Superconductors Under Pressure
Magnesium diboride (MgB2) exhibits superconductivity at 40 K, a temperature higher than that thought possible for metals to remain in the superconducting state. This finding has sparked efforts aimed at unraveling the mechanism for this unexpectedly high transition temperature Tc. Monteverde et al. (p. 75; see the Perspective by Campbell) report on the pressure and temperature dependence of the electrical resistance of MgB2. They see a parabolic decrease in Tc with pressure and thermally activated behavior at low temperatures that suggest similarities to the high-Tc cuprates.
Breath Figures Caught in Polymers
Nanoporous polymers can be made from diblock copolymers, which can join chains of otherwise immiscible polymers that are then forced to phase-separate at the nanoscale. They can also be physically templated with hard colloidal particles. Srinivasarao et al. (p. 79) show that an even simpler process can by used. They passed moist air across a thin film of polystyrene dissolved in a volatile solvent. The condensed water formed arrays, or “breath figures,” that evaporated to create opalescent films containing three-dimensional ordered arrays of holes. The hole size could be tuned by changing the airflow over the surface. This effect may also contribute to nanoporosity in more complex patterning schemes that also rely on evaporative drying in air.
Bending Light the "Wrong" Way
The “bending” of a pencil in a water glass reflects the different speeds of light in air and water, and this difference can be expressed in terms of refractive indices. For most materials, the components of the refractive index affecting the electrical permittivity and magnetic permeability of the light are both positive. Recent work has suggested that materials can be prepared in which both components are negative. Working in the microwave regime, Shelby et al. (p. 77; see the Perspective by Wiltshire) present scattering data as direct experimental verification of such “left-handed” materials.
The extratropical North Atlantic Ocean is a region in which sea surface temperature (SST) and atmospheric pressure patterns are clearly different before and after 1950. These differences have been reflected in trends in Northern Hemisphere land-surface temperatures, weather patterns in Europe and the Middle East, and the corresponding marine and terrestrial ecosystems. Hoerling et al. (p. 90) present evidence that North Atlantic climate change since 1950 and the precipitation and heating changes that have accompanied it are linked to increases in SST in the Pacific and Indian Oceans.
Progress Toward an AIDS Vaccine
Many AIDS vaccines that have attempted to neutralize the virus with antibodies have been unsuccessful. An alternative approach is to try to produce a vaccine that confers broad cellular immunity instead. Amara et al. (p. 69) show that vaccination of rhesus macaques with a DNA vaccine based on multiple HIV proteins, followed by a boost with a recombinant, attenuated vaccinia virus, protected the animals from a pathogenic immunodeficiency virus. The viral challenge was done 7 months after the boost, when the immune response had declined to baseline memory levels and was mucosal in nature, thus mimicking the predominant mode of natural infection. The vaccine did not protect animals from infection but from development of AIDS: Viral RNA was reduced to 1000 copies or less per milliliter of plasma, there was no loss of CD4 cells, and the lymph node architecture was preserved.
Out of India
After its separation from Madagascar during the Cretaceous, the Indian landmass was an island drifting northeastward across the Indian Ocean until its impact with Asia during the early Tertiary. Zoogeographic evidence has suggested that modern land vertebrates might be derived from ancestral isolated Indian groups that dispersed from India after its merger with Asia. Bossuyt and Milinkovitch (p. 93) performed a molecular analysis of ranid frog phylogeny that supports this hypothesis. Several frog lineages currently endemic to southern India diverged before the collision event, and at least three other lineages, including those in Africa, Europe, and the Americas, dispersed out of India.
Panda Park Perils
The Wolong Nature Reserve in southwestern China was set aside for the protection of the giant panda a quarter of a century ago, and its 200,000 hectares host up to one-tenth of surviving wild pandas. Liu et al. (p. 98) used satellite imagery to document the changes in vegetation cover before and after establishment and compared the resulting patterns with those in the surrounded unprotected areas. They see a disturbing trend of increased human activity that has degraded and fragmented the pandas' habitat in the reserve since protected status was given.
