Editors' Choice

Science  22 Oct 2004:
Vol. 306, Issue 5696, pp. 577

    Fields of Atacama

    The Atacama Desert, in the rain shadow of the Andes of Chile, is one of the driest and most lifeless environments on Earth and is considered a test environment for astrobiology. It also contains an abundance of nitrate minerals, enough that its soils have been mined for fertilizer for nearly two centuries. The origin of these nitrates has been debated since Charles Darwin visited here on his Beagle voyage, with suggestions ranging from bacterial fixation, to atmospheric deposition of sea salt, to volcanic inputs, guano, and more. Michalski et al.examined the three oxygen isotopes (16O, 17O, and 18O) of soil nitrates and sulfates to characterize their origin. Biological processes and most inorganic reactions produce fractionate isotopes depending on their mass difference. In contrast, photochemistry in Earth's atmosphere yields a distinctive relation among the isotopes that is independent of their mass. Most of the nitrate and other salts in the Atacama appear to originate from conversion of gaseous nitrogen and other compounds to particles in the atmosphere followed by deposition. In wetter places, these salts would be processed further by biological activity. Deposition over the past 2 million years is probably sufficient to have produced the large deposits observed. — BH

    Geochim. Cosmochim. Acta 68, 4023 (2004).


    Neutral Atom Register

    Several implementations have been proposed for quantum computation. Each scheme will require the precise setting and manipulation of the quantum units on which the information is to be stored and processed, whether these are atoms, ions, molecules, or flux qubits, etc. However, which scheme will eventually win out is not so clear. Schrader et al.describe the experimental realization of such a quantum register, involving a linear chain of neutral cesium atoms confined to an optical dipole trap. The use of neutral atoms is particularly attractive because the tools for atom storage, trapping, transport, and readout are already available, and also because the excitation levels in the atoms can be tuned individually by introducing a magnetic field gradient along the length of the chain. As hoped, each individual atom in the chain could be addressed and coherently manipulated. Although the size of the present register is quite modest—only five atoms long—it should be readily scalable to a chain of several hundred trapped atoms. — ISO

    Phys. Rev. Lett. 93, 150501 (2004).


    Arrive Early, Appear Late

    Chickenpox is caused by the varicella-zoster virus (VZV), which infects via the respiratory tract before producing the telltale skin rash 2 to 3 weeks later. It is currently assumed that this latency results from incubation of the virus in lymphoid tissue and organs such as the liver, before being transported to the skin as the final stage of a secondary viremia.

    Ku et al.challenge this model by showing that the virus, in fact, quickly hitchhikes its way to the skin by infecting T cells. Using immune-deficient mice that had received grafts of human skin, the authors observed that transferred human T cells appeared in the grafts as early as 24 hours after being infected with the virus. Predominantly, these proved to be memory T cells, which is consistent with the observation that such cells are significantly more susceptible to infection with VZV. Potentially, the high rates of recirculation to peripheral tissues shown by memory T cells would benefit the virus by allowing it to access the skin efficiently. On arrival, innate antiviral mechanisms were induced, suggesting that the delay in the appearance of VZV pox lesions may arise from the virus having to overcome local defenses. — SJS

    J. Exp. Med. 200, 917 (2004).


    Anaerobic Bullets

    The spore-forming Clostridium species of bacteria are notorious for the frightening toxins they can generate, including botulinum and tetanus toxoid. They also prefer to live where there is little oxygen. Starting with early observations by William Coley in the 19th century of cancer remission in some patients surviving postoperative infection, Agrawal et al.harnessed these two characteristics of Clostridium novyi-NT, for experimental cancer therapy. They injected mice bearing large established tumors with spores of an attenuated strain of the bacterium and showed that the spores exclusively germinated within the hypoxic cores of the tumors and destroyed all but the peripheral oxygenated cells. These cells subsequently continued to expand, but about 30% of the mice were cured of the cancer in the absence of radio- or chemotherapy. This happened because a potent immune response to the cancer cells was generated, such that all the cured mice subsequently rejected transplanted tumor cells of the same type and kept the bacteria under control. Similar success was observed in rabbits and rats, but it is unknown whether humans would generate similarly effective immune responses. — CA

    Proc. Natl. Acad. Sci. U.S.A. 101, 15172 (2004).


    Understanding Timothy Syndrome

    Timothy syndrome is a rare human disorder characterized by diverse physiological and developmental defects, including heart arrythmias, webbing of fingers and toes, and autism. Splawski et al.identify a single mutation in a gene that encodes the calcium channel Cav1.2 as being responsible for the syndrome. The mutation occurs in a highly conserved region that is important for voltage-gated channel inactivation resulting in prolonged influx of calcium into cells. Other calcium channel-based disorders affect distinct organ systems. However, the mutated splice variant of Cav1.2 is widely expressed in tissues and organs, consistent with the scope of disease abnormalities. Mutant channels are sensitive to calcium channel blockers, suggesting the potential use of such drugs in treating this genetic disorder, which is often fatal by 2.5 years of age. The study also suggests that aberrant calcium signaling in the brain may contribute to autism, a condition whose molecular mechanism is not well understood. — LDC

    Cell 119, 19 (2004).


    Adding O to Olefins

    Allylic oxidation, in which O is added to a carbon adjacent to a C-C double bond, is a key step in accessing a wide variety of useful organic compounds. Catino et al. have shown that a Rh dimer bound by four caprolactam ligands can catalyze this reaction at loadings as low as 0.1% relative to the carbon substrate. Using t-butyl hydroperoxide as the oxygen source, the authors convert 12 different cyclic olefins to enones. The reactions are complete within 1 hour and selectively form allylic C-O double bonds in the presence of alkyl, phenyl, carbonyl, and nitro groups. It seems that the crucial feature of the catalyst is a low one-electron oxidation potential, which leads to a mixed-valence Rh(II)-Rh(III) intermediate that the authors confirmed spectroscopically. This system improves on alternative methods that require stoichiometric metal oxidants or high catalyst loadings. — JSY

    J. Am. Chem. Soc. 10.1021/ja045330o (2004).


    Erasing DNA Methyl Marks

    Methylation of DNA plays an important role in the epigenetic regulation of gene expression, including preventing the activation and spread of invading nucleic acids in the genome, such as transposable elements. These patterns of 5-methyl C (5meC) in DNA undergo a genome-wide reprogramming in early embryos and primordial germ cells, yet the mechanism by which the methyl marks are removed independently of DNA replication is not known.

    Morgan et al. now show that the enzyme Activation-Induced Cytidine Deaminase (Aid), which together with other family members is known to deaminate C residues in DNA and/or RNA, is also able to deaminate methylated C residues in single-stranded DNA, resulting in a mismatched pairing of G against the transformed T. This G:T pair can be repaired back to the Watson and Crick G:C pair, with the loss of the methyl group, or the mutant T can be fixed in the sequence. Provocatively, Aid and its relative Apobec1 are expressed in the specific tissues and at the time when 5meC reprogramming occurs. Furthermore, the genes for both are located in close proximity to, and coexpressed with, a cluster of genes involved in tissue pluripotency, leading the authors to speculate that Aid and Apobec1 may, through their 5meC deaminase activity, play a role in epigenetic reprogramming. — GR

    J. Biol. Chem. 10.1074/jbc.M407695200 (2004).

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