Editors' Choice

Science  23 Jan 2009:
Vol. 323, Issue 5913, pp. 438

    Filming Chalcogenides

    1. Phil Szuromi

    In atomic layer deposition (ALD), thin films are deposited with two alternating chemical reactions. Because both reactions stop once they have exhausted the reactive groups on the surface, growth rates are highly controlled. Extending ALD growth to new materials systems requires the identification of fast reactions that saturate and that avoid the use of highly toxic reagents. Pore et al. report an ALD route for the growth of metal tellurides and selenides, which include germanium antimony telluride (GST), a material used in phase-change memories; and copper indium selenide, a photovoltaic material. Bis(trialkylsilyl) compounds of Te and Se can be used with metal salts of Ge, Sb, and Cu, as well as other metals, to grow films in a controlled manner. GST films could be grown by mixing the precursor metal chlorides, and in the case of copper, the use of copper(II) pivalate allowed for temperature-controlled tuning of the film stoichiometry. — PDS

    J. Am. Chem. Soc. 131, 10.1021/ja8090388 (2009).


    Creating a Spinoff

    1. Gilbert Chin

    How might one acquire a new catalytic ability? Other than by stealing an enzyme (akin to lateral gene transfer), one could take advantage of an existing but suboptimal active site. Sánchez-Moreno et al. describe the promiscuous behavior of the decidedly un-sexy protein dihydroxyacetone kinase (DHAK), whose generally accepted role is to use ATP to phosphorylate the metabolite dihydroxyacetone; the product, dihydroxyacetone phosphate, can be used in lipid and carbohydrate biosynthesis, whereas the unphosphorylated substrate is toxic. What these authors document is that DHAK (shown at left) also catalyzes the FAD-AMP lyase reaction, in which the cofactor FAD is split into AMP and a molecule of riboflavin 4′,5′-phosphate. A key point is that the lyase reaction proceeds with roughly the same catalytic efficiency as the kinase reaction, albeit significantly more slowly and with a requirement for Mn2+ instead of Mg2+. Intriguingly, the intracellular concentration of Mn is much lower than that of Mg, yet several other carbohydrate-handling enzymes, such as pyruvate kinase, are also manganese-dependent. — GJC

    ChemBioChem 10, 10.1002/cbic.200800573 (2009).


    Methane Mystery

    1. H. Jesse Smith

    Methane is a powerful greenhouse gas, implicated as a cause of many episodes of climate change in the past and also as a major factor in Earth's contemporary radiative balance. Nonetheless, we still lack an adequate understanding of the global atmospheric methane budget, and in particular the strength of its sources. Recent research suggested that plants were a major source of methane, supplying as much as almost half of the methane released to the atmosphere; the results implied an unknown biochemical pathway for methane production in plants. Nisbet et al. have performed additional experiments, in chambers that allow careful control of the growing environment, which show that plants do not produce appreciable quantities of methane; genetic analyses moreover revealed no genes that would indicate the presence of any known methane biosynthesis pathway. Instead, the authors suggest that previous observations of methane production may have been reflecting the spontaneous breakdown of plant material under high ultraviolet stress conditions or the transpiration of water containing dissolved methane. They conclude that plants are not a major source of global methane production. — HJS

    Proc. R. Soc. London Ser. B 10.1098/rspb.2008.1731 (2009).


    One Shell Fits All

    1. Marc S. Lavine

    Nature has evolved remarkably complex structures from a limited number of materials in order to obtain multifunctionality. For example, the shell of a turtle needs to be stiff in order to protect the internal organs from trauma. However, it also needs to be sufficiently flexible to enable efficient respiration and locomotion. As such, the shell needs to be compliant at low cyclical loads but simultaneously stiff and tough to resist sudden large impacts. Krauss et al. show that turtle shells contain densely packed inner and outer cortical sheets, and these two layers in turn sandwich sparsely located porous trabecular bone tissue. Attached to the spinal column are rigid ribs, which are separated by regions of suture (shown above). The suture has a seesaw or wavy geometry and is not mineralized, but instead is composed of aligned collagen fibers that are angled relative to the direction of the suture. This arrangement allows the fibers to remain in tension when the shell is loaded in either tension or compression. At low loading, the suture compresses only slightly, allowing flexibility in the movement of neighboring bones, but at high loading, the bones can lock together to strongly resist deformation. — MSL

    Adv. Mater. 10.1002/adma.200801256 (2008).


