Extending the Art of Concealment
Stealth technology relies on impedance matching that absorbs impinging radiation, thereby denying the observer a back-reflected signal. However, an observer on the far side of the “hidden” object can still see a shadow. Using properties of metamaterials, artificial materials with tunable dielectric and magnetic properties, Pendry et al. (p. 1780, published online 25 May) and Leonhardt (p. 1777, published online 25 May) present theoretical studies proposing that such materials could be used to steer electromagnetic radiation around an object, subsequently allowing the radiation to proceed as if it had not been scattered from the object at all (see the 26 May news story by Cho). This sophisticated version of stealth would be both reflectionless and shadowless.
Microquasars are binary star systems with twin radio-emitting jets that resemble those of quasars, albeit on smaller scales. Radiation from these jets arises from particles moving at relativistic speeds in high magnetic fields, but little is known about the jets' composition or how they are formed. Albert et al. (p. 1771, published online 18 May; see the Perspective by Mirabel) have used the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescope to monitor monthly variations caused by very high energy gamma rays (>100 gigaelectron volts) from a microquasar. A comparison of the phases of the gamma-ray variability with those of radio waves and x-rays shows that the gamma-ray emission peak does not coincide with the time when the two stars are closest to one another, which suggests that there is a strong orbital modulation of the emission processes. Further analysis of the emission favors an underlying leptonic over a hadronic process.
Phosphate ultimately limits biological productivity in the ocean, and much of what is understood about past productivity depends on knowing how phosphorus (P) was distributed in the sea. Unfortunately, reconstructions of ocean phosphate contents have always relied on indirect proxies that can be affected by other factors such as temperature, carbonate ion concentration, and incomplete preservation, and so their utility often is limited by their inherent uncertainties. Montagna et al. (p. 1788; see the Perspective by Boyle) present evidence that the cold-water coral Desmophyllum dianthus incorporates P into its skeleton in amounts proportional to the concentration of P in ambient seawater. Such a direct proxy would make robust reconstructions of long-term variations in ocean P content possible and allow changes in the residence times and the sources of deep-water masses to be detected.
Unmasking Spiral Arms
The Milky Way is a spiral galaxy, but its precise shape and even the number and extent of spiral arms has been difficult to discern. For example, the brightness of the 21-centimer hyperfine transition of neutral atomic hydrogen (HI) falls off rapidly away from the Galaxy's center and fluctuates greatly over the sky, so it has been hard to pick out spiral arms against this background. Levine et al. (p. 1773, published online 1 June) applied a technique similar to “unsharp” masking to previous survey maps that essentially removes a coarse template of the large-scale emission and reveals finer details. The maps show spiral structures out to distances of 25 kiloparsecs from the Galactic center that fit a logarithmic spiral form.
Art or other forms of symbolic expression are found in many early human sites that date to about 50,000 years ago, but earlier evidence of such modern cultural behavior has been sparse. Vanhaeren et al. (p. 1785; see the news story by Balter) now describe a few gastropod shells apparently modified for jewelry that were collected previously from two inland sites in western Asia and North Africa. Both sites date to older than 100,000 years ago, about 25,000 years earlier than similar but more abundant drilled shells found in South Africa. Examination shows that these shells were drilled by humans, presumably for threading and wear.
Phasing Out Fat
The protein known as TRB3 is a pseudokinase (a kinase-like protein that lacks kinase activity) synthesized in fasting animals. TRB3 modulates insulin signaling and is related to a Drosophila protein that coordinates mitosis and morphogenesis during development. Qi et al. (p. 1763; see the Perspective by Neels and Olefsky) find that overexpression of TRB3 in mice confers resistance to diet-induced obesity. This effect appears to be the result of decreased activity of acetyl-coenzyme A carboxylase (ACC) and consequent decreased synthesis of fatty acids. TRB3 directly interacts with ACC to promote its degradation via the E3 ubiquitin ligase constitutive photomorphogenic protein 1. Understanding the regulation of lipid metabolism may promote therapeutic strategies for the control of obesity.
Limits on Cultural Variation
Behavior does not always agree with claimed intent—hence, “Do as I say, not as I do.” In order to assess variations in the assessment of fairness and punishment across the breadth of humanity, Henrich et al. (p. 1767; see the news story by Bhattacharjee) have complemented existing evidence from questionnaire-based surveys by adapting three economic games—the ultimatum game, third party punishment, and the dictator game—and by sampling 15 small-scale societies distinctly dissimilar to the commonly used pool of students in industrialized countries. Individuals across populations become more willing to administer punishment (even when they must “pay to punish”) as inequality increases, and this willingness co-varies with altruistic behavior.
