Editors' Choice

Science  06 Apr 2007:
Vol. 316, Issue 5821, pp. 19

    Perennial Infection

    1. Andrew Sugden

    Although grazing and fire have been proposed as explanations for the remarkable success of exotic annual grasses in California, where they have established themselves among the native perennials over wide swathes of the landscape despite being inferior competitors for resources, active management based on these factors has failed to stem the invasions. It is known that disease can alter the competitive balance between species in ecological communities, and Borer et al. have developed a model showing quantitatively how invasion has been mediated by viral disease (barley and cereal yellow dwarf viruses, which are a major pathogen in crops, including wheat, barley, and oats). They find that the key to the success of the annual grasses is that virus is horizontally transmitted by aphids, rather than vertically via seeds; hence, seed survival is unaffected, and each generation suffers infection anew. In contrast, perennial grasses serve as long-term reservoirs for the virus and experience deleterious effects on survival and on lifetime seed production, thus facilitating the invasion by annuals. — AMS

    Proc. Natl. Acad. Sci. U.S.A. 104, 5473 (2007).


    Probing Quantum Memories

    1. Ian Osborne

    Can't quite place a name to the face, or associate a singer with a song? You know, or at least hope, that the information lies intact somewhere in your head, needing only the correct memory trick or stimulus to retrieve it. For quantum communications, where information is transmitted along quantum channels and stored in quantum memories, it is necessary that the stored information be robust and retrievable. However, quantum memories are known to decay because of decoherence, and physicists therefore have to develop their own set of tricks to probe and measure how reliable these memories are. Staudt et al. look at quantum information stored in an optical memory, where the information is encoded in the coherent transfer of the phase and amplitudes of light pulses onto a suitable solid-state medium. They use a photon-echo technique whereby a sequence of pulses initializes the memory cell, encodes the data onto it, and uses a read pulse to generate a stimulated echo pulse which replicates the stored information. The advantage of this scheme is that, though memories may be lost, if they are recalled they remain undistorted. — ISO

    Phys. Rev. Lett. 98, 113601 (2007).


    Step by Step

    1. Gilbert Chin

    Recent exponential growth in databases as a consequence of big-science projects such as genome sequencing and structural genomics has, in some environments, credentialed bioinformatic analysts and relegated experimental work to the back-benchers. Nevertheless, the significance of mutations can be hard to predict without actually making the proteins and assessing their behavior.

    Kona et al. have taken this approach in trying to understand the role of a Cd2+-binding cysteine in the Escherichia coli enzyme KDO8P synthase in comparison to an asparagine in the Aquifex aeolicus version of the same enzyme. The reaction they catalyze is an aldol condensation of phosphoenolpyruvate and arabinose 5-phosphate. This enzymatic step is a critical one in the bacterial biosynthetic pathway leading to lipopolysaccharides and hence is a potential drug target. A comparison of the structures enabled them to make a series of mutations bridging the metallo- and nonmetallo-KDO8P synthases; follow-up kinetic and structural analyses yielded several insights. The cysteine-coordinated metal fulfills the same function as the asparagine carboxamide in binding and orienting a water molecule for attack on the si side at C2. Even though the metallo- and non-metallo-KDO8P synthases produce the same chemical intermediate, probably via the same reaction pathway, the binding constants of the substrates and products differ, which may reflect an evolutionary adaptation to changes in metabolite concentrations. — GJC

    Biochemistry 46, 10.1021/bi6024879 (2007).


    A Less Radical Pathway

    1. Jake Yeston

    Enediyne molecules, in which two doubly bonded carbons tether two sets of triply bonded carbons, have been known for over 30 years to cyclize to the intriguing para-benzyne biradical. This species has been observed in many cases to behave as a benzene ring with two diametrically opposed trivalent carbons, which each react rapidly with hydrogen or halogen atom sources.

