This Week in Science

Science  18 Feb 2000:
Vol. 287, Issue 5456, pp. 1165
  1. Black Hole Sun

    An accretion disk of rapidly spiraling gas and particles may form around a black hole. This disk can give off intense x-ray emissions as the gravitational energy of the material is lost before it is swallowed by the black hole. Modeling by Zhang et al. (p. 1239) of the low- and high-energy x-ray spectra of two Galactic superluminal sources, GRS 1915+105 and GRO J1655-40, suggests that the high-energy x-ray emissions are related to a very hot and thin corona around the disk, and that the low-energy x-ray emissions are related to Compton scattering of central disk photons in a warmer disk layer that encompasses the colder central disk. This modeled structure resembles the structure of the solar atmosphere, which suggests that similar physical processes may account for the formation and stability of accretion disks in general.

  2. Getting Their Iron

    Marine microorganisms have evolved sophisticated mechanisms for sequestering iron, which is often only present in trace amounts in the surrounding seawater. Like terrestrial microorganisms, they secrete siderophores, low-molecular-weight compounds that scavenge iron from the environment and facilitate its uptake into the microbial cells. Martinez et al. (p. 1245) show that two families of siderophores isolated from different genera of marine bacteria spontaneously form vesicles upon addition of Fe(III). This iron acquisition mechanism may be a distinctive feature of marine bacteria.

  3. On the Ground and in the Air

    Satellite-based microwave measurements of lower tropospheric temperatures have shown no change or even a slight cooling trend since 1979, which conflicts with ground-based observations of surface temperature that indicate warming. A research article and a report discuss how these different measurements can be reconciled (see the Perspective by Parker). Santer et al. (p. 1227) show that approximately 30% of the difference in the trends of satellite-based measurements of lower tropospheric temperature and ground-based measurements of surface temperature for the past 20 years can be explained by the different spatial coverage of the methods. Their model results show that the remaining differences are greater than can be explained by natural variations alone, and that the best model simulation is one that includes the influences of anthropogenic factors and volcanic aerosols. Gaffen et al. (p. 1242) have collected vertical temperature profiles of the tropical troposphere made with weather balloons during this period and found that both satellite and surface measurements apparently are correct. The differences between the two time series are due to variations in the vertical temperature profile of the troposphere.

  4. Recognizing a Stop Sign

    Proteins that are destined for integration into a membrane or for secretion across a membrane generally carry a signal sequence at their amino-terminus—a stretch of ∼20 amino acids containing both hydrophobic and basic side chains. As the nascent peptide emerges from the ribosome, the signal sequence is bound by the signal recognition particle, a complex of protein and RNA, and protein synthesis is halted momentarily. The entire assembly is then ferried to a membrane where peptide synthesis continues with co-translational membrane insertion. Batey et al. (p. 1232; see the cover and the Perspective by Walter et al.) describe the 1.8 angstrom structure of the core of the signal recognition particle that reveals the intimate contacts between two RNA loops and two protein helices docked into the minor groove of the RNA.

  5. Liver Transplant Lifelines?

    Many liver transplant patients die each year because of the chronic shortage of suitable donor organs. Two reports discuss successful approaches in rodent models of liver disease that may one day buy human patients time by slowing liver damage (see the news story by Hagmann). Transplantation of hepatocytes, which can provide temporary metabolic support, is hampered by a similar shortage of transplantable hepatocytes. Kobayashi et al. (p. 1258) have constructed a line of hepatocytes that can be reversibly immortalized and therefore grown up in large quantities in vivo. Subsequent removal of the immortalizing gene minimized possible oncogenic side effects. Transplantation of the cells was effective in treating rats with acute liver failure. In an independent study, Rudolph et al. (p. 1253) suggest a different therapeutic strategy for prolonging the life of patients awaiting liver transplants. Mice with abnormally short telomeres (the DNA sequences at the end of chromosomes) are especially prone to develop liver cirrhosis when the liver is injured. The development of cirrhosis in these mice was prevented by the administration of a gene encoding an essential component of telomerase, the enzyme that makes telomeres.

  6. Steroid Receptors Hit and Run

    In the presence of hormone, steroid receptors modulate transcription by binding to specific response elements in chromatin and recruiting co-activators or co-repressors. McNally et al. (p. 1262) have observed the interaction of glucocorticoid receptor, labeled with green fluorescent protein, with response elements in living cells. With ligand continuously present, the receptor exchanges rapidly between chromatin and the nucleoplasmic compartment. This finding is inconsistent with the classic view that the receptor remains bound to chromatin as long as ligand is present. Instead, it supports the “hit and run” mechanism in which the receptor interacts transiently and recruits secondary factors to form a stable complex that modulates transcription.

  7. Images Remembered

    What differentiates the familiar image from the unfamiliar one? Henson et al. (p. 1269) have examined this age-old question in psychology by using functional brain imaging to measure the amount of neuronal activity triggered by single presentations of stimuli. What they find is a complex interplay between the factors of repetition and familiarity; that is, the second viewing of an unfamiliar drawing portraying a possible object would generate an enhanced neural response that would reflect the process of creating an internal memory. If the drawing could not represent an object or if it recalled an already memorized object, however, the neural response would decline, as a simple manifestation of stimulus repetition.

  8. A Hot Spot for Protein Binding

    Are there regions within proteins with characteristics that make them particularly well suited to interaction with other proteins? DeLano et al. (p. 1279) addressed this question by selecting peptides from a randomized library that would bind to the constant fragment (Fc) of immunoglobulin G. Although it should have been possible to isolate peptides that interacted with virtually any part of the Fc molecule, selection for interactions of high affinity yielded primarily a single peptide. In biological contexts, the Fc fragment binds four other proteins that all have very different structures. However, these proteins all bind to the same site on the Fc molecule, as did the peptide found during selection. The nature of this preferred binding site—its accessibility, hydrophobicity, and limited number of sites for polar interactions—helps define the essential properties of such interaction domains. Understanding these characteristics in more detail should be useful for predicting protein function or design of interacting ligands.