Getting a Headful
Double-stranded DNA viruses pump their genome into preassembled procapsids until the particles are filled to capacity with internal pressures higher than corked champagne. How is this internal “headful” density signaled to the outside packaging machinery? Insight comes from a 17 angstrom resolution asymmetric reconstruction of the infectious P22 virion by Lander et al. (p. 1791, published online 18 May). DNA is tightly spooled around the P22 portal, which is in a different conformation from the isolated portal. The authors suggest that DNA tightens around the portal as packaging density increases. When the headful density is reached, a conformational switch signals the termination of packaging.
Flies and jetlag
Circadian clocks can be reset to a new phase by a brief exposure to light, but the molecular details of this resetting are not clear. In Drosophila, a light-sensitive protein cryptochrome undergoes a conformational change in response to light and binds to a clock component, the protein TIMELESS (TIM). This interaction then triggers TIM degradation and effectively resetting the clock. By screening mutant flies that show reduced sensitivity to this light-induced resetting, Koh et al. (p. 1809) identify a gene, termed jetlag, that is necessary for degradation of TIM after the light pulse. JETLAG exists in a complex with TIM and increases its ubiquitination, a tag that marks the protein for degradation. Thus, JETLAG is an F-box protein that targets TIM for ubiquitination and consequent rapid degradation in response to light.
TopoIIβ Gets to Work
DNA topoisomerases regulate conformational changes in DNA topology by catalyzing DNA strand breakage and rejoining. Topoisomerase IIβ (TopoIIβ) alters DNA conformation near gene promoters, and associates with sequence-specific transcription factors and chromatin modifying and remodeling factors. Ju et al. (p. 1798; see the Perspective by Haince et al.) now show that DNA TopoIIβ generates double-strand breaks at transcriptional promoters when these genes are activated in a signal-dependent manner. Subsequently, DNA repair enzymes are activated, and there is a resultant exchange in histone composition and local chromatin structure.
Moving to the Seaside
Humans have settled by coasts since prehistoric times. Recent impacts of such settlement have been far better documented than historical and prehistorical effects. Lotze et al. (p. 1806) quantified detailed historical baselines for 12 estuarine and coastal ecosystems in North America, Europe, and Australia since the onset of human occupation. Patterns of change were surprisingly similar at all sites. Overexploitation and habitat loss alone explained ∼95% of all species declines, extinctions, and consequent shifts in diversity and ecosystem functioning. Significant recovery in upper trophic levels was seen where those impacts were restrained, indicating that well-targeted management can reverse destructive trends.
In a fractal structure, units with a given shape assemble into ever larger arrays of that same shape. Among the earliest fractal forms described mathematically was the Sierpinski gasket, which is composed of regular polygons with hollow centers. Newkome et al. (p. 1782, published online 11 May) have prepared a molecular version of the hexagonal Sierpinski gasket by combining two types of terpyridyl ligand building blocks in solution with Ru and then Fe ions. The ligands, with alternating twofold and threefold symmetry, self-assemble with metal ions at the junctions to yield a hexagonal fractal structure ∼10 nanometers in diameter, which the authors visualized by transmission electron and scanning tunneling microscopies.
Distinct and Diverse
Natural RNA virus communities collected from the Canadian Pacific coast are diverse, but a few types predominate. In two localities, Culley et al. (p. 1795) found viruses that have not been reported from aquatic environments before. In one community, two abundant genotypes of the order Picornavirales were found that did not resemble any of the established families of RNA viruses, but probably infect protists. In the other community, a different sequence dominated that probably belongs to a member of an unknown group of viruses related to the Tombusviridae, which infect plants. Other sequences were diverse, but rare, with some evidence for cripaviruses that probably parasitize crustaceans. The two communities sampled did not resemble each other closely, although they shared the same characteristic predominance of a few genotypes, suggesting “boom or bust” ecology.
Dok-7 and Synapse Construction
The synapse between motor neurons and skeletal muscles forms during development when the neuron contacts the muscle and secretes a protein called agrin. Agrin, in turn, causes phosphorylation of a muscle-specific kinase MuSK, ultimately resulting in clustering of acetylcholine receptors on the muscle membrane at the site of nerve contact. Okada et al. (p. 1802) now describe Dok-7, a protein that can bypass the requirement for agrin and that is required for MuSK activation, binding to and activating MuSK through a phosphotyrosine binding domain. Mice engineered to lack Dok-7 do not form clusters of neurotransmitter receptors, and a mutant form of Dok-7 found in patients with congenital myasthenic syndrome prevents normal receptor clustering and junction formation. Thus, Dok-7 is a MuSK binding protein required for the formation of the postsynaptic side of the neuromuscular junction.