Periodic Table in the Sky
Despite the key role that aerosols play in ozone destruction and global cooling, most measurements have been performed on retrieved samples, which can become contaminated or can undergo aggregation. Murphy et al. (p. 1664) report in situ aerosol data from measurements aboard research aircraft that span the troposphere and stratosphere at altitudes between 5 and 19 kilometers. More than 28,000 individual particles were characterized in terms of their size and chemical composition. At least 45 different elements are present in these particles, including meteoritic material and mercury in stratospheric aerosol and organic material in tropospheric aerosol. Such measurements may allow the origins of particular types of aerosols to be traced.
Human Influence on Climate
Efforts have been made to identify the human influence on climate during the 20th century, but controversy persists whether natural variability and anthropogenic forcing have been attributed adequately in these studies. Wigley et al. (p. 1676) analyze the auto- and cross-correlation structure of hemispheric mean temperature from observations and from several different climate model simulations. They show that significant external forcing exists that cannot be attributed purely to solar forcing but rather to a combination of solar and anthropogenic forcing. The study provides strong evidence for significant anthropogenic forcing on global climate.
Metals make fine mirrors in many wavelength ranges, but for certain applications low-dielectric materials may be preferred. Fink et al. (p. 1679) show theoretically that stacking alternating layers of materials with different dielectric constants can produce materials that are reflectors over a particular range of wavelengths, and they demonstrate high reflectivity for wavelengths from 10 to 15 micrometers with a material made from alternating layers of polystyrene and tellurium. This approach opens up the possibility of tuning the wavelength range of coatings for transparency and reflectivity.
Quantum Interference of Atomic Tunnel Arrays
Atoms can be cooled and trapped to a fraction of a degree above absolute zero to form an unusual state of matter, a Bose-Einstein condensate (BEC). Manipulation of the BEC can allow quantum effects to be observed macroscopically. Anderson and Kasevich (p. 1686; see the Perspective by Burnett) allowed a rubidium BEC to fall gravitationally through a periodic array of laser traps (standing waves). The trapped atoms tunnel coherently through all of these potential wells created by the laser standing waves at the same time. The interference of the tunneling atoms manifests itself as a train of phase-coherent atomic pulses. This behavior is effectively that of a mode-locked atom laser.
A Different Twist
Polyacetylene, the simplest linear conjugated polymer, is of particular interest for electrical applications because of its high conductivity upon doping. However, processing of these polymers has proved difficult and has limited its applications. Akagi et al. (p. 1683) show that by performing an existing synthetic protocol in a chiral nematic liquid crystal environment, polyacetylene films form that contain helical fibrils consisting of bundles of acetylene chains. Iodine doping led to high conductivities. These helical structures may find application as electrically conducting molecular wires and may also have interesting magnetic and optical properties.
The origination of angiosperms (flowering plants) impacted the evolution of insects and other animals and greatly affected Earth's climate (particularly the CO2 budget). Angiosperms have typically been thought to have arisen in the Cretaceous, but Sun et al. (p. 1692; see the cover and the Perspective by Crepet) now describe an angiosperm from probable Jurassic rocks in China that may extend their origins back several tens of millions of years.
HIV-1 Reverse Transcriptase in Action
The reverse transcriptase (RT) enzyme of HIV-1 (human immunodeficiency virus-1) copies the viral genome into DNA that can then be integrated into the host cell's genome. Although inhibitors of the RT have been a prime target for antiviral therapy, mutations confer resistance to the RT inhibitors currently in use. Huang et al. (p. 1669; see the news story by Balter) present the crystal structure of HIV-1 RT in complex with both the double-stranded DNA template-primer and the deoxynucleoside triphosphate substrate. The enzyme does not specifically register to the double-stranded DNA, which often leads to poor crystallization; this problem was overcome by covalently linking the DNA to RT. The active site structure suggests how mutations can reduce sensitivity to inhibitors and cause cross-resistance.
The Fall and Rise of the Prairie Chicken
Understanding the consequences of small population size is essential for conserving biodiversity. Westemeier et al. (p. 1695; see the Perspective by Soulé and Mills) chart the decline of a once common grassland bird of the midwestern United States—the greater prairie chicken—by demonstrating the links between small population size, isolation, fitness loss, and decreased genetic variation. They also report the probable saving of the population by supplementing their numbers from large, genetically diverse populations.
Closing the Cycle
Progression through the cell cycle is governed in part by the anaphase promoting complex, which causes the controlled degradation of kinases and phosphatases that are involved in orchestrating DNA replication and cell division. Zachariae et al. (p. 1721) describe a protein, Hct1, that controls the activity of the anaphase promoting complex and restricts its activity to the correct phase of the cell cycle.
Calcium and Nerve Growth
Calcium has an important regulatory role in control of growth and navigation at the tip of developing or regenerating nerve cells. However, the relative contributions of Ca2+ that enter through channels in the plasma membrane versus Ca2+ released from internal stores has not been established. Takei et al. (p. 1705) found that type 1 inositol 1,4,5-trisphosphate receptor (IP3R), a channel that releases Ca2+ from intracellular stores, was abundantly expressed in neurons from chick dorsal root ganglion. Inhibition of Ca2+ release through IP3Rs caused growth arrest and retraction of neurites. Proper neurite extension appears to require release of Ca2+ from intracellular stores through the IP3R, probably in coordination with influx of extracellular Ca2+.
