A new assessment of the number of plant species threatened by human activity, by Pitman et al. (p. 989), indicates that at least 30% of all plant species are at risk of extinction this century.
The innate immune system responds rapidly to pathogens, either by obstruction and direct killing of microbes or by activation of the adaptive arm of the immune system. Defensins have feet in both camps, and two studies add insight into how they work (see the Perspective by Ganz). Although chemokines play a role in the inhibition of human immunodeficiency virus-type 1 (HIV-1) replication by CD8 T cells, the full identity of the factors involved has been elusive. Zhang et al. (p. 995; see the 27 September news story by Cohen) used mass spectrometry and protein chip technology to examine culture supernatants from CD8 cells isolated from patients who are long-term nonprogressors to AIDS. On the basis of amino acid sequencing and antibody recognition, they identified a set of defensins that only appeared upon T cell activation. Antibodies to these molecules blocked viral inhibition by CD8, and commercial preparations of β-defensins-1 and −2 inhibited different HIV-1 isolates. Biragyn et al. (p. 1025) find that one defensin, β-defensin-2, could activate dendritic cells (DCs) by binding Toll-like receptor-4 (TLR-4), a cell-surface pattern-recognition protein hitherto considered to be limited to recognizing pathogen-derived molecules, such as the Gram-negative bacterial endotoxin, lipopolysaccharide (LPS). In DCs, the interaction of β-defensin-2 with TLR-4 induced much the same cellular activation program as LPS, with the stimulation of costimulatory molecule and pro-inflammatory cytokine expression.
Protecting the Heart
Within the mitochondria of heart cells, ion channels control the flux of different ions and change the physiological status of the mitochondria, which in turn affect the relative health of the heart cell. Xu et al. (p. 1029) now describe the role of a calcium-activated potassium channel in the inner mitochondrial membrane of guinea-pig heart cells in protecting the cells from ischemia. A drug that opened the channel could protect the heart from infarction.
Taking Advantage of the Host
The tick-borne protozoan parasites Theileria spp. (related to the malaria parasites, Plasmodium spp.) cause a cancer-like disease in cattle that is of major economic importance in Africa and Asia. Heussler et al. (p. 1033) show how this pathogen subverts its host's signaling pathway for its own good. The schizont stage of the parasite infects the B and T cells of the immune system and immortalizes them into tumor-like cells capable of metastasis. Transformation seems to occur because the parasite activates the nuclear factor-κB (NF-κB) pathway and prevents apoptosis. This activation cannot be blocked by treatment with drugs that affect steps in the NF-κB signaling pathway upstream of multisubunit IκB kinase (IKK). The authors show that the intracellular foci of Theileria are associated with recruitment and aggregation of the IKK signalsome complex. Such aggregation seems to be sufficient to activate the kinase and subsequently activate NF-κB.
Polycomb Group Methylation
The covalent modification of histones, proteins that help package genomic DNA into chromatin, plays a critical role in gene expression and the segregation of chromosomes. The Polycomb (Pc) group of proteins have been implicated as chromatin-modifying proteins, and Cao et al. (p. 1039) now show that one of the two known Pc complexes, ESC-E(Z), specifically methylates histone H3 on residue lysine 27. This modification recruits the second complex, Polycomb repressive complex 1, likely through Polycomb protein itself, and shuts down gene expression.
When two immiscible liquids are violently mixed, it is possible to obtain droplets of the minor component inside the major one, at least for short periods of time. The emulsion can be stabilized by the addition of colloidal particles, as they will aggregate at the droplet surfaces, thus lowering the surface free energy. Working with colloids with narrow particle distributions, Dinsmore et al. (p. 1006) show that the particles can be fused together to form a capsule with tailored pore sizes. The capsules can be designed to entrap drugs or cells that are then soluble in the minority component of the emulsion. Once caged, the structures are stable enough for storage or transfer to new solvent media.
In the developing vertebrate, cells spun out along the midline from the moving point of gastrulation will later become segmented into somites, which will go on to form muscle and other tissues. With the use of advanced imaging techniques, this process, which has long been thought to resemble a knife cutting through a loaf of bread, is now seen to be a much more complex choreography of cell movements and changes in gene expression. Kulesa and Fraser (p. 991) observed cell movements in the live chick embryo and found that as each somite cleaves from the younger segmental plate, cells move caudally and rostrally across the incipient somite border. Domains of gene expression thought to relate to somite determination do not specify fate of all individual cells, as cells move into and out of differing domains of expression.
Early transcription studies have suggested that the TATA-binding factor TBP is a general transcription factor that is required for transcription from RNA polymerase I (pol I), pol II, and pol III promoters. However, more recent studies showed that TBP and TBP-related factors display distinct roles in early embryonic gene transcription. Martianov et al. (p. 1036) now inactivate mouse TBP by homologous recombination and show that after the disappearance of maternal TBP, blastocysts undergo growth arrest and apoptosis. Mouse embryos that lack TBP die at the blastocyst stage, shortly after depletion of the maternal supply of TBP. RNA polymerase II transcription continues in the absence of TBP; however, RNA polymerase I transcription is severely impaired upon loss of TBP. These results reveal a TBP-independent RNA polymerase II transcriptional mechanism and suggest that although TBP may be essential for cell division and transcription initiation, it is not required for maintaining transcription in vivo.