    Perrin et al. have observed a surprisingly different mode of reactivity, which is more consistent with nucleophilic attack at one of the unsaturated carbons than with radical atom abstraction. Their studies show that slight heating of an enediyne in the presence of lithium halide salts and acid results in a halide and proton adding to opposite ends of the resultant benzene ring. Isotopic labeling reveals that even as weak an acid as dimethylsulfoxide can serve as the proton donor, implicating a highly basic phenyl anion intermediate formed after halide attack. The reaction is high-yielding for chloride, bromide, and iodide salts, and shows kinetics consistent with p-benzyne formation as the rate-limiting step. These findings offer a compelling rationale for the puzzling isolation from marine sources of monochlorinated cyanosporaside isomers whose structures were inconsistent with established radical or electrophilic chlorination pathways. — JSY

    J. Am. Chem. Soc. 129, 10.1021/ja070023e (2007).


    SERS from Sharp Silver

    1. Phil Szuromi

    Surface-enhanced Raman scattering (SERS) is observed on a variety of silver and gold surfaces where nanoscale roughness creates high local fields, and giant enhancements have been observed in “hot spots” created between two nanoparticles. However, even single nanoparticles can create fields large enough to enable single-molecule detection.

    To better understand the origin of this effect, McLellan et al. have deposited silver nanoparticles of various shapes on silicon substrates that have registration marks. Scanning electron microscopy was used to determine the orientation of the particles so that the effect of laser polarization on SERS spectra could be studied. For nanocubes, the SERS intensity of adsorbed 1,4-benzenedithiol varied greatly with the direction of polarization, and the spectra were more intense when the field cut across the cube's corners; more rounded truncated cubes showed little variation with polarization direction. Similar effects were seen in simulations of the local fields for these particles. — PDS

    Nano Lett. 7, 10.1021/nl070157q (2007).


    The Evolution of Origins

    1. Guy Riddihough

    Prokaryote genomes are generally organized as a single circular chromosome with a single origin of DNA replication; most eukaryotes, on the other hand, have multiple chromosomes, each with multiple replication origins. This latter feature has recently been found in a number of archaea, including Sulfolobus species, which have several origins on a single chromosome. Might these have arisen simply by duplication?

    Robinson and Bell show that origins that are conserved across Sulfolobus species share the gene copG, encoding a plasmid copy-number control protein, as well as a number of stress response genes. Furthermore, one of the two origins in the archaeal Aeropyrum pernix bears a striking resemblance to two origins found in a distantly related Sulfolobus species; several genes and evidence of a putative prokaryotic viral integration site are conserved. Among the genes is a protein that is similar to RepA, a bacterial plasmid initiator protein, as well as the yeast replication initiation protein Cdt1. Altogether, this evidence points to a captured extrachromosomal element, possibly a virus/plasmid hybrid, as the source of the supernumerary origins. A hybrid phage/eukaryotic replication initiation site on the yeast 2μ plasmid hints at a similar genesis for the multiple origins on eukaryotic chromosomes. — GR

    Proc. Natl. Acad. Sci. U.S.A. 104, 5806 (2007).

  7. STKE

    Better Bones Without Bax

    1. Elizabeth Adler

    At about age 50, the depletion of ovarian follicles through apoptosis leads to the loss of cyclic ovarian function in women. Although aging female mice do not undergo menopause, they do suffer a depletion of ovarian follicles and health complications similar to those of postmenopausal women. After their earlier finding that oocyte loss was mitigated in mice lacking the proapoptotic protein Bax, Perez et al. investigated aging Bax-deficient female mice and found them to be leaner and more active than their wild-type counterparts. They retained more of their hair, developed fewer cataracts, experienced less wrinkling of the skin, and had stronger bones. Although older Bax knockout mice failed to become pregnant, they did ovulate in response to gonadotropin, and when their ovarian tissue was grafted into young wild-type females, the oocytes produced viable pups. Finally, behavioral analyses indicated that the knockouts were less anxious and more attentive than wild-type mice. — EMA

    Proc. Natl. Acad. Sci. U.S.A. 104, 5229 (2007).