Responding to Blue Light
The responses of plants to red and blue light involve a cascade of receptors and other proteins that translate the light signal into a growth response. Now, Christie et al. (p. 1698) elucidate further the transduction of the blue light signal into the physiological response. The protein encoded by the NPH1 locus in Arabidopsis binds a flavin chromophore and responds to blue light with increased autophosphorylation, thus clarifying the identity of the chromophore and the initial steps in response to blue light.
Granting Immune Privilege
Some sites in the body, such as the anterior chamber of the eye, are “immune privileged”–immune responses do not take place. These tissues were thought to be protected by their constitutive expression of Fas ligand (FasL), which induced apoptosis in meandering T cells. However, in many experimental systems where FasL is inserted as a protective element, inflammation is incited. Chen et al. (p. 1714) found that FasL can activate granulocytes to kill other cells. Fluid from the eye inhibits this killing—and the active agent appears to be transforming growth factor-β (TGF-β). TGF-β could prevent the rejection of tumors that was instigated by FasL in vivo. Thus, immune-privileged sites may require FasL to kill the T cells and TGF-β to thwart the inflammation.
Phagocytosis Two Ways
Cells such as macrophages ingest foreign particles through phagocytosis. Receptor-particle interactions lead to progressive wrapping of particles by the cell membrane, which are then internalized for subsequent degradation. The process requires large morphological changes in the cell to accommodate the particle. Caron and Hall (p. 1717) examined the role of a family of proteins that control the actin cytoskeleton. Cells can use two different mechanisms to promote actin rearrangment during phagocytosis, which may explain the varied cellular responses to phagocytosis mediated by different cell surface receptors.
Is cultural evolution responsible for the markedly low levels of mitochondrial diversity in matrilineal species of whales? As Whitehead reports (p. 1708; see the news story by Vogel), females of these species, which include pilot, sperm, and killer whales, spend their entire lives with close female relatives under circumstances conducive to cultural development, such as a large body size, low travel costs, dispersed and patchy food sources, and the efficient transmission of sound. How would this cultural inheritance—defined as information learned from the same species that causes variation in behavior—affect mitochondrial diversity? Whitehead proposes that the selection of particular cultural traits causes an incidental reduction in diversity at neutral linked genetic loci.
More Magnetism in Electronics
Magnetic storage media have long held a place in electronics, but most electronic circuitry on chips relies on charging effects to turn devices on and off. Prinz (p. 1660) reviews recent progress in creating devices that rely on magnetoresistive effects, which use magnetism to switch electrical signals. Such effects, already in use in newer read-write heads for magnetic memories, may be used in nonvolatile circuitry that would “remember” their states even when the power goes off.
Lower Crustal Flow After Landers
The magnitude 7.3, Landers earthquake was mainly a strike-slip earthquake along four poorly defined fault segments in the Mojave Desert of southern California. After the main event, postseismic slip in the horizontal and vertical direction was measured by space geodesy. Local deformation within one fault segment has been explained by a change of pore-fluid pressure as the stressed region relaxes, but the broader scale deformation remains difficult to explain. Deng et al. (p. 1689) developed a three-dimensional viscoelastic flow model that can explain the horizontal and vertical deformation better than an afterslip model. Their viscoelastic flow model suggests a weak, low-viscosity layer in the lower crust beneath the fault trace as the main mechanism for the postseismic deformation. Identifying the source of the deformation will improve our understanding of earthquake hazards and related deformation in the Basin and Range region that may be caused by flow in the lower crust.
Kinase Cascade in Mitosis
Control of the cell cycle depends on an elaborate series of protein phosphorylation and dephosphorylation events. Qian et al. (p. 1701) add to the complexity by identifying and cloning a protein kinase that phosphorylates and activates the Xenopus polo-like kinase Plx1. Plx1 functions to regulate the protein phosphatase Cdc25, which dephosphorylates and activates the cyclin-dependent kinase Cdc2, which in turn promotes progression of cells into mitosis. Polo-like kinases also appear to function in control of spindle assembly and regulation of the proteolytic machinery that controls exit from mitosis. The newly identified enzyme, polo-like kinase kinase 1, is itself activated by phosphorylation, indicating that it is part of a cascade of kinases that control mitotic events.
Adding Hox Clusters During Evolution
The hox genes, which are so critical for pattern specification during development, are aligned in clusters on the chromosomes. Although lower chordates have only one cluster, tetrapods have four, and, as now shown by Amores et al., (p. 1711) zebrafish have seven. The additional clusters likely arose early in the evolution of fishes, perhaps by whole genome duplication, and may have provided the tools to support the diversification of fishes.
Biomass Decline in Amazonian Forest Fragments
W. F. Laurance et al. (Reports, 7 Nov. 1997, p. 1117) found that fragments of the rain forest in the Amazon experienced a large loss of above-ground tree biomass that was not offset by recruitment of new trees.
D. Cowles comments that trees of less than 10 centimeters in diameter were excluded from the analysis, which may have introduced “a bias toward measuring losses while undersampling the gains that partly offset the losses.” J. B. Kauffman et al. also state that the data in the report “seem inadequate to ascertain losses of total above-ground biomass and carbon pools.” Data from their tropical forest plots in Brazil show “a statistically significant negative correlation between the understory:overstory biomass ratio and the overstory tree biomass.”
In response, Laurance et al. provide “estimated above-ground dry biomass for small trees and lianas (woody vines) in the central Amazon.” They state that the effects described in the comments “are likely to be of limited importance” and conclude that “biomass collapse in forest fragments is a real—and worrisome—phenomenon.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/282/5394/1611a