Editors' Choice

Science  19 Mar 2004:
Vol. 303, Issue 5665, pp. 1731

    Pulsing for Polytypes

    Silicon carbide (SiC) is a wide-band gap semiconductor and one of the more promising materials for use in high-temperature high-power devices because of its thermal, chemical, and mechanical stability. Still, there are obstacles to its fabrication as epitaxial films because it requires a high substrate temperature (around 1500°C), and it's hard to control the polytype that grows—usually a mixture is obtained. These polytypes represent variations in the stacking of the SiC layers, and because they alter the band gap of the material, finding a method to control the growth is vital.

    Kusumori et al. have developed a pulsed laser deposition technique for fabricating heteroexpitaxial films of the 3C, 2H, and 4H polytypes with a substrate temperature of only 1100°C. Unlike other deposition methods, laser ablation ejects species containing considerable kinetic energy, which is converted to thermal energy on impact with the sapphire substrate. Thus, by altering the frequency of the laser pulses it is possible to raise the temperature at the site of deposition while keeping the rest of the substrate at a lower temperature. As they varied the laser pulse frequency from 1 to 2 to 5 Hz, the polytype changed from 3C to 2H to 4H. — MSL

    Appl. Phys. Lett. 84, 1272 (2004).


    Location, Location, Location

    One of the notable advances in biomedical research has been the application of recombinant DNA techniques to establish the association of human diseases and genetic mutations, beginning with the mapping of the Huntington's disease gene to chromosome 4 two decades ago. On the other hand, filling in the causal links between a nucleotide change (or insertion or deletion) and the outward manifestation of dysfunction has not always been as rapid as anticipated. In particular, it has been challenging to explain how different mutations in the same gene can yield distinct phenotypes.

    Inoue et al. describe how the location of mutation within the gene SOX10, which encodes a transcription factor, dictates whether the less severe Waardenburg syndrome 4 (WS4) or the more severe PCWH disorder (encompassing peripheral demyelinating neuropathy, central dysmyelinating leukodystrophy, Waardenburg syndrome, and Hirschsprung disease) ensues. Many of these mutations (nonsense) introduce premature termination codons (PTCs) and hence would result in truncated protein products. As predicted, all of the mutant proteins display dominant negative effects on wild-type SOX10 transcriptional activity in vitro, but with about equal potency. There is, however, a surveillance mechanism for PTCs called nonsense-mediated decay (NMD), a process in which messenger RNAs (mRNAs) that contain a termination codon lying upstream of an intron are recognized as faulty and are degraded. The intron requirement explains why mutations in exon 5, the last exon and only one in SOX10 not followed by an intron, produce mRNAs that apparently escape the NMD pathway and are translated into aberrant proteins with severe consequences. — GJC

    Nature Genet. 36, 361 (2004).


    Access to Help

    Differentiation of CD4+ helper T cells is described as taking one of two routes: Naive T cells can become either T helper type 1 (TH1) cells or TH2 cells, with distinctive patterns of cytokine expression. Looking at genome-wide transcriptional profiles of helper T cells at different stages of differentiation, Lu et al. observed that of the distinct sets of transcribed genes, those regulating replication became prominent soon after activation, while resting-state genes were silenced. This led the authors to suggest that a large-scale increase in chromatin accessibility accompanying early events would allow transcription factors to coordinate the expression of genes needed to secure the subsequent differentiation of the helper T cell.

    Cote-Sierra et al. reexamined the specific role of interleukin-2 (IL-2) in TH2 differentiation. In contrast to the previously held assumptions that this cytokine acts simply as a growth factor for differentiating T cells, IL-2 appeared to assist directly the differentiation process by stabilizing the accessibility of the IL-4 locus. Both studies suggest that factors influencing activation and proliferation also set the stage for transcription events that secure the fate of the T cell. — SJS

    Proc. Natl. Acad. Sci. U.S.A. 101, 3023; 3880 (2004).


    Seventeen Cell Lines

    Embryonic stem cells hold promise both for the study of early development and as a source of differentiated cell types that may be used in cellular therapies for human diseases. Yet human embryonic stem cells are not widely available. Now, Cowan et al. have increased the number of human embryonic stem cell lines by 17. Although not approved for research supported by National Institutes of Health funding, these cell lines are available for noncommercial research purposes. Each of them, derived from donated embryos, requires a feeder layer of mouse fibroblasts and has been developed to respond to standardized cell culture techniques. — PJH

    N. Engl. J. Med. 10.1056/NEJMsr040330 (2004).


    Boiling Planets

    The extrasolar planet Osiris orbits the star HD 209458 and is only 150 light years from Earth. It is fortuitously aligned with respect to us, so that the planet transits in front of the star, allowing direct measurements of the planet's radius, mass, and atmospheric composition. Osiris completes an orbit in less than 4 days, and because it is so close to its parent star, hydrogen boils off the hot upper atmosphere; Osiris belongs to a planetary class called “hot Jupiters” because of its heated atmosphere.

    Vidal-Madjar et al. observed four transits with the STIS spectrograph on the Hubble Space Telescope. They detected an evaporating envelope of oxygen and carbon around Osiris. The oxygen and carbon must be escaping from the lower atmosphere, but the typical mechanism for escape, eddy diffusion, that works for Jovian planets does not match their observations. Instead, oxygen and carbon are being hydrodynamically dragged along with the much more abundant hydrogen, and all three species appear to be escaping at a ratio consistent with their composition in the lower atmosphere. Amazingly, in less than a decade, astronomers have progressed from detecting the presence of extrasolar planets to probing their atmospheres from top to bottom. — LR

    Astrophys. J. 604, L69 (2004).


    Mosquitoes Complement Malaria

    Not all strains of the mosquito Anopheles gambiae are able to host or transmit malaria parasites with equal efficiency. Even in susceptible strains, most parasites are killed and cleared before they can be passed on to the next host. Blandin et al. have investigated a thioester-containing protein (TEP1) that was previously discovered to be important in mosquito immune responses to bacterial pathogens and that has homology to mammalian complement factors. Double-stranded RNA knockdown experiments rendered both susceptible and refractory strains of mosquito highly vulnerable to parasite establishment. Blandin et al. realized there were two alleles: TEP1r, which occurs in the refractory strains of mosquito that encapsulate parasites in melanin, and TEP1s, which regulates the resistance to parasites observed in susceptible mosquitoes. The difference probably lies in the kinetics of binding to the parasite, with TEP1r being faster than TEP1s. It is known that TEP1 is a glycoprotein secreted into the hemolymph by hemocytes, and in bacterial infections TEP1 apparently tags the pathogen surface with its C-terminal domain via the thioester bond, rendering it liable to phagocytosis. Something similar probably happens to the malaria parasites. — CA

    Cell 116, 661 (2004).


    Monitoring Mercury

    As a product of industrial activity, over 300 tons of mercury are added to the atmosphere each year in Europe alone. With direct consequences for health and the surrounding environment, such heavy-metal emissions are coming under closer scrutiny. Laser-based detecting systems look at the spectroscopic signature of backscattered light from excited particulates in the atmosphere surrounding emission sources, and they offer an alternative to the time-consuming and labor-intensive chemical analysis of air samples. Exciting mercury, however, requires radiation in the deepUV, a wavelength range in which long-term, stable laser-light sources are not readily available. Sjöholm et al. use an optical parametric oscillator as the UV light source and demonstrate in a ground-based system the ability to monitor accurately the movement and flux of mercury plumes over a range of 800 m. The surveillance technique should provide valuable information for those involved in regulating the impact of these potentially hazardous emissions. — ISO

    Opt. Express 12, 551 (2004).

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