Origin and Significance of Archean Quartzose Rocks at Akilia, Greenland
Fedo and Whitehouse (Reports, 24 May 2002, p. 1448) reinterpreted 3.8 billion year old quartzose rocks in Greenland—previously thought, based on carbon isotope signatures, to harbor evidence for Earth's oldest life—as metasomatized ultramafic igneous rocks, an origin that, they maintained, invalidates claims that the rocks record early biological activity. In separate comments, Mojzsis and Harrison and Friend et al. argue that rare earth element signatures and other geochemical patterns of these rocks are more similar to those of ancient sedimentary banded iron formations than to those of ultramafic igneous rocks, and that field relations suggest that Fedo and Whitehouse did not analyze a key unit sampled in earlier studies. Fedo and Whitehouse respond that these earlier studies, and the current comments, have relied heavily on a single sample that has not been adequately described in the literature, and that a more comprehensive evaluation of field and geochemical data from the Akilia locality supports the interpretation of these rocks as metasomatized ultramafics.
The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/298/5595/917a
Transformations that preserve single crystals of organic molecules usually must avoid large motions of the lattice that would shatter the crystal and have mainly been seen in systems where double bonds convert to single bonds or where guest molecules move through pores. Atwood et al. (p. 1000; see the Perspective by Steed) observed a host-guest transformation where the addition of the guest to a nonporous host led to large-scale cooperative motion of the atoms, and thus maintained a single crystal. Once the transformation was initiated, it proceeded to equilibrium, even when the host was only partially saturated with guest molecules.
Faster Growing Grains
When carbon steel is cooled from the melt, it forms a polycrystalline structure that undergoes several solid-state transformations. Using newly developed x-ray techniques and a synchrotron source, Offerman et al. (p. 1003; see the Perspective by Militzer) studied the transformations as they occur in individual crystal grains and found that current models do not capture the full physics of grain growth. Grain nucleation occurs much faster than previously thought. These observations may lead to enhanced methods for carbon steel forming.
Less Indirect Effect
The indirect aerosol effect—the modification of cloud coverage and properties by atmospheric aerosols—is the largest uncertainty in the calculation of the net solar energy budget. Lohmann and Lesins (p. 1012) propose a more precise estimate of the anthropogenic indirect aerosol effect by constraining predictions from complex climate models with observations of aerosol and cloud properties made by the POLDER satellite. By considering the effects of aerosols on both cloud albedo and cloud lifetime, their estimate of the indirect aerosol effect, 0.8 watts per square meter (W m−2), is much smaller than previous values derived from climate model simulations alone (1.4 W m−2).
Silent Earthquake in Japan
The Tokai region of central Japan, about 200 kilometers south of Tokyo, has been accumulating strain since its last large earthquake in 1854, and there are concerns that another large-magnitude event may occur in the eastern Tokai region in the near future. Ozawa et al. (p. 1009) used global positioning system data to study ground deformation and found evidence for aseismic slip (also called a silent earthquake) as great as 2 centimeters in the western Tokai region in 2001. The aseismic slip in the western Tokai region is altering the stress state and may increase the probability of an earthquake in the eastern Tokai region.
Moths Making Traveling Waves
Nonlinear processes should be capable of producing a variety of spatiotemporal patterns in animal population dynamics, such as traveling waves, but experimental examples have been rare. Bjørnstad et al. (p. 1020; see the Perspective by Ranta et al.) report their aerial survey data from the European Alps of the long-term defoliation dynamics (covering more than four decades) of the larch budmoth, a species that exhibits extreme local cyclicity in abundance. Statistical modeling of spatiotemporal data with spatially extended theoretical models reveal the presence of regular traveling waves of budmoth outbreaks every 8 to 9 years.
An anaerobic bacterium that reductively dechlorinates trichloroethane, a problematic groundwater contaminant, has been discovered by Sun et al. (p. 1023; see the Perspective by Gossett). These organisms are phylogenetically different but related to other dechlorinating anaerobes. A bioaugmentation study described in this work shows that these organisms could be used in the cleanup of trichloroethane-contaminated groundwater sediments.
Intertidal Hot Spots
The intertidal zone is believed to be a bellwether of climate change because organisms there are thought to live very close to their thermal-tolerance limits. Helmuth et al. (p. 1015) show that the common intertidal mussel species Mytilus californianus responds to patterns of high-temperature exposure across a latitudinal gradient along the west coast of the United States in a manner more complex than might be expected. They find that in most cases, thermal stress is likely to be most severe at northern locations and may lead to localized extinctions rather than a general shift of the species to higher latitudes as warming progresses.
The Nature of Nurturing
The evolution of complex organs such as eyes or placentas presents a dilemma because they demand a large number of individual adaptations to function property, so it can be difficult to envision how they could all fall into place. The live-bearing fish genus Poeciliopsis displays a continuum of placental development, which ranges from little or no provisioning (yolk feeding) to more extensive maternal provisioning, that allows evolution of this organ to be studied. Reznick et al. (p. 1018; see the news story by Morrell) show that there have been three independent origins of extensive maternal provisioning in the genus. The upper estimates for the evolution of extensive postfertilization provisioning range from 0.75 to 2.34 million years. Such relatively rapid evolution has been predicted in theoretical models.