers or sliders.

  11. Latch and Release

    In prokaryotes, cotranslational protein targeting to the membrane is regulated by two GTPases, Ffh and FtsY, that are analogous to the signal recognition particle and its receptor in mammalian cells. Focia et al. (p. 373) have determined at 2.05 Å resolution the structure of a complex of the NG domains of Ffh and FtsY from Thermus aquaticus in the presence of a nonhydrolyzable GTP analog. The complex is remarkably symmetric with both nucleotides in a shared catalytic chamber, providing a mechanism for reciprocal activation. The nucleotides are integral to the interface that holds the proteins together, and their hydrolysis would cause disengagement of the components to unlatch the targeting complex.

  12. Designer Glycoproteins from Bacteria

    Glycosylation is a common posttranslational modification of proteins in eukaryotes and affects a wide range of protein functions. Glycoproteins are typically produced as a mixture of glycoforms. Zhang et al. (p. 371) have genetically incorporated β-GlcNAc-serine at a defined position into myoglobin expressed in E. coli. The introduced sugar moiety can be recognized by a saccharide binding protein, and can also be used as a substrate by a galactosyltransferase. The approach should facilitate production of homogeneous glycoforms of proteins.

  13. Language Acquisition in Humans and Monkeys

    Humans can construct an infinite variety of sentences from a finite vocabulary. Grammar deals with how words relate to each other and are combined to make sentences. A finite-state grammar specifies local or near-neighbor relationships and more complex phrase-structure grammar includes hierarchical or nested arrangements. Obviously, humans (even children) can learn both types of grammars. Fitch and Hauser (p. 377; see Perspective by Premack) show that tamarin monkeys can learn finite-state but not phrase-structure grammars.

  14. Representation of Odor in the Brain

    Molecular and imaging experiments have expanded our understanding of the functional anatomy of the Drosophila olfactory system, in particular by revealing many organizational parallels with the mammalian olfactory bulb. However, until now, the single-neuron electrophysiology needed to bridge the gap between genes and behavior in Drosophila has been lacking. Wilson et al. (p. 366) succeeded in carrying out whole-cell patch-clamp recordings from Drosophila antennal lobe neurons in vivo. They found that projection neurons are tuned considerably more broadly and produced more complex responses than their afferent olfactory sensory neurons.

  15. Pathological Mechanism of Myotonic Dystropy

    Myotonic dystrophy type 1 is an inherited disorder of the muscles and many other body systems caused by expansion of CTG trinucleotide repeats in a gene encoding a specific protein kinase. The repeats form an unstable region in the gene, but the mechanisms underlying pathogenesis are still not understood. Ebralidze et al. (p. 383) demonstrate that expanded RNA CUG repeats sequester a number of transcription factors away from the chromatin to ribonucleoprotein complexes. This process leads to a reduction in the expression of several genes, including the chloride ion channel CIC-1. Artificial overexpression of the transcription factor Sp1 in affected cells restored RNA levels for the CIC-1 gene. This reduction of the effective concentration of transcription factors may contribute to the pathology of the disease.

  16. Manufacturing Gums

    Galactomannans are plant-derived carbohydrate gums useful in diverse processes such as ice cream manufacture and concrete flow. Dhugga et al. (p. 363) have isolated the gene responsible for mannan synthesis from the guar plant. The mannan synthase gene is a member of the cellulose synthase supergene family. As well as paving the way toward a more convenient source of this useful gum, this story yields fresh insight into the complex biosynthetic processes that produce the cell wall and highlights how a family of enzymes can evolve to process different substrates, to acquire different subcellular localizations, and to respond to different regulatory controls.

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