This Week in Science

Science  20 Dec 2002:
Vol. 298, Issue 5602, pp. 2279
  1. In Brevia

    Bacterial microfossils in carbonate rocks, similar in many respects to modern cyanobacteria, are shown by Kazmierczak and Altermann (p. 2351) to be 2.5 billion years old.

    CREDIT: KAZMIERCZAK AND ALTERMANN
  2. Stable Semiconducting Analogs of Zeolites

    The microporous zeolites oxides are generally insulators, but analogous compounds based on sulfur and selenium that use cations such as In, Ge, and Sn as the central atom instead of Si or Al are semiconductors that could have unusual electronic and photophysical properties. However, nanoporous analogs have often exhibited poor thermal stability that limits their use in applications such as catalysis. Zheng et al. (p. 2366) found that they could achieve high thermal stability (up to at least 380°C) in several different compounds by mixing tetravalent and trivalent cations.

  3. Two Looks at the LDL Receptor

    The low-density lipoprotein receptor (LDL-R) plays a key role in cholesterol homeostasis, and mutations in its gene can lead to familial hypercholesterolemia. The receptor binds LDL at neutral pH; after internalization, it releases LDL at low endosomal pH. The lipoprotein is subsequently degraded while the receptor is recycled. Rudenko et al. (p. 2353; see the Perspective by Innerarity) have determined the structure at 3.7 angstrom resolution of the extracellular portion of LDL-R at pH 5.3. The ligand binding domain folds back as an arc over the epidermal growth factor (EGF) precursor homology domain to facilitate an intramolecular interaction with features that are consistent with reversible pH-regulated binding. This interaction likely occurs in the endosomes to displace bound LDL. Multidomain proteins like LDL-R are thought to fold domain by domain from the amino- to the carboxyl-terminus as they are translated. Jansens et al. (p. 2401) examined the folding of LDL-R by analyzing the production of conformation-specific epitopes and by looking at the pattern of disulfide bond formation in nascent molecules. They found instead that each domain folded separately during translation and that different domains appeared to fold posttranslationally and out of the expected sequence.

  4. A Mixed Message

    An 18-year time series of the ocean carbon cycle near Bermuda reported by Gruber et al. (p. 2374; see the Perspective by Quay) shows that changes in sea surface temperature and the depth of the winter mixed-layer cause CO2 uptake there to be highly variable. Because these physical properties are closely associated with the North Atlantic Oscillation, the authors suggest that the entire subtropical North Atlantic Ocean carbon cycle may fluctuate annually by as much as 50%.

  5. Reducing Schemes

    Geological evidence suggests that Earth's early atmosphere had little free oxygen, but direct evidence of oxygen content during Earth's history has been unavailable. Two reports take advantage of the better record of sulfur isotope values for much of the Precambrian to infer the abundance of oxygen and the sulfur chemistry of the early Earth (see the Perspective by Wiechert). Isotopic fractionation is normally mass dependent, but exceptions are known, including the mass-independent photochemical fractionation of sulfur that occurs today in the upper atmosphere where ozone is absent. For sulfur, evidence for mass-independent effects that reflect a paucity of atmospheric oxygen has been found in Archean crustal rocks. Farquhar et al. (p. 2369) now report evidence of mass-independent isotope effects in sulfide inclusions in ancient diamonds, which are derived from deep in Earth's mantle. These data provide direct evidence that these diamonds were sampling material that reflected processes in Earth's atmosphere and had been subducted into the mantle during the Archean. Archean sulfide rocks are much less depleted than are modern sulfides in 34S, an isotope that reflects bacterial processing. In culture experiments, Habicht et al. (p. 2372) show that the isotope data limit Archean ocean sulfate concentrations to 200 micromolar or less, considerably lower than previously thought. These data also imply that the atmosphere had little free oxygen but an abundance of the greenhouse-gas methane produced through the action of more prevalent methanogens.

  6. Name That Tube

    Single-walled carbon nanotubes can be described by an (n,m) naming convention that specifies different angles for rolling up a graphene sheet into a tube. The properties of the tubes change for each (n,m) combination, and Bachilo et al. (p. 2361) have identified a signature for the semiconducting nanotubes based on their absorption, fluorescence, and Raman spectra. The authors pose questions about current one-electron theories of nanotube-excited states and also present an analytical method that may aid in the understanding of nanotube synthesis.

    CREDIT: BACHILO ET AL.
  7. Where We Come From

    Understanding the distribution of genetic variation among human populations, where a population can be a cultural or geographic stratification, is important for studies ranging from evolution to clinical medicine. Rosenberg et al. (p. 2381; see the Perspective by King and Motulsky) used short tandem repeat polymorphisms to characterize 1056 individuals from all continents. Only a small amount of the variation resulted from differences between populations; more than 90% of the variation was between individuals within given populations. Nevertheless, the authors developed an algorithm for cluster analysis that could assign nearly all individuals studied to a major geographic grouping without other information.

