Editors' Choice

Science  13 Jun 2008:
Vol. 320, Issue 5882, pp. 1396

    Clearing up Cloudy Data

    Aerosols have a huge influence on climate, largely through how they affect clouds by what is termed their indirect effect. The indirect aerosol effect is the sum of two distinct phenomena: first, the response of cloud drop density and size to changes in aerosol properties; and second, the response of cloud albedo to changes in cloud drop density and size. Both components must be known to determine the whole effect, but most of the experimental and field studies conducted to date have addressed only the first process, leaving large uncertainties. Roberts et al. describe a method to quantify both phenomena at once, thereby enabling direct observation of aerosol-cloud-albedo interactions. They use a stack of three autonomous unmanned aerial vehicles below, at, and above cloud level, to simultaneously measure the radiometric and microphysical properties of individual clouds. Their results therefore pave the way for resolving the largest current source of uncertainty in the quantification of the radiative forcing of climate. — HJS

    Proc. Natl. Acad. Sci. U.S.A. 105, 7370 (2008).


    Caged Protection

    Aerogels are mesoporous materials whose very low densities lead to unusual thermal and acoustic properties. However, their high porosity makes them brittle and has limited their use to a few specialized applications. Leventis et al. show that by crosslinking vanadia aerogels with isocyanates to form a conformal coating, they achieve a composite material with an idealized morphology. Unlike more common silica aerogels, mesoporous vanadia consists of entangled wormlike fragments, which fused together at contact points in the polymer-coated composite. In compressive tests, the silica and vanadia aerogels showed similar trends, with the polymer crosslinked materials exhibiting greater strength and toughness. However, one important difference was that the silica aerogels cracked and fractured when highly compressed, whereas the vanadia counterparts could carry a high load even at 91% strain and showed excellent properties under cryogenic conditions. Although vanadia is too expensive to use on a large scale, the authors envision that with the right combination of aerogel morphology and polymer-aerogel interactions, strong aerogels could be prepared from silicon, iron, or aluminum oxides. — MSL

    J. Mater. Chem. 18, 2475 (2008).


    Small-Cluster Coexistence

    When metal clusters are adsorbed on metal oxide surfaces, their properties can change, especially if they interact with defect sites and undergo charge transfer. Simic-Milosevic et al. explored the extent of such charge transfer in a model system, in which magnesium oxide films [three monolayers (MLs) of MgO(001)] were grown on a Ag(001) surface; Au was then deposited at low coverages (0.03 ML) at very low temperatures (5 K), and its structure was studied with a scanning tunneling microscope (STM). Although most of the Au was present as isolated atoms, two types of dimer could be observed that either lay flat on the surface (shown at left) or stood upright. Manipulations with the STM tip could form the flat dimers from the atoms and then convert them to the upright form. Density functional theory calculations indicated that the upright form is more stable and adsorbs as a neutral species onto surface oxygen sites, whereas the flat form is negatively charged and places the Au atoms over Mg. These results show that the spread in energy between different electronic states may be low enough to allow substantial coexistence on a surface. — PDS

    J. Am. Chem. Soc. 130, 10.1021/ja8024388 (2008).


    Monarch Menace

    Parasites are known to harm their hosts, although from an evolutionary perspective it is not intuitively obvious why an organism that depends on another for its survival and transmission would risk killing its partner. Virulence may be an unavoidable outcome of a parasite using host resources to replicate, which causes damage and provokes costly immune responses; parasites would thus be expected to limit their replication to a submaximal level. To test this hypothesis, de Roode et al. collected data on the migratory North American monarch butterfly and a spore-forming protozoan parasite. As expected, greater parasite replication and greater spore loads reduced the probability of a butterfly emerging successfully from a chrysalis and also reduced the subsequent mating success and life span of female butterflies. The tradeoff was that female monarchs with the lowest parasite loads transferred spores to only 20% of their eggs, even though the fecundity of the parasitized butterflies was unaltered by spore load. Significant differences in virulence were observed between eastern (less tolerant of virulence) and western monarchs. Because eastern migration is 10 times longer, butterflies carrying highly virulent genotypes of parasite could die of the effects of parasitism en route. — CA

    Proc. Natl. Acad. Sci. U.S.A. 105, 7489 (2008).


