A (Not Quite) Perfectly Still Landscape
Mars Pathfinder landed and surveyed the boulder-strewn area of Ares Vallis. On the basis of the morphology of the rocks and surface, it was suggested that the current landscape was created about 2 billion years ago through catastrophic flooding and dust storms and that little further processing has occurred. Horz et al. (p. 2105) now identify an impact-produced crater of about 25 centimeters in diameter on one particular rock (Stimpy) and suggest that other fractures and angular shapes observed on other rocks are related to small-scale impacts. The authors then calculated a realistic range of sizes and velocities for iron and chondritic projectiles that could enter the martian atmosphere and create the small craters and fractures. Thus, it seems that dynamic processes may still be altering the cartoon-character shaped rocks of Ares Vallis, and these small-scale features can provide additional understanding about the present and past martian environment.
Charge Density Waves and Defects
Many metallic systems exhibit charge-density waves (CDW), which are modulations of the valence charge density that lower both the energy and the symmetry of the system. Weitering et al. (p. 2107) have studied the effects of defects on the two-dimensional CDW formed by an overlayer of tin on the (111) face of germanium, which forms domains of threefold symmetry. Point defects (substitutions of germanium for tin in the overlayer) were found to “pin” a particular orientation of the CDW. Moreover, the distribution of defects was nonstatistical, which suggests that defects moved to accommodate the CDW at its onset temperature. Such hopping occurred at temperatures well below those for the atoms to be mobile, which indicates that the formation of CDW lowers the barrier for such motion.
Micromanaging Polyethylene Synthesis
Catalysts made from early transition metals such as zirconium and titanium have long been used to polymerize ethylene and other small alkenes. Kageyama et al. (p. 2113; see the Perspective by Lehmus and Rieger) show that when a titanium compound (titanocene) is incorporated within the nanometer-scale channels of mesoporous silica fibers, polyethylene of very high molecular weight (>6 million) is formed when a methylalumoxane cocatalyst is present. The polyethylene, which is extruded from the channels as filaments with diameters of 30 to 50 nanometers, forms highly ordered, extended-chain crystals.
The Gravity of the Situation
Long Valley Caldera in California is a large, nearly circular collapsed roof of a magma reservoir that formed during a catastrophic eruption about 73,000 years ago. The magma reservoir continues to be replenished today, and the area has been monitored for changing patterns in surface and subsurface deformation related to magmatic and hydrothermal activity (the heating of the local water system by the hot magma reservoir). Battaglia et al. (p. 2119) used extensive gravity measurements of Long Valley Caldera conducted in 1982 and 1998 along with surface deformation measurements to distinguish the progress of magmatic activity from hydrothermal activity. They found a positive residual gravity signal that is related to the intrusion of magma about 10 kilometers beneath the resurgent dome. These results can be used to assess the growth of the magma reservoir and to estimate the potential for an eruption.
The Mantle's Noble Gases
Noble gases such as neon and xenon can be used to trace the evolution and interaction between Earth's mantle and atmosphere because nuclear processes can produce characteristic anomalies in their isotopic distributions. Caffee et al. (p. 2115) measured the isotopic composition of xenon in carbon dioxide-rich gases from wells in Colorado, New Mexico, and southern Australia. They found excesses of xenon isotopes compared to Earth's atmosphere that were consistent with excesses that have been measured in meteorites. They suggest that the noble gases are derived from a lower mantle source that has not be substantially degassed from its primordial, solar-like composition and that this source has remained isolated from mixing by convective processes. Their results have important implications for the age of Earth, the timing of the atmosphere's formation, and the subsequent evolution of our planet.
RNAs Inside the Machine
Ribosomes are the cellular entities that translate information, encoded as nucleotide sequences, into protein-based machines that carry out the catalytic, structural, and signaling functions necessary for life. In bacteria, these entities are themselves composed of 50 to 60 distinct protein molecules and three distinct RNA molecules that are arranged into a complex, with a mass of approximately 2.5 million daltons (see the Perspective by Liljas and the news story by Pennisi). Information is provided in the form of messenger RNA (mRNA, which is essentially a copy of the nucleotide sequence of a gene). The keys to decoding are supplied by transfer RNA (tRNA) molecules that bind specifically to the mRNA at one end and carry amino acids at the other end; these amino acids are then assembled into peptide sequences. An overall view at 7.8 angstroms of the ribosome and the binding interactions with tRNA are presented by Cate et al. (p. 2095; see the cover), and a close-up analysis of the arrangement of the largest RNA molecule is described by Culver et al. (p. 2133).
