This Week in Science

Science  28 Jul 2006:
Vol. 313, Issue 5786, pp. 405
  1. To Be or Not to Be Germ Cells


    Plants and animals manage the development of their germline cells rather differently; male germlines are seemingly inducible in plants, but are set aside early in animal embryogenesis. Haerizadeh et al. (p. 496) now offer insight into the molecular processes by which plants manage the determination of their germ cells by identifying a repressive transcriptional modulator and the DNA motif to which it binds. The modulator, germline restrictive silencer factor (GRSF), represses expression of male germline genes in all cells that are not germ cells. GRSF is found in diverse plant species and its function parallels that of an animal protein, REST, which represses neuronal genes in nonneuronal cells.

  2. Round and Round

    Long-term variation in terrestrial insolation (the amount of energy that Earth receives from the Sun) results from the precession of Earth's axis of rotation, the variability of the obliquity of the spin axis, and the eccentricity of Earth's orbit around the Sun, which lead to signals with periods of ∼20,000, 41,000, and 100,000 years, respectively. However, these cycles do not always appear in various paleoclimate records in the way predicted by current models. For example, the 41,000-year obliquity signal in geological records dominates between 3 million and 1 million years ago, rather than the expected ∼20,000-year precessional signal. Two studies offer different solutions to this problem (see the Perspective by Paillard). Raymo et al. (p. 492, published online 22 June) maintain that the ∼20,000-year cycles are largely absent from sea-level records of that period because ice volume changes in the Northern and Southern polar ice sheets are always opposite in sign and essentially cancel each other. Huybers (p. 508; published online 22 June) argues that models have been incorrectly using peak summer insolation to calculate melting, rather than the correct quantity, the integrated amount of solar energy received during the course of the season.

  3. Nonlinear Metamaterials

    The ability to tune the optical response of metamaterials through structure design has already shown some exciting effects in the linear optical regime such as negative refraction and superlensing. Klein et al. (p. 502) now extend the properties of metamaterials to the nonlinear regime. The magnetic response of an array of split-ring resonators exhibits a large second-harmonic signal when exposed to incident infrared light.

  4. Kicking Out Kirchhoff's Laws


    In classical electronics, Kirchoff's laws tell us how to add resistances, impedances, and currents in order to determine current flow within a circuit. Gabelli et al. (p. 499, published online 13 July) show that for a fully coherent circuit consisting of a quantum resistor (point contact) and quantum capacitor in series, Kirchhoff's laws no longer describe the resistance of the system. In addition to highlighting the difference in electronic transport behavior between quantum and classical systems, the results should prove useful for understanding the electronics in future solid-state implementations for quantum computers.

  5. Minimalism in Data Representation

    Reduction of data dimensionality, which creates more compact representations with fewer variables, is important in computer vision, document analysis, and other pattern-recognition tasks. Hinton and Salakhutdinov (p. 504; see the Perspective by Cottrell) devised a way of using neural networks to convert high-dimensional data into lower dimensional codes. They pretrained the internal encoding layers to establish an approximation to the desired network parameters. Decoding the low-dimensional representation to reconstruct the original data shows that the method provides better fidelity than conventional linear techniques.

  6. Companion Group for Neptune

    The solar system contains several reservoirs of small bodies—the icy Kuiper Belt objects at its far edges, the main belt asteroids, and the Jovian Trojans, which co-orbit with Jupiter. A fourth reservoir of Neptune Trojans, a set of asteroids that follow or lead Neptune in its orbit by ∼60°, has now been found by Sheppard and Trujillo (p. 511, see the Perspective by Marzari; published online 15 June). One of these appears to be in a highly inclined orbit relative to the main plane; these small bodies are part of a thick disk and did not form in situ from the breakup of a larger object, but originate from earlier times. The Neptune Trojans have similar red colors, which suggests that they have a common origin. They also differ from the more distant Kuiper Belt objects, which implies that these two reservoirs had different formation mechanisms.

  7. Toward Diversity Despite Environmental Homogeneity

    What is required to initiate divergence between cohabiting populations sharing the same resources? Maharjan et al. (p. 514, published online 6 July) found that multidirectional divergence could arise within a genetically homogenous bacterial population in a chemostat. Diversity apparently was generated even in an unstructured environment lacking obvious ecological niches. After 90 generations, experimental Escherichia coli cultures had differentiated into at least five different lineages that were distinguished by metabolic properties and nutrient uptake patterns. The differentiation took place on a time scale comparable to that of infectious disease progression.