    Fragile DNA

    The iconic image of DNA is a double helix, yet it can also adopt more exotic conformations such as the guanine (G) quadruplex, where four strands of G-rich DNA align (intra- or intermolecularly) in a stacked rectangular arrangement via Hoogsteen bonds between the Gs. Although the in vitro evidence for such structures is plentiful, the extent (or, indeed, the consequence) of their existence in vivo is less clear. It is known that runs of Gs are vulnerable to deletion and that a protein linked to Fanconi anemia, FancJ, can help to protect such fragile sites.

    In Caenorhabditis in which the FancJ homolog dog-1 is deleted, Kruisselbrink et al. show that the G4 DNA tracts most prone to deletion have the characteristics of a canonical G quadruplex and that G3 sites—those missing one of the four legs—are not fragile. Furthermore, within the G-rich sequences, those with the greatest ability to fold into a quadruplex are most likely to suffer deletion, occasionally at a very high frequency. The small size of the deletions, less than 300 base pairs, suggests that G4 DNA preferentially stalls the replication machinery on the lagging strand, the tangled mess being extricated only by removal of the offending region up to the nearest upstream Okazaki fragment. The loss of FancJ in Fanconi anemia patients may sensitize over 300,000 predicted G4 sites in the genome to deletion, and thence produce a predisposition to cancer. — GR

    Curr. Biol. 18, 10.1016/j.cub.2008.05.013 (2008).


    Coping with Heavy Metal

    Cadmium seems to be useless to plants but manages to hitchhike its way into plant cells through transporters used to import iron, calcium, and zinc. Various processes serve to sequester and thus detoxify cadmium; plants contaminated by cadmium are an unfortunate source of heavy metal in human and animal diets. Dutilleul et al. find that the expression of selenium-binding proteins was increased in Arabidopsis seedlings after exposure to cadmium, and the overexpression of the selenium-binding proteins served to protect Arabidopsis from the toxic effects of high levels of cadmium. Binding studies indicate direct interactions between cadmium and selenium-binding protein, the function of which in the normal unstressed plant remains unclear. Better insight into how plants deal with toxic metals could contribute to developing plants that are able to detoxify soils or to reducing trace toxicity in the food chain. — PJH

    Plant Physiol. 147, 239 (2008).


    Making Sense of EPO Receptors

    Erythropoetin (EPO) signaling contributes to organ development, as well as to the differentiation of erythrocytes. The abundance of the EPO receptor (EPO-R) increases after removal of one lung (pneumonectomy) in dogs. In the same system, Zhang et al. report that the abundance of EPO-R appears to be regulated by the sense EPO-R transcript (sEPO-R), as well as by either the antisense transcript (asEPO-R) or by proteins encoded by two open reading frames (ORFs) within the antisense transcript. Both transcripts were detected in the normal canine lung; the abundance of sEPO-R increased modestly after pneumonectomy, whereas that of asEPO-R and EPO-R more than doubled. Coexpression of sEPO-R and asEPO-R increased the abundance of EPO-R in transfected human cells as compared to sEPO-R alone. The putative ORF1 protein was not needed to elevate EPO-R in transfected cells. In contrast, EPO-R production increased when a construct that included ORF2 and 300 base pairs 5′ of the ORF2 start site was cotransfected with sEPO-R; however, when the upstream region was not included, EPO-R abundance did not increase. The enhancing effect was restored by mutating the start codon of ORF2. The authors propose that asEPO-R has several regulatory elements, with the RNA having stimulatory effects and the ORF2 protein negative effects on EPO-R synthesis. — NRG

    Proc. Natl. Acad. Sci. U.S.A. 105, 7612 (2008).