This Week in Science

Science  04 Apr 2003:
Vol. 300, Issue 5616, pp. 13
  1. Close Quarters for Nanowires

    Although it is possible to fabricate wires with diameters on the nanometer scale, it can be very difficult to place them close together because the standard fabrication procedures can cause wire damage. Melosh et al. (p. 112) use molecular beam epitaxy to control the growth of a master that is then selectively etched, tilted, and coated with metal. The master can then be placed on a substrate to deposit a series of closely spaced nanowires.

  2. High Humidity in the Holocene

    The tropics and subtropics were generally warmer during the early to middle Holocene than they are today, and were subject to larger amplitude changes in the hydrological cycle. What were hydrological conditions like in the anthropologically important eastern Mediterranean region? Arz et al. (p. 118) reconstruct Holocene surface ocean properties and continental rainfall based on two sediment cores from the northern Red Sea. They find evidence for significantly increased regional rainfall, and conclude that this humid phase was related to moisture export from the eastern Mediterranean, probably involving a “Mediterranean monsoon”-type circulation.

  3. A Profusion of Prehistoric Plants

    Plant diversity is exceptionally high in the tropical rainforests, but whether this is a fairly recent development (in the Pleistocene or perhaps Miocene) has been debated. Wilf et al. (p. 122; see the Perspective by Knapp and Mallet) describe an assemblage of plant fossils from a volcanic lake deposit in South America that dates to 52 million years ago. This locality was at mid-latitudes but marks a time when Earth's climate was unusually warm. This collection shows that plant diversity in South America was as high as or higher than it is today and thus supports the notion that neotropical diversity is ancient.

  4. Touching Stardust

    Grains of material such as nanodiamonds have been found in meteorites that came from stars beyond the solar system. Messenger et al. (p. 105; see the Perspective by Tielens) have now identified six circumstellar silicate grains (crystalline and amorphous) in interplanetary dust particles collected from the stratosphere. The oxygen isotopic signature of four grains suggests that they came from red giant branch, asymptotic giant branch, or metal-poor stars. The stellar sources for the other two grains are still undetermined. Stellar observations combined with further work on these rare samples will help resolve the parent stars' characteristics and chemistry in relation to that of the Sun. The presence of the more delicate silicate grains compared to nanodiamonds suggests that the interplanetary dust particles are derived from more primitive and less disturbed solar system sources, such as short-period comets.

  5. Add the Base, Close the Gate

    Members of the chloride channel family ClC are present in organisms ranging from bacteria to animals. In ClC channels, ion conduction and gating are tightly coupled. Dutzler et al. (p. 108) compared their structures of wild-type and mutant Escherichia coli ClC channels and found that a glutamate residue acts as a gate. When it is unprotonated the glutamate side chain occupies a chloride-ion binding site and closes the channel. However, when it is protonated, the side chain flips out of the way and the channel opens. Thus, extracellular pH can regulate gating. The open channel contains a queue of three chloride ions, not unlike the ion pathway in the structurally dictinct potassium channel.

  6. Blind Faults in Los Angeles

    Blind thrust faults are buried faults that do not reach the surface and are difficult to identify. Dolan et al. (p. 115) studied borehole and seismic data for the Puente Hills blind thrust fault, located beneath metropolitan Los Angeles. They found evidence for four earthquakes (moment magnitudes between 7.2 and 7.5) along this fault during the past 11,000 years. The occurrence of such large-magnitude earthquakes indicates the importance of blind thrust faults in assessing seismic hazards of metropolitan areas with complex tectonics.

  7. The Cost of Charisma

    Mate choice is frequently influenced by physical cues, which are used to indicate general vigor to prospective partners, but such secondary sexual traits can impose a cost. Faivre et al. (p. 103) find a trade-off between the role of carotenoids in bill coloration and immune responses. In male blackbirds, where bright orange bills are preferred by females, activation of the immune system diminished bill color intensity by depleting carotenoids from keratin. Bill coloration is a secondary sexual trait in zebra finches, too, and Blount et al. (p. 125; see the news story by Pennisi) observe that more intense coloration produced by supplementing dietary carotenoids correlated with preferred selection by females and with increased immune response.

