This Week in Science

Science  04 Apr 2008:
Vol. 320, Issue 5872, pp. 15
  1. Yummy Antibiotics


    As a food source, antibiotics would seem to be a poor choice for bacteria. However, Dantas et al. (p. 100) readily cultured bacteria from the soil that can consume antibiotics as sole carbon sources. Importantly, these bacteria are from several genera, some of which are closely allied to human and livestock pathogens, and are also generally extremely resistant to many antibiotics. Consumption is not restricted to antibiotics derived from natural products but also includes synthetic ones, as well as new-generation molecules, such as levofloxacin. This heretofore unrecognized source of antibiotic-metabolizing bacteria represents a potential reservoir of antibiotic resistance genes for pathogenic bacteria.

  2. Aztecs' Lay of the Land

    Just as modern governments require careful land surveys and records of value for taxation, the Aztecs were diligent bookkeepers when it came to landholdings and real estate transactions. How exactly did the Aztecs calculate land parcel areas from the recorded dimensions of the plots? Williams and del Carmen Jorge y Jorge (p. 72) examined the records of the Codex Vergara and were able to determine the indigenous computational techniques used by the Aztecs to calculate land area.

  3. Chance Aspects of Cell Fate

    Ranging from bacteria to humans, cell fate is generally reached through a hard-wired program. However, in a review on stochasticity and cell fate, Losick and Desplan (p. 65) describe how the surroundings and cell lineage may have a lesser impact on cell fate than normally assumed. Instead, the differentiation pathway may be stochastically or randomly attained. Examples are seen in the entry of Bacillus subtilis into a state of competence or the generation of alternative colorvision photoreceptors in Drosophila melanogaster. There are varied reasons as to why a cell lacks a deterministic program, for example, “bet hedging” in bacteria to anticipate adverse changes in the environment. A stochastic mechanism for cell fate may be advantageous or, in some cases, necessary for the organism's or species survival.

  4. Cobalt Atoms Singled Out

    Magnetization curves, the response of a magnetic material to an applied magnetic field, have long been used to characterize sample properties that determine a material's usefulness in applications such as memory storage. Characterization techniques have had to develop in parallel to the miniaturization of magnetic memory, even down to the single magnetic atom adsorbed on a surface (an adatom). However, previous work looked at the adatoms in a somewhat artificial environment in which they are strongly coupled to a magnetic surface. Meier et al. (p. 82; see the cover) have now developed a spin-polarized scanning tunneling microscopy system to probe the magnetization of single cobalt adatoms on the more technologically relevant system of a nonmagnetic, metallic surface.

  5. Early Condensation of Graphite Whiskers


    Graphite whiskers can be condensed from high-temperature plasmas in the laboratory but have not been seen naturally. These whiskers have been postulated to be responsible for affecting the brightness of type 1a supernovae (used as a distance measure in astronomy) and the cosmic microwave background. Fries and Steele (p. 91; see the Perspective by Bland) have found several graphite whiskers in some of the most primitive and highest-temperature grains in several meteorites. They identified the whiskers using Raman imaging and electron microscopy. Thus, these whiskers likely condensed from the early hot solar nebula and may be expected in other stellar systems.

  6. Probing Post-Perovskite Conductivity

    Deep in Earth's mantle, near the core, silicate perovskite transforms to post-perovskite. Understanding the properties of post-perovskite is critical to inferring the nature of the deep mantle and the flow of heat, magnetic flux, and momentum from the core to the mantle. Using a diamond anvil cell, Ohta et al. (p. 89) show that the electrical conductivity of post-perovskite at deep mantle pressures and temperatures is much higher than for most oxide insulators. These results imply that there is a strong electromagnetic coupling between the core and mantle, which affects the rotation of Earth, as observed in decadal changes in the length of the day.

  7. Working Back to Pluripotency

    Recent studies have shown that adult mouse and human fibroblasts can be reprogrammed to a pluripotent state after the viral integration of four transcription factors. However, questions remained as to the origin of those cells, whether specific genomic integration sites are needed, and how tumorigenicity might be reduced (see the Perspective by Bang and Carpenter). Embryonic stem cells and cells of human malignancies are characterized by low levels of let-7 microRNAs (miRNAs), a phenomenon thought to be functionally linked to pluripotency and oncogenesis. In both cases, precursors of let-7 are detectable but processing to the miRNA mature form is inhibited. Viswanathan et al. (p. 97, published online 21 February) identify Lin28 as an RNA binding protein that selectively inhibits let-7 miRNA processing. Lin28 was recently shown to function with three other factors to reprogram human fibroblasts to pluripotent stem cells. This work suggests that the regulation of miRNA processing may be critical in the dedifferentiation that occurs in both reprogramming and oncogenesis.