  9. Mitochondria Make the Cut

    Mitochondria, the powerhouses of the cell, exist in multiple copies in eukaryotic cells. They possess their own genome that is essential both for their efficient functioning and for their propagation from generation to generation during cell division. However, the division process itself that produces daughter mitochondria is not well understood. Beech et al. (p. 1276; see the Perspective by Martin) discovered the existence of a protein encoded by a nuclear gene of an alga that is targeted to the mitochondria and is likely to play a role in mitochondrial division. The protein is related to a bacterial protein known to be involved in bacterial division, and thus this finding may provide a missing link in our knowledge of mitochondrial evolution.

  10. Double Take

    Mutations are generally thought to occur through substitutions of single nucleotides in DNA. Averof et al. (p. 1283) now document the occurrence of double-nucleotide substitutions in a wide range of organisms at a rate that is higher than would be expected by chance. These doublet substitutions may take place at different rates in different cell types and at different sites, with implications for models of molecular evolution and phylogenetic reconstruction as well as mutational mechanisms of human disease.

  11. Balancing Act

    Assessment of the potential effects of glaciers on climate requires an understanding of polar ice sheet dynamics. The traditional view of polar ice sheets—that a fast flow structure like an ice stream controls ice discharge from a basin that acts only as passive source—has recently been challenged by observations of a handful of sites in Antarctica. Bamber et al. (p. 1248) have calculated the “balance velocity” for the entire “grounded” portion of the Antarctic Ice Sheet (as opposed to ice sheets projecting above the sea). This quantity, which is the depth-averaged velocity required at any point to maintain the ice sheet in a state of balance, is used to construct a flow map of the entire grounded portion of the ice sheet. Their results show that a complex system of ice streams and tributaries penetrates as far as 1000 miles into the interior of the ice sheet, which challenges the view that the Antarctic plateau is homogeneous and slow moving, and also illustrate the limitations of numerical models that have not yet reproduced such complex flow.

  12. Core Properties from an Iron Resonance

    Earth's core is dominated by iron and possibly some iron alloys. Experimental work has suggested that ϵ-iron, a high-pressure hexagonal close-packed structure, is the main phase in the core, but little is known about its properties under the high pressure and temperature conditions of the core. Lübbers et al. (p. 1250) have developed a useful method for measuring the bulk properties of ϵ-iron. They took advantage of the large cross section available from the resonance of iron-57 to measure nuclear inelastic absorptions of synchrotron radiation in polycrystalline samples of ϵ-iron at pressures of 20 to 42 gigapascals at room temperature in a diamond anvil cell. The resulting spectra contain sidebands that they used to calculate the density of phonon states in the iron and to estimate the bulk vibrational properties of the material free from magnetic or electronic lattice effects. The availability of the phonon density of states allows many other useful properties to be calculated, such as mean sound velocities, which are important for understanding seismic wave propagation, and lattice contributions to the material's specific heat.

  13. A Parkinson's Mouse

    In Parkinson's disease, the loss of neurons in the nigrostriatal pathway that produce dopamine leads to the characteristic symptoms of muscle rigidity and tremor. Mutations in the protein α-synuclein have been implicated in inherited forms of Parkinson's disease. Although a valuable primate model exists there is no mouse model. Masliah et al. (p. 1265) now report that overexpression of wild-type human α-synuclein in mice results in the deposition of inclusions containing α-synuclein in the substantia nigra, hippocampus, and neocortex of transgenic animals. These inclusions were associated with loss of dopaminergic neurons in the basal ganglia and motor impairments typical of Parkinson's disease.

  14. Sparse Coding of Visual Information

    Neurons communicate with a rather limited vocabulary of action potentials, which are depolarizations of the membrane that release neurotransmitters onto target neurons. The frequency of these depolarizations can vary, but their amplitude remains the same. Populations of neurons can produce somewhat more complex ensembles of messages where, for example, every other neuron firing would signify something different than every third neuron firing. Coding of pictures in the visual system has been thought of as a continuum between highly discriminating neurons (such as ones that respond only to Vermeers) and broadly appreciative neurons that respond, for example, to all types of paintings but not to sculptures. Both codings are inefficient, as the former method would require an enormous number of rarely active neurons and the latter would result in many neurons firing at all times. Vinje and Gallant (p. 1273) describe recordings from V1, the first stage of visual processing in the brain, which suggest that a sparse or intermediate coding system is in use that improves efficiency while maintaining information.

  15. Peg3 and the Conflict Hypothesis

    In a mouse study, Li et al. (Reports, 9 Apr., p. 330) found that the paternally expressed imprinted gene Peg3 regulates fetal growth and maternal behavior in offspring. That observation, Li et al. suggested, may be consistent with the parental-conflict hypothesis for genomic imprinting. Hurst et al. comment that the maternal-behavior connection may not fit with the conflict model. “[T]he paternally expressed Peg3 affects the behavior of daughters, not of the current mate,” note Hurst et al., and genes transmitted to the daughter from either parent have the same chance of being transmitted to the daughter's offspring. Smits et al., in a separate comment, concur: “The confusion may have arisen from the involvement of three generations, whereas the classical theory of parental conflict deals with two generations only.”

    Li et al. respond that Peg3's effect on maternal behavior may indeed not fit easily into the conflict hypothesis, but that the effect of the imprinted gene on fetal growth in utero clearly does. Judging the evolutionary significance of imprinting for behavior, they conclude, will require additional experiments. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/287/5456/1167a

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