Seeds of Change
Much can be learned about environmental impacts by looking at changes in plant seedings. A small number of “keystone” species can be critical to the functioning of their native ecosystems. The kangaroo rat of the Chihuahuan desert in the United States is a case in point. Ernest and Brown (p. 101; see the Perspective by Bond) used a 20-year data set recording the response of granivorous desert rodents to the experimental removal of kangaroo rats. They document a major and persistent decrease of energy flowing through the rodent community. Smaller rodents could not take up the slack, and previously rare large-seeded plants began to replace smaller seeded plants. Only when a new rodent species colonized the plots, after 18 years, did energy use approach that on control plots. Changes in plant growth parameters in response to atmospheric CO2 concentration have been intensely studied in recent years to look for the potential effects of anthropogenic global change. In a 3-year experiment in a loblolly pine forest, LaDeau and Clark (p. 95; see the news story by Tangley) found a large, rapid, and sustained increase in seed production in response to increased CO2, as well as early onset of reproductive maturity. The composition of forests could undergo large changes as a result of differential responses of species to rising CO2.
Fruit Flies at a Ripe Old Age
Life-span is partly controlled by the genetic makeup of an organism. In the nematode Caenorhabditis elegans, mutations in the daf pathway, which normally regulates an inactive hibernation-like life phase, can prolong life-span dramatically. The daf pathway is homologous to the insulin pathway of higher organisms, a tantalizing link to the ability of caloric restriction to increase rodent life-span. By mutating the genes in the insulin-like pathway of fruit flies, Tatar et al. (p. 107) and Clancy et al. (p. 104) generalize the participation of this pathway in life-span control. Mutation of the InR gene (homologous to the mammalian insulin receptor and the daf-2 gene) increased by 85%, and mutation of chico, an insulin receptor substrate, prolonged fly life-span by 52%. The insulin-like signaling pathway and its control of organismal metabolic activity is thus likely to be a general regulator of the rate of aging in a broad range of species (see the news story by Strauss and Fabrizio et al. in the 5 April Science Express).
Cracking the Chromatin Code
Covalent modifications on the amino-terminal tails of the histone proteins are thought to be involved in the specification of higher order chromatin structures that are intimately involved in processes such as gene transcription, DNA replication, and repair. For example, heterochromatin plays an important role in silencing gene expression. The protein Clr4 has been suggested to be involved in heterochromatin formation and can methylate the lysine-9 residue of the histone H3 tail. Nakayama et al. (p. 110; see the Perspective by Berger) now show that Clr4-directed methylation of histone H3 corresponds with heterochromatin assembly in vivo, which is consistent with the role of Clr4 in epigenetic silencing. H3 methylation results in localization of Swi6, a homolog of the Drosophila heterochromatin protein 1. Furthermore, Clr3, a histone H3-specific deactylase, is also required for H3 methylation, Swi6 localization, and heterochromatin formation, supporting the hypothesis that a histone modification “code” exists for the establishment of chromatin structures.
Clones in the Community
Staphylococcus aureus is a common pathogen of humans and a major public health concern, yet most of the people who carry this bacterium display no disease symptoms. Day et al. (p. 114; see the Perspective by Lipsitch) have discovered that specific ancestral genotypes of S. aureus that circulate among humans within a defined geographical area are disproportionately common causes of severe disease. Even when there is no disease outbreak, the abundance of hypervirulent clones suggests that factors that promote the spread of a clone of S. aureus also promote its virulence. The loss of virulence in less abundant isolates appears to be the outcome of recombination sometime in their ancestry.
Attacking a Problem at the Source
The deliberate addition of nitrogen into the environment, such as through fertilizers, can especially impact terrestrial ecosystems in which primary production is nitrogen-limited. Understanding how nitrogen is cycled in these environments, an essential part of evaluating the impacts of anthropogenic nitrogen release, has been hindered by the tremendous complexity of the ecosystems. Peterson et al. (p. 86) attack a part of this problem by applying stable nitrogen-isotope tracer techniques to streams from all across the United States. They find that headwater streams play a particularly important role in regulating water chemistry.
HIV-1 RNA Editing, Hypermutation, and Error-Prone Reverse Transcription
Examining human immunodeficiency virus-type 1 (HIV-1) transcripts in virus-producing cells, Bourara et al. (Reports, 1 Sept. 2000, p. 1564) observed cytosine-to-uracil (C-to-U) and guanine-to-adenine (G-to-A) changes that they attributed to post-transcriptional RNA editing. In a comment, Berkhout et al., focusing in particular on the G-to-A event observed by Bourara et al. at site 181, argue that “known editing mechanisms … cannot easily explain” some of the observed changes and propose “an alternative mechanistic model” based on error-prone HIV-1 reverse transcription to account for those changes. Araya and Litvak, in their response, suggest that the Berkhout et al. model is implausible because it requires multiple events with a low cumulative probability. They further argue that the fact that changes were observed only in “transcripts generated by transcription-competent provirus,” and not in the proviral sequence itself, strongly favors post-transcriptional RNA editing as the cause. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/292/5514/7a