Nanotubes Play the Field
Theoretical work has predicted that the band structure of carbon nanotubes should display a dependence on the magnetic flux threading through the tube, a manifestation of the Aharonov-Bohm effect. Two groups report on the magnetic field dependence of carbon nanotubes that verify these predictions. Coskun et al. (p. 1132) present transport measurements on short multiwall carbon nanotubes, and provide evidence for the interconversion from metallic to semiconducting behaviour. Working with single-wall carbon nanotubes, Zaric et al. (p. 1129) obtained magneto-optical spectra that show a shift and splitting of the optical absorption features with increasing magnetic fields.
The Ancestors Were Blowing in the Wind
The close affinities of the floras of the Southern Hemisphere islands and continents have long been assumed to be the result of long-distance dispersal of spores and seeds by wind. Muñoz et al. (p. 1144) use satellite scatterometer data, which provide a detailed, continuous record of wind speed and direction, to model wind connectivity between southern landmasses. The distribution patterns of mosses, liverworts, lichens, and ferns indeed show significant correlations with wind connections.
A Split LIP
Large igneous provinces (LIPs) were formed by voluminous magmatic events caused by mantle upwelling during a relatively short period of time. Throughout the past 550 million years of Earth's history, several LIPs formed before or during supercontinental breakups, which suggests that these magma upwellings may create weaknesses in the continents that lead to their sundering. Hanson et al. (p. 1126, published online 22 April 2004) have found a new LIP, part of it in the Kalahari craton of Africa and part of it in the Laurentian craton of North America. This LIP, which formed about 1108 million years ago during the aggregation of the Rodinia supercontinent, not only provides important constraints on the location and motion of continental landmasses but also suggests an intriguing role for LIPs in sticking continents together.
Roundup Ready by Another Route
Transgenic technology has allowed development of crop plants insensitive to the herbicide glyphosate (Roundup). The widely adopted glyphosate-tolerant crops support improved farming technologies with lessened environmental impact. Castle et al. (p. 1151; see the news story by Stokstad) now develop a different sort of glyphosate-tolerant plant, one that detoxifies glyphosate as it is taken up. The detoxifying enzyme was derived by directed evolution in vitro to increase the effectiveness of a weakly detoxifying bacterial enzyme.
Probing Protonated Water
The structure of protonated water underlies an understanding of aqueous acid-base chemistry and the anomalously high rate of proton diffusion in water. Two infrared spectroscopic studies, in which absorption of vibrational energy into OH stretches was detected by a loss of clusters from the beam source by subsequent fragmentation, attempt to unravel the structure of small water clusters containing a single proton (see the Perspective by Zwier). Miyazaki et al. (p. 1134, published online 29 April 2004) conclude from their analysis that small clusters (about 10 or fewer water molecules) form chains; that from about 10 to 21 molecules, two-dimensional (2D) networks form; and above that number, nanometer-scale 3D cages are created. Shin et al. (p. 1137, published online 29 April 2004), looking at similarly sized clusters from beam sources at different temperatures, focused on the structure of the 21-molecule clusters associated with a particular stable dodecahedral structure. A doublet feature in the free OH stretch region observed for clusters with 10 to 20 water molecules collapsed to a singlet for clusters with 21 and 22 water molecules. Comparison with calculation explains this difference in terms of a surface-bound H3O+ group, but calculations cannot account for the simplicity of the 22-molecule cluster's spectrum or the absence of lines at 2500 wave numbers for the H3O+ species.
Meltwater Mud Tracks
The Last Glacial Maximum (∼10,000 years ago) likely ended with a massive pulse of meltwater that would have caused a rapid sea level rise of 10 to 15 meters, but there is insufficient evidence to establish either where (or even if) this event occurred. Clark et al. (p. 1141) present observations from the Irish Sea coast that confirm the existence of this upward surge in sea level, evaluate its possible source, and discuss how the effects of this event on North Atlantic deepwater formation may have been transmitted to the Southern Hemisphere. They identify an erosional channel more than 10 meters deep, filled with marine mud of essentially the same age, and conclude that this feature resulted from the rapid infilling of the channel (in less than 500 years) in response to a rapid rise of sea level.