  8. Unmixing Apoptotic Signals

    The BAX and BAK proteins have been said to provide a “mitochondrial gateway” to apoptosis or cell death because stimuli that cause death through effects on mitochondria require the action of one or the other of these proteins at the outer mitochondrial membrane. Scorrano et al. (p. 135; see the Perspective by Demaurex and Distelhorst) show that these proteins have a similar role as an essential gateway through which apoptotic signals affect Ca2+ release from the endoplasmic reticulum (ER) and consequent changes in concentrations of Ca2+ in the mitochondria that lead to cell death signals. Cell lacking both BAX and BAK had decreased concentrations of Ca2+ in the ER. The authors selectively restored Ca2+ regulation in the mitochondria or the ER and distinguish three different classes of apoptotic stimuli. Stimuli like arachidonic acid or C2-ceramide require action of BAX or BAK at the ER. The so-called BH3-only apoptotic proteins require only restoration of BAX or BAK at the mitochondria to cause death. Finally, intrinsic cellular stress signals require BAX and BAK function in both organelles.

  9. No Backsliding

    There are two models for the mechanism by which heat shock proteins (Hsp) aid protein translocation across the inner mitochondrial membrane. In one, Hsp hydrolyzes ATP and actively pulls the protein through the membrane; in the other, Hsp uses ATP to prevent translocating peptides from slipping back, like a molecular ratchet. Liu et al. (p. 139) provide compelling in vitro evidence that the latter model is likely to apply.

  10. Forced to Close

    Dorsal closure occurs in the Drosophila embryo and resembles movements in vertebrate development and wound healing. Hutson et al. (p. 145; see the Perspective by Martin and Parkhurst) describe the relative contributions of various tissues and components required to drive the key morphogenetic process of dorsal closure. They used laser microsurgery and mathematical modeling methods to determine the force contributions from four different cell movements: amnioserosal contraction, supracellular purse-string contraction, lateral epidermal stretching, and filopodial zippering. The robustness of this process comes from a balance of these various forces involved. The quantitative model was then applied to a dorsal closure mutant to identify how the different force components were affected.

  11. Pushing Back the Edge

    The dynamics of the leading edge of motile cells have been probed by Zicha et al. (p. 142) by co-expressing two distinctly colored fluorescent actins, one of which is bleached while the other serves as a reference. Actin moved rapidly from the base of the lamellipodia to the tip, possibly via hydrodynamic flow that would transport unpolymerized actin forward, implying that both polymerization and hydrodynamic flow could provide the force required to drive the leading edge forward.

  12. Assembling a Silencer

    The chromosome-wide repression of gene expression seen in X chromosome inactivation is intimately linked to both the noncoding RNA Xist and with the state of the chromatin on the X chromosome. The Polycomb (Pc) complex, recently shown to add a methylation mark to the histone H3 lysine 27 residue (H3 K27), has been implicated in regulation of X inactivation. Plath et al.(p. 131) now show that the transient presence of the Pc complex and the H3 K27 methyl mark correlate with the initiation of X inactivation in both extra-embryonic and embryonic tissues. Both the Pc complex and the H3 K27 methyl mark are recruited to the X chromosome by the Xist RNA, yet they are not sufficient for X chromosome silencing. The authors speculate that the Pc complex and the H3 K27 methyl mark may facilitate the initial association of Xist with the X chromosome or with other factors that are recruited to mediate silencing.

  13. Coping with Reader's Block

    Messenger RNA (mRNA) molecules carry information in the form of nucleotide sequence; the ribosome reads this information and makes a protein. There are distinct start and stop signals, but what if the ribosome starts reading a defective mRNA that lacks a stop signal? Using cryoelectron microscopy, Valle et al. (p. 127; see the Perspective by Moore et al.) observe the ribosomal rescuer in action. The tmRNA first inserts its transfer RNA (tRNA)-like domain into the ribosome and then switches it onto its own mRNA-like open reading frame. Not only does this serve to liberate the stalled ribosome, but it also adds a short peptide tag to the defective protein, targeting it for degradation.