  8. Organizing Purine Biosynthesis

    How primary metabolic enzymes are organized in living cells has long been a topic of discussion. There have been suggestions that multienzyme complexes may facilitate flux through the pathway, but scant evidence in support of such organization. An et al. (p. 103) use fluorescence microscopy to show that enzymes in the purine biosynthesis pathway colocalize to form clusters in the cytoplasm of HeLa cells. Formation of these “purinosomes” is regulated by changes in purine levels and results suggest that purinosomes form to satisfy a cellular demand for purine. Similar dynamic regulation may apply to other metabolic enzyme complexes.

  9. Structure for Self-Splicing


    Splicing by autocatalytic group II introns is essential for gene expression in plants, fungi, and yeast. Group II introns are also of great interest as model systems because they are thought to be evolutionary predecessors of the eukaryotic spliceosomal apparatus. Toor et al. (p. 77; see the perspective by Piccirilli) describe the structure of an intact, self-spliced group II intron at 3.1 angstrom resolution. The structure is consistent with a two-metal ion mechanism for catalysis and, together with previous biochemical studies, supports the hypothesis that group II introns and the spliceosome share a common ancestor.

  10. I've Got Rhythm

    Low-frequency electroencephalogram oscillations are very common in the cortex, but their functional significance has largely been unclear. In most studies low frequencies are desynchronized by attention. Now, however, Lakatos et al. (p. 110) present data from area V1, the primary visual cortex, that indicate a potential mechanism behind selective attention in the case of rhythmic stimuli. When stimuli are presented at a predictable, low-frequency rate, the low-frequency oscillations entrain to the low-frequency stimulus, and the higher frequencies, the so called gamma band oscillations, are modulated in phase with the low frequencies. Cells then become most excitable at times when the stimulus is expected. During visual attention, ongoing entrainment and incoming stimulation support each other and lead to particularly strong responses.

  11. Mental Time Travel in Rats?

    The notion of how time is represented in episodic memory is an elusive concept. In work on scrub jays, episodic-like memory has been tested by tasking the birds to remember whether one of two food items appeared 4 hours previously or 4 days previously. Now, in experiments with rats, Roberts et al. (p. 113) sought to learn whether the “when” component of episodic-like memory really is a sense of “when,” or if it is merely a sense of elapsed time. They contrasted scenarios in which the animals were required to remember exactly when the food item event occurred or, in a different condition, how long ago the food item event occurred. Their results show that the rats can only remember elapsed time and are consistently subject to chance when required to remember precisely when an event happened. The authors thus argue that episodic-like memory in rats is very different from human episodic recall.

  12. Single-Molecule Resequencing

    A significant move toward personalized medicine would come from the ability to cheaply and reliably sequence human genomes. Given the availiability of a consensus sequence, short read-length technologies can be used to achieve resequencing, but the problem remains to develop technologies that reliably achieve high throughput at low cost. A step in this direction comes with a single-molecule sequencing-by-synthesis technique described by Harris et al. (p. 106). The method allows more than 280,000 DNA strands to be simultaneously sequenced to read lengths of about 25 bases. It was used to resequence the M13 genome with 100% coverage and efficient detection of single-base mutations.

  13. Catalysis Is More Than Skin Deep

    The nature and the working environment of heterogeneous industrial catalysts complicate the investigation of their mechanisms with surface science techniques, which often work best under vacuum conditions and with crystalline samples. An added difficulty is the limited number of methods that can monitor hydrogen. Teschner et al. (p. 86) have combined two techniques that overcome some of these limitations—in situ photoelectron spectroscopy and prompt gamma activation analysis—to study the role that subsurface carbon and hydrogen play, respectively, in the selective hydrogenation of alkynes to alkenes by palladium. These catalysts have an induction period before they become selective that corresponds to displacement of the subsurface β-hydride phase (which promotes full hydrogenation to alkanes) by carbon from the alkyne.

  14. Tropical Dust

    Airborne dust, which is transported long distances from its origins by winds, has important effects on many processes, such as cloud formation, precipitation, and marine productivity, all of which in turn can affect climate. Because the distribution of dust sources and wind patterns over thousands of years is irregular, the correlation of dust fluxes and climate remains incompletely understood. Winckler et al. (p. 93) present a 500,000-year-long record of dust deposition across the equatorial Pacific Ocean, a region from which few data previously existed. Their results indicate that dust fluxes in the Equatorial Pacific were two-to-three times higher during glacial maxima than during warm intervals and that dust fluxes were more than three times greater in the west than in the central or eastern regions. These data should help climate models better represent the effects of marine biogeochemical cycles.