This Week in Science

Science  20 Feb 1998:
Vol. 279, Issue 5354, pp. 1105
  1. Cross antagonism by malaria strains

    The complexity of the interaction between the malaria parasite and the immune system reaches a new level in a study combining laboratory experiments, field study, and mathematical modeling. Gilbert et al. (p. 1173; see the news story by Williamson, p. 1136) describe a pair of variants of Plasmodium falciparum (the parasite that causes malaria) that differ at a single amino acid in a key immunogenic epitope: These variants inhibit the immune response to each other in vitro, a process known as cross antagonism. Consistent with this being of biological importance, the distribution of parasite variants in African children is radically different from that predicted under conditions where strains are randomly mixed. Tying together these empirical findings, the authors have developed a mathematical model that illustrates how antagonism can alter parasite population structure. Such findings are also important for vaccine development.

  2. Underlying T cell receptor recognition

    Unlike B cells, which can directly bind to antigen through antibodies, T cells need to have the antigen broken up and inserted into a pocket on major histocompatibility complex (MHC) antigens, which are borne on the surface of “antigen presenting cells.” Garcia et al. (p. 1166) have resolved an MHC-self peptide-T cell antigen receptor (TCR) complex. They compare it to previous structures of the same MHC and peptide complexes without the TCR and of a MHC-foreign peptide-TCR structure. This structure gives us insight into alloreactivity, mechanisms for TCR bias toward class I or class II recognition, and models for how the limited contacts of the peptide with the TCR could affect T cell maturation and activation.

  3. Pop records

    The rare gas budget and chemistry of the mantle provides key information on Earth's degassing and early accretion history. Atmospheric contamination, however, has hampered efforts to measure rare gases in mantle samples. One important sample is the so-called “popping rock,” a sample of ocean ridge basalt that evidently trapped mantle gases and elevated pressures in vesicles sealed from atmospheric contamination. Moreira et al. (p. 1178) analyzed neon, argon, and xenon abundances and isotopes in this rock and found that the rare gases abundances in Earth's mantle are like those found in chondrites, rather than solar values, and that Earth's atmosphere can be largely explained by mantle degassing.

  4. Extracting sulfur from Shoemaker-Levy 9

    When comet Shoemaker-Levy 9 fragmented and then collided with Jupiter, observers noted the unexpected and so far unexplained production of organosulfur compounds in the jovian atmosphere. Kaiser et al. (p. 1181) performed molecular cross beams experiments and ab initio calculations to understand the reaction sequence that starts with carbon and hydrogen sulfide and ends with organosulfur components. They identified some intermediate reactions that might account for some of the observed organosulfur compounds formed during the impact, thus shedding some light on the dark, sulfur-rich blotches that were created in the jovian atmosphere.

  5. Vesicle inventory

    Although the contents of single vesicles in cells may elicit a biological response, identifying the contents of single vesicles is a daunting analytical challenge. Chiu et al. (p. 1190) report that single vesicles with attoliter volumes (10−18 liters) can be manipulated by optical trapping into a capillary electrophoresis separation column, and then lysed and labeled so that compounds can be identified by laser-induced fluorescence (peaks were identified with know standards and by analyzing the contents of populations of vesicles with mass spectrometry). Variations in the contents of vesicles from the atrial gland of the mollusk Aplysia californica were observed, such as large changes in their taurine content.

  6. Pushing back carbon-14 dating

    The radiocarbon time scale (and thus carbon-14 dates) must be calibrated in order to obtain accurate calendar ages. Trees rings provide a good calibration to about 10,000 years ago. The calibration curve also provides information on ventilation of the oceans (a main carbon reservoir) and the strength of Earth's magnetic field and the cosmic ray flux (both of which affect carbon-14 production in Earth's upper atmosphere). Kitagawa and van der Plicht (p. 1187) dated fossils in an annually varved lake in Japan to obtain a calibration back to 45,000 years ago. The results are consistent with some other attempts at calibrating the record in less detail and indicate several episodes of ocean ventilation and two large spikes, perhaps indicating a high flux of cosmic rays, nearby supernova, or collapse of the geomagnetic field.

  7. Interlayer tunneling and superconductivity

    Electron motion in layered cