This Week in Science

Science  31 Mar 2000:
Vol. 287, Issue 5462, pp. 2373
  1. Upon Several Layers of Reflection

    Mirrors are made conventionally by coating a substrate with a reflective metal, but in more advanced designs, multiple layers of transparent dielectric materials can be assembled and designed to give very high reflectivity or wavelength selectivity. Such multilayer mirrors usually have an Achilles heel that arises from Brewster's Law—while such mirrors exhibit very high reflectivity for p-polarized light at normal incidence, they rapidly lose reflectivity when viewed at an angle. Weber et al. (p. 2451; see the cover and the news story by Service) show that multilayer stacks of alternating birefringent and optically isotropic polymer layers (in some cases, hundreds of layers of submicrometer thickness) can create broadband and color mirrors that are highly reflective at all angles, as well as polarizers that work in reflection.

  2. Solar Convection

    The sun is divided into a hot inner core, a radiative zone, a convective zone, a photosphere, and a very hot outer atmosphere. The thin boundary between the radiative and convective zones, called the tachocline, is assumed to generate the solar dynamo by rotational shear that produces strong magnetic fields. Howe et al. (p. 2456; see the Perspective by Gough) have detected variations in the rate of rotation at the tachocline by combining 4 years of data on modes of oscillations derived from the ground-based observatories of the Global Oscillation Network Group (GONG) and the space-based Michelson Doppler Imager (MDI) on the Solar Heliospheric Observatory (SOHO) spacecraft. The rotation of the sun near the tachocline varies at low latitudes over a period of 1.3 years and may indicate dynamic coupling between the radiative and convective zones.

  3. Minimal Hydration

    Scientists studying ion hydration often resort to investigating how just a few water molecules interact with the ion in the gas phase. In conjunction with theoretical studies, this approach can yield insights into the structure of the hydration shell and interactions in the condensed phase. Weber et al. (p. 2461; see the Perspective by Scoles and Lehmann) used cluster spectroscopy and ab initio theory to establish the detailed structure of the hydration shell around the superoxide anion, O2-. The minimum hydration shell involves just four water molecules, each of which is engaged in a single hydrogen bond to the superoxide.

  4. Caught in the Act

    To better understand how genes are regulated, it is important to understand the kinetics behind transcription (the mechanism by which the DNA template is converted into RNA), but this generally has relied on bulk experiments where events are averaged. These analyses do not readily allow for the dissection of the various stages encountered by an elongating complex. Davenport et al. (p. 2497; see the Perspective by Buc) have used an optical trap method and video microscopy to map in real time the transcription stages and rates of single molecules of Escherichia coli RNA polymerase. The paused state is shown to be a kinetic intermediate between transcription elongation and arrest. The presence of a switch between transcriptionally competent states may provide a target for gene regulation.

  5. By Land or by Sea

    Fossil fuel burning currently adds about 6 billion tons of carbon dioxide (CO2) to the atmosphere each year. Half of that is incorporated into the oceans and the land biosphere, but how much CO2 does each of those sinks absorb? Battle et al. (p. 2467) address that question by combining two methods of determining the net CO2 fluxes into and out of the terrestrial biosphere by measuring the change of O2/N2 in the atmosphere and by measuring stable carbon isotopic composition of atmospheric CO2. The highly variable fluxes they find for the land biosphere between 1991 and 1997 are a stark contrast to the stable behavior of the 1980s.

  6. Hot and Dry in Tibet

    Xenoliths are foreign fragments of rock with textures and compositions different than those of their host rocks. Some xenoliths represent samples of the lower crust or mantle rapidly brought to the surface in volcanic eruptions. Hacker et al. (p. 2463) studied a suite of mafic and metasedimentary xenoliths found in Tibetan lavas and determined that these xenoliths had seen high temperatures (about 800° to 1000°C) under anhydrous conditions. These results suggest that the metasedimentary xenoliths indicate a dry, but hot lower crust that mixed with mafic xenoliths from deeper mantle melts. Such a model calls into question the possibility of a regionally extensive hydrous melt zone in the lower crust beneath the Himalayas formed by convective thinning of the underlying mantle.

  7. Controlling Insect Pests

    The sterile insect technique (SIT) is a popular method for controlling insect pests and disease vectors; large numbers of sterile males are bred and released to compete with wild males for females, thus reducing the population of the next generation. However, breeding insects on the required scale is often expensive and problematic. Thomas et al. (p. 2474) describe a major improvement to SIT using a transgenic Drosophila system that can provide both the efficient sexing and the female-specific lethality required. When transferred to other systems, this new technique is likely to become an important weapon in the armory used against insect pests.

  8. Use It or Lose It

    Monocular deprivation studies have established that during early postnatal development there are periods of plasticity in the visual system; activity-dependent refinement of neuronal connections from earlier, more diffuse stages occurs. It has been thought that these connections, once established, are maintained throughout life. However, Chapman (p. 2479) shows that blockade of retinal activity abolishes existing ocular dominance structures in the lateral geniculate nucleus of the ferret. This result implies that activity-dependent competition is necessary not only for the establishment of specific connections in the visual system, but also for their maintenance.

  9. A Deterioration of Division

    What happens inside cells as we age? High-density oligonucleotide arrays analysis of messenger RNA from young, middle-aged, and old humans for over 6000 genes reveal classes of genes that are altered in dividing fibroblasts. Ly et al. (p. 2486; see the news story by Marx) report that with age, many of the genes with altered expression are involved in the G2-M phase of the cell cycle. The authors suggest that a decreased ability to replicate properly with age would lead to higher rates of somatic mutation that then would affect the expression of specific genes contributing to the aging phenotype.

  10. Taking Up Residence

    In brucellosis, the pathogenic bacterium Brucella abortus invades and establishes a long-lasting interaction with the host mammalian cell. In leguminous plants, the symbiont Rhizobium meliloti invades and takes up residence in root nodule cells. LeVier et al. (p. 2492) now report that these very different host-bacterial relationships both require a putative membrane transporter encoded by the bacA gene. The BacA protein may assist the invading bacteria to evade the host cell's defense mechanisms.

  11. The Sound of Silence

    Double-stranded RNA is able to regulate gene expression by sequence-specific post-transcriptional interference (RNAi). Grishok et al. (p. 2494; see the Perspective by Sharp and Zamore) show that in Caenorhabditis this RNAi phenomenon can be transmitted to the offpsring, and they identify the genes involved. Their findings suggest the existence of a sequence-specific silencing factor, which may be a small RNA (20 to 23 nucleotides long), and implicate the genes rde-1 and rde-4 in its formation.

  12. Identity Crisis

    Most of the time, our self proteins remain unnoticed by our immune systems. Autoimmune responses occur when this tolerance of self is broken. When B cells become activated instead of tolerant, they produce antibodies. Kouskoff et al. (p. 2501) now report a system in which B cells are tolerant to self until a bacteriophage carrying an epitope recognized by the B cells (but not similar in sequence to the self-antigen recognized by those B cells) is introduced into the mouse. This T cell-independent activation of formerly tolerant B cells raises the question of how B cells are normally able to distinguish self-antigens from foreign, T cell-independent antigens.

  13. Liquid Disks in the Inner Core

    The presumed structure of Earth's core (a solid inner core surrounded by a liquid outer layer, both composed mainly of iron) is based on the properties of seismic waves that sample the core. As more seismic data are collected, the structure required to fit the data has become more complex. Singh et al. (p. 2471) have fit a relatively simple effective medium model to a specific set of seismic data, namely compressional and shear wave velocity, and attenuation and compressional wave anisotropy. By adding 3 to 10% by volume of flat disks of liquid, aligned along the equatorial plane of Earth, to a solid iron inner core, they could fit the seismic data and estimate that the unknown shear wave anisotropy in the inner core could be large. Thus, the authors have provided evidence for small volumes of liquid iron in the inner core, which has broader implications for the evolution of the core and the geodynamo.

  14. Cryptic Control of Hunchback

    The instructions for the development of higher organisms, whether fly or man, is many times more complex than the engineering specs of the most powerful computer. With one illustrative example, Wimmer et al.(p. 2476) shed light on this assembly problem. Bicoid (Bcd) is a maternal morphogen that specifies body pattern in Drosophila, in part by regulating the expression of Hunchback (Hb), an evolutionarily more ancient morphogen, to form thoracic segments. Deletion of the second of hb's two promoters removes the sensitivity to Bcd, and maternal Hb then controls zygotic hb expression and can direct the formation of thoracic segments. Thus, the second Bcd-sensitive promoter likely was added later in evolution to confer Bcd regulation on hb, supplanting the older hb regulation, which is revealed only after artificially de-evolving one regulatory part of hb by removing the second promoter.

  15. Priming the Machine

    DNA polymerases extend oligonucleotides hybridized to a DNA template strand, but cannot initiate replication of single-stranded DNA. Initiation requires the synthesis of RNA primers by replication-priming RNA polymerases (primases). Keck et al. (p. 2482) (see the Perspective by von Hippel and Jing) have determined the structure of the polymerase domain of the Escherichia coli primase DnaG. The active-site architecture of the core structure is unrelated to other DNA or RNA polymerases, but is related to the “toprim” fold found in a variety of metal-binding phosphotransfer proteins. The polymerase domain contains a groove lined with invariant residues that is likely to bind single-stranded DNA, and the synthesized DNA-RNA hybrid would be held in a shallow depression where interactions might limit the primer size. The authors propose that the polymerase is positioned to accept single stranded DNA directly from the helicase.

  16. Middle Miocene Hominoid Origins

    Drawing on fossil material from the Tugen Hills, north central Kenya, Ward et al. (Reports, 27 Aug.) identified a new Middle Miocene hominoid genus, Equatorius, that incorporates all material previously referable to Kenyapithecus africanus and that may cast new light on the origin of the great ape-human clade. Begun argues that the samples attributed to Equatorius cannot be reliably distinguished from the Eurasian genus Griphopithecus, and that the biogeographic pattern marked by the fossil record suggests that “African Middle Miocene hominoids represent successive radiations of forms dispersing from Eurasia.” Kelley et al. respond that Equatorius and Griphopithecus differ in some key details of dental morphology and question Begun's “out-of-Eurasia hypothesis” in light of imprecise fossil age dating and the lack of Eurasian Early Miocene catarrhine lineages.

    Separately, Benefit and McCrossin suggest that Ward et al. included in the Equatorius hypodigm two genera—Nacholapithecus kerioi and K. africanus—that differ in their morphologies, adaptations, and relationships with other hominoids. The samples analyzed by Ward et al., Benefit and McCrossin conclude, may represent not a new hominoid genus but a combination of Kenyapithecus and Nacholapithecus. Kelley et al. acknowledge the separate taxonomic status of N. kerioi, but stress that “the material now included in Nacholapithecus did not contribute to the formal diagnosis of Equatorius.” Thus, they argue, “differences between N. kerioi and ‘K.africanus … have no bearing on the validity of Equatorius, other than to better characterize the distinctiveness of the latter.

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