Cell Death and Autoimmune Disease
In mammals, cells that undergo apoptosis are marked for degradation by macrophages with phosphatidylserine. Hanayama et al. (p. 1147; see the Perspective by Züllig and Hengartner) report the phenotype of mice lacking milk fat globule EGF factor-8 (MFG-E8), a protein secreted by macrophages that binds to apoptotic cells and appears to promote their engulfment. The knockout mice showed engulfment defects; in particular, uptake of apoptotic lymphocytes by macrophages in germinal centers of the immune system was compromised. The animals also had enlarged spleens and signs of autoimmune disease.
PARK'd in Mitochondria
Parkinson's disease (PD) is a progressive neurodegenerative disorder that most often affects the elderly. Although the common forms of PD are likely to arise from the interaction of multiple genes and environmental factors, important insights into pathogenesis have emerged from genetic analysis of rare families with inherited forms of PD. Valente et al. (p. 1158, published online 15 April 2004; see the Perspective by Greenamyre and Hastings) have now identified the culprit gene on chromosome 1p36 (the PARK6 locus) that causes early-onset PD in a small group of European families. This gene, PINK1 (for PTEN-induced putative kinase 1), encodes a protein with a serine-threonine kinase domain that is disrupted by the PD mutations. Intriguingly, PINK1 is localized in mitochondria, an organelle that has been linked previously to PD pathogenesis by a variety of biochemical experiments.
Genetic Variation Between Breeds of Dog
There are more than 400 phenotypically distinct breeds of domestic dogs. Parker et al. (p. 1160; see the news story by Pennisi) have used molecular markers to define an independent classification of purebred dogs based on patterns of genetic variation. This classification supports a subset of traditional groupings but also reveals previously unrecognized connections among breeds.
Gambling, Regret, and the Human Brain
How do we feel about decisions when we are unsure of the possible consequences of our actions? Camille et al. (p. 1167) compared the emotional reactions to decision outcomes when a gambling task was performed by normal subjects or by patients with damaged orbitofrontal cortices. Healthy people showed an overall preference for winning more than less, as well as both disappointment and regret upon losing. Patients with lesions in the orbitofrontal cortex, however, still showed a preference for winning, but diminished disappointment, and no regret.
TrIPing from Bacterium to Plant Cell
Many species of bacteria exchange DNA and inject various virulence factors using the conjugation apparatus known as the Type IV secretion system. Similarly, Agrobacterium tumefaciens causes tumors in plants by inoculating them with transfer DNA (T-DNA), via a Type IV secretion system, and thus has become an important biotechnological tool. Cascales and Christie (p. 1170; see the Perspective by Lybarger and Sandkvist) examined the transport of T-DNA by a T-DNA immunoprecipitation assay (TrIP). The results define the translocation pathway for a DNA substrate through the conjugation machinery, specifying contributions of each subunit of the secretory apparatus to substrate passage.
Phosphatase Culprits in Cancer
The identification of genes that are mutated in cancer cells can provide important clues to tumor pathogenesis. Working with a large collection of human colorectal tumors and matched normal tissue, Wang et al. (p. 1164) have systematically sequenced the entire family of genes coding for an important group of cellular signaling enzymes called protein tyrosine phosphatases. Sequence alterations in phosphatase genes were found in about 25% of the tumors, and biochemical analysis confirmed that at least a subset of these alterations had functional effects on phosphatase activity. In addition to identifying potential tumor suppressor genes, this comprehensive sequencing approach may have future applications in the design of personalized cancer therapy.
Similar But Not the Same
Resistin and adiponectin are two serum proteins secreted by adipocytes. Adiponectin exists in low- and high-molecular-weight (LMW and HMW) forms, and the relative amount of the HMW species correlates with systemic responsiveness to insulin and with thiazolidinedione-induced improvement in animal models of obesity-associated insulin resistance. Patel et al. (p. 1154) describe the crystal structure of resistin, which bears no sequence similarity to adiponectin, yet also exists in a small form consisting of three monomers intertwined via a coiled coil and in a larger hexamer formed by the tail-to-tail association of two trimers. A further intriguing similarity is that the smaller, trimeric form of resistin can act as a potent antagonist of insulin effects on rat liver.