    Sophisticated Movements

    1. Peter Stern

    The ability of human beings to exert fine motor control is astonishing. Just think of the precision movements executed by a brain surgeon or a concert pianist or even a top-flight tennis player. What enables us to control the muscles of our upper extremities to such an unmatched degree?

    To establish the anatomical underpinnings of these abilities, Rathelot and Strick injected a strain of rabies virus into single shoulder and arm muscles of monkeys and also reanalyzed previously published results from similar injections into finger muscles. Retrograde transneuronal transport of the virus demonstrated that the primary motor cortex (area M1 in the brain) contains two subdivisions. A rostral region of M1 contains corticospinal cells that project to spinal interneurons; these cells do not make direct connections onto motoneurons and represent a phylogenetically old part of M1 that is standard for most mammals. In contrast, a caudal region of M1 contains neurons that do synapse directly onto motoneurons; this region represents a phylogenetically younger part of M1 that is present only in some higher primates and in humans. The direct access to motoneurons afforded by corticomotoneuronal cells enables this younger part of M1 to bypass spinal cord mechanisms and to sculpt idiosyncratic patterns of motor output that are revealed as highly skilled movements. — PRS

    Proc. Natl. Acad. Sci. U.S.A. 106, 918 (2009).


    Controlling the Masses

    1. Helen Pickersgill

    The regulation of gene transcription is complex and occurs at many different stages, of which one is the transformation of chromatin from an open and more active structure to a closed and inactive state. This regulatory challenge is amplified when the target genes are present in hundreds of copies on several different chromosomes. Ribosomes are composed of proteins and RNAs, and the number of genes encoding ribosomal RNAs (rRNAs) ranges from several hundred in mammals to several thousand in plants. They are organized in tandem repeats that reside in nucleolar organizing regions on chromatin, and the activity of these rRNA genes is finely tuned to support cell growth. However, not all rRNA genes are actively transcribed at the same time, and Sanij et al. show that decreasing the amount of upstream binding factor (UBF) causes methylation-independent and reversible silencing of rRNA genes. UBF depletion led to chromatin remodeling, although total rRNA production was only partially reduced, as there was an increase in transcription from the remaining active genes. The authors found that during differentiation, the active pool of rRNA genes actually decreases, along with the levels of UBF, suggesting that UBF may regulate the ratio of active versus inactive rRNA genes during development. — HP*

    J. Cell Biol. 183, 1259 (2008).

    • *Helen Pickersgill is a locum editor in Science's editorial department.


    Fly Fishing with a Hedgehog

    1. Barbara R. Jasny

    The complexity of gene regulation mechanisms is also evident in the analysis by Amano et al. of the temporal and spatial patterns of Sonic hedgehog (Shh) expression. Shh is required for proper limb formation in mice. In the limb bud, it is expressed exclusively in a group of posterior mesenchymal cells called the zone of polarizing activity (ZPA). The authors found that colocalization of the Shh coding region and the long-range enhancer of its expression in limb bud, MFCS1, frequently occurs in the ZPA of day-10.5 limb buds but not by day 12.5, when Shh has been turned off. Chromosome conformation capture assays confirmed that MFCS1 and Shh interact directly in the limb bud. This interaction appears to indicate transcriptional competence; for active transcription to occur, a chromosome loop involving Shh needs to form, and evidence of loop formation was seen only in the ZPA. Such loops have been observed in other systems, and the authors propose that the interaction of MFCS1 and Shh promotes a looping-out of the chromosome that renders Shh accessible to transcription factors. This change in chromosomal configuration was observed in only a fraction of the ZPA cells at any one time; the resultant intermittent exposure of Shh may ensure proper control of levels of Shh needed for limb morphogenesis. — BJ

    Dev. Cell 16, 10.1016/j.devcel.2008.11.011 (2009).

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