  8. Self-Correction Ensures Faithful Transcription

    As DNA is being read for the synthesis of nascent RNA, occasional errors in nucleotide incorporation are made by the elongating RNA polymerase molecule. However, there is a system in place to ensure the fidelity of the transcription process. This system utilizes cleavage factors and, as reported by Zenkin et al. (p. 518; see the Perspective by Cramer) for bacterial transcription, an intrinsic mechanism in which the misincorporated nucleotide participates in its own excision by providing active groups and coordination bonds to the RNA polymerase active center. This process in turn stimulates nucleolytic activity for the release of the mismatched base. Thus, transcription machinery relies on the RNA product itself to ensure fidelity of transcription.

  9. Mastering Venomous Bites and Stings


    Mast cells play multiple roles in protective immunity to infection, but also have less desirable roles in autoimmune and allergic conditions. The tissue injury caused by mast cell activation arises through the release of mediators that perform a range of inflammatory functions, which have been thought to exacerbate the effects of venomous bites or stings. However, Metz et al. (p. 526; see the news story by Marx) manipulated a genetic mast-cell deficiency in mice to show that mast cells can protect against certain snake and bee-sting venoms by releasing carboxypeptidase A, which can break down the most dangerous component of several types of venom.

  10. Ancestry and Adaptation of HIV

    The origins of the human immunodeficiency virus type-1 (HIV-1) can be traced to the simian immunodeficiency virus (SIV) in primates. Nevertheless, the strain of SIV most closely related to HIV has been detected only occasionally in captive chimpanzees, and a recognizable reservoir in wild apes has been lacking. Studying endangered primates in their natural habitat, Keele et al. (p. 523, published online 25 May) detected antibodies directed against SIV, as well as SIV nucleic acid in fecal samples, from wild-living species of chimpanzee, and found infection prevalences as high as 35%. Comparisons of the HIV and SIV genomes and phylogeographic clustering of the newly identified SIV strains allowed the origins of present-day human HIV-1 virus groups to be traced to distinct chimpanzee communities. In a related Review, Heeney et al. (p. 462) discuss HIV in humans and consider how host factors are contributing to the evolution of resistance to the virus.

  11. Of Mice and Fear

    The neurotransmitter serotonin (5-HT) modulates a diverse array of functions related to homeostasis and responses to the environment. Despite its importance, little is known about the brain structures or the postsynaptic receptors that mediate 5-HT effects. Weisstaub et al. (p. 536) created 5-HT2A receptor (5HT2AR) knockout mice and found that these animals exhibited less anxiety and more disinhibition in a conflictual situation. This behavior could be reversed by selectively restoring 5HT2ARs in the cortex. However, restored 5HT2AR expression in a subcortical region such as the thalamus produced no difference between rescued and knockout mice.

  12. Shedding Light on Melanoma Risk

    Exposure to ultraviolet radiation is a well-established risk factor for melanoma in light-skinned populations, but genetic factors also play a role. Studying melanomas that occur on skin with little chronic Sun damage, Landi et al. (p. 521, published online 29 June) discover an interaction between two genes that contributes to cancer risk. Individuals with allelic variants of the gene encoding the melanocortin-1 receptor (MC1R), which contributes to phenotypic traits such as fair skin, freckling, and red hair, have a much greater risk of developing melanomas with mutations in the BRAF oncogene. The mechanism underlying this association is unclear but appears to be independent of MC1R's effects on pigmentation.

  13. Seeing Is Believing

    In order for vertebrate photoreceptors to exhibit their exquisite sensitivity that allows them to distinguish stimulation by a single photon, the sensor, rhodopsin, must have a very reproducible response. Rhodopsin propagates the signal from absorbed photons by activating transducin, but is then inactivated by phosphorylation. Doan et al. (p. 530) measured the response of single rhodopsin molecules to single-photon absorption events in preparations of mouse photoreceptors. Multiple phosphorylation events provide independent inhibitory signals that together may provide the remarkable reproducibility of the amplitude and duration of rhodopsin activation observed in the vertebrate eye.

  14. Direct Target Gene Binding by Signaling Kinases

    Components of regulatory signaling pathways can control gene expression by phosphorylation of transcriptions factors, which in turn control expression of target genes. Members of the mitogen-activated protein kinase (MAPK) family physically associate with target genes. Pokholok et al. (p. 533; see the Perspective by Edmunds and Mahadevan) show that this phenomenon is much more common than previously expected. The authors performed a genome-wide screen for binding sites for members of the MAPK and cyclic adenosine monophosphate-dependent protein kinase (PKA) families. The MAPK proteins tested were physically associated with their target genes, in some cases at the promoter, but in others along the transcribed region of the genes. The PKA family member Tpk1p was found associated with the transcribed portion of essentially all actively transcribed genes, whereas another family member, Tpk2p, was associated specifically with the transcribed region of genes encoding ribosomal subunits. Such analysis of target gene binding may reveal previously unrecognized targets of kinases and the signaling pathways in which they act.

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