  8. Muscles That Save Face

    The muscles of the head and the trunk in vertebrates are specified by different genetic pathways, but much more has been known about the transcription factors involved in skeletal muscle development than those that control head muscle formation. Lu et al. (p. 2378) now show that when the genes for the transcription factors MyoR and capsulin are inactivated in mice, specific subsets of face muscles are eliminated.

    CREDIT: LU ET AL.
  9. Gas-Driven Transcriptional Regulation

    Gene expression in mammalian cells is controlled by environmental and metabolic cues, but little is known about the molecular mechanisms that sense and respond to these cues. Dioum et al. (p. 2385; see the Perspective by Boehning and Snyder and the 22 November Prize Essay by Rutter) have discovered one intriguing mechanism in a study of NPAS2 (neuronal PAS domain protein 2), a transcription factor implicated in the control of circadian rhythms. NPAS2 was shown to bind heme as a prosthetic group, and the heme, in turn, was shown to function as a gas-regulated sensor. In experiments with purified proteins, carbon monoxide (CO) was identified as a candidate ligand for this sensor. Exposure to CO inhibited the dimerization of NPAS2 with BMAL1, the protein that confers the transcription factor with DNA binding activity.

  10. Transcriptional Regulation via ACTive Transport

    Molecules called kinesins are critical for the movement of chromosomes, organelles, and messenger RNAs within cells. Macho et al. (p. 2388) now show that the testis-specific kinesin KIF17b can function in the transport of the coactivator ACT and affect transcriptional activity. ACT is a coactivator of CREM-mediated transcription in mammalian spermatids. KIF17b acts as a dose-dependent inhibitor of ACT-dependent transcription by excluding ACT from the nucleus. This work begins to address the molecular mechanism by which germ-cell gene expression is regulated.

  11. Coordinate Metabolism? Yes, Sir2!

    The highly conserved Sir2 family of proteins are involved in regulating chromosome stability and cell aging in eukaryotes, as well as gene silencing in both eukaryotes and archea. Sir2 has nicotinamide adenine dinucleotide (NAD+)—dependent deacetylase activity and its substrates include histones and the tumor suppressor p53. Starai et al. (p. 2390) show that the Salmonella enterica acetyl-CoA synthetase is turned off by post-translational acetylation of a lysine residue in the active site. The Sir2 homolog CobB switches on the activity of acetyl-CoA synthetase by deacetylating this residue. The conservation of these proteins suggests that lysine acetylation is a common regulatory mechanism in prokaryotes and eukaryotes, in the latter case providing a link between the acetylation of histones and the physiological state of the cell.

  12. Additional Early Steps in Photosynthesis

    Carotenoids increase the range of wavelengths that can be used for photosynthesis by absorbing light energy and efficiently transferring it to chlorophyll. Carotenoid photophysics was classically interpreted in terms of two low-lying excited singlet states, S2 and S1. However, recent theoretical and experimental studies suggested that an intermediate excited state might be involved in energy transfer. Cerullo et al. (p. 2395) used ultrafast spectroscopy to identify directly a short-lived excited state (Sx) that mediates the internal conversion between the S2 and S1 states.

  13. Oxygen in a Line-Up

    Confinement of molecules into lower dimension spaces may alter their physical properties and reactivity. Oxygen is an interesting candidate in this regard because of the magnetism arising from its unpaired electrons, as well as its photophysical properties. Kitaura et al. (p. 2358) report that they have confined O2 in one-dimensional chains within the pores of copper coordination polymer based on x-ray diffraction evidence.

  14. Switching Signals

    B cells mount distinct responses by altering the class of antibody they secrete, and this class switching is also evident at the B cell surface, where membrane-bound antibodies cooperate with common signaling components to form B cell receptors (BCRs). Although these transduction modules are the same in each BCR, one receptor, the immunoglobulin G (IgG)-containing BCR, possesses distinct signaling capabilities from the others. Wakabayashi et al. (p. 2392) find that these capabilities arise through the selective activation of CD22, a transmembrane protein known to down-regulate B cell activation. Ligation of IgG-BCR failed to induce both efficient CD22-dependent recruitment of the phosphatase SHP-1 and inhibition of the extracellular signal-regulated kinases (ERK1 and ERK2) and calcium signaling. By protecting itself from the inhibitory effects of CD22, IgG-BCR may regulate efficient class-switching, as well as enhanced responsiveness of IgG+ memory B cells.

  15. Is Schizophrenia Linked to Chromosome 1q?

    Levinson et al. (Reports, 26 April 2002, p. 739) conducted a multicenter study of 779 families with schizophrenia and found no evidence of genetic linkage to chromosome 1q—contrary to several previous reports implicating this region in the disease. In separate comments, Macgregor et al. and Bassett et al. assert that locus heterogeniety can explain the divergent results and that failure to replicate linkage may indicate more about the relative merit of different study designs than about the genetics of schizophrenia. Levinson et al. respond that although schizophrenia susceptibility genes may exist on 1q, classical heterogeneity cannot explain a substantial portion of disease cases. They further emphasize the importance of large, multicenter linkage studies to help confirm and localize the multiple interacting loci likely to be involved in schizophrenia.

    The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/298/5602/2277a