The Bigger the Cell, the Bigger the Fly
Cells of a specific type maintain a characteristic cell size, and in multicellular organisms, this effect has an influence on the size of organs or the complete animal. Even as more knowledge about control of the cell division cycle is accumulated, the mechanisms by which cells control cell size remain poorly understood. Montagne et al. (p. 2126) provide evidence that implicates a role for a signaling pathway that controls translation in the control of cell size. Drosophila that lack DS6K, the homolog of the mammalian p70 ribosomal protein S6 kinase, develop slowly and are only about half the size of their normal counterparts. The change in size of the fly is a cell-autonomous event that reflects changes in cell size while cell number remains constant. In a Perspective, Leevers discusses how these results, together with recent findings that cell size is influenced through signaling from the insulin receptor (which also controls the activity of DS6K), begin to define a critical pathway that regulates the size of individual cells.
Suppressing Cancer and Autoimmunity?
Mutations of the Pten tumor suppressor gene, which encodes a phosphatase, occur in both sporadic and inherited human cancers. Di Cristofano et al. (p. 2122) show that mice carrying heterozygous mutations in Pten develop lymphomas and suffer from autoimmune disease in a manner similar to that seen in mice with inactivating mutations in Fas or Fas ligand. T and B cells from the Pten heterozygous mice were impaired in Fas-mediated apoptosis. These results implicate Pten in the Fas response and suggest that it may play a role as a suppressor of autoimmunity as well as cancer.
Inhibiting Pathways Instead of Molecules
One of the medically important roles that the phosphatase calcineurin plays in regulating T cell responses is the dephospho-rylation of the transcription factor NFAT, which then can move into the nucleus and initiate transcription and cell activation. Current immunosuppressive drugs inhibit calcineurin, and therefore inhibit all of its functions, which may contribute to the known toxicity of the drugs. Aramburu et al. (p. 2129; see the news story by Hagmann) have made a more specific inhibitor of the NFAT activation pathway by identification of a peptide that binds with high affinity to the site on calcineurin to which NFAT binds, thereby inhibiting the association of the two proteins and specifically blocking only the NFAT pathway in the cell. This result demonstrates that interaction sites may make good targets for specifically inhibiting a particular signaling pathway and provide alternatives for the development of therapeutics.
It's the Muscles and the Motion
The primary motor cortex subserves movements yet it is unclear what properties are encoded by these neurons—does firing of these neurons correlate with a pattern of muscle activation or with the direction of the desired movement in space? Kakei et al. (p. 2136) have designed a task to dissociate these parameters for a series of wrist motions. By recording from neurons in the motor cortex of a monkey during this task, they identify two main populations, one in which the neuronal activity does appear to be connected to muscle activation independent of the relation to extrinsic spatial coordinates and another in which the neuronal firing appears to be tied closely to the direction of movement in space. Thus, they suggest that both muscle and spatial representations are encoded, although further work will be needed to establish how they are combined to guide actual movements.
Considering Neonatal Gene Therapy
The treatment of certain disorders by gene therapy techniques would in some cases require fetal interventions. Before such procedures can be attempted, sufficient success would need to be seen in animal trials. Zanjani and Anderson review the current state of in utero gene therapy methods, such as transfer of genes by transplantation of hematopoietic stem cells. In an accompanying Policy Forum, Sugarman discusses the ethical issues involved in moving from animal studies to human therapies.
Cuprates and Quantum Critical Points
Classical phase transitions are governed by changes in temperature and pressure; in the vicinity of critical points, thermal fluctuations can dominate the behavior of the system. In the quantum regime, critical points occur in the limit of zero temperature and energy—indeed, energy and temperature become interchangeable. Using a new technique for measuring and analyzing angle-resolved photoemission spectra with improved resolution, Valla et al. studied quasiparticle excitations in an optimally doped high-temperature bismuth superconducting cuprate. The temperature and frequency dependence of the self-energy and behavior of single-particle excitations suggest that the system exhibits quantum critical behavior.
Molecular and Fossil Gaps
M. Foote et al. (Reports, 26 Feb., p 1310) analyzed the rates of extinction and preservation in fossil lineages to address discrepancies between fossil and molecular dates for the origins of key lineages. Their analysis was inconsistent with the proposed early origination of eutherian mammals deduced from molecular data [S. B. Hedges and S. Kumar, Nature 392, 917 (1998)].
Hedges and Kumar comment that “several critical aspects of [Foote et al.'s] model are flawed” and then re-assess and attempt to reconcile the molecular and fossil data. J. D. Archibald questions Foote et al.'s treatment and placement of a key group of possible early placental mammals, the zhelestids. Finally, T. H. Rich et al. comment that one reconciliation of the molecular and fossil age estimates—that crown group taxa were evolving in a geographically isolated and poorly sampled region—may be supported by a new fossil find in Australia.
In response, Foote et al. defend their analysis and treatment of the zhelestids and argue that the new fossil finds, even if accepted, are insufficient to explain the gap in many lineages. They state that even “accepting Hedges and Kumar's calibration … the gap [between the fossil and molecular records] is implausible.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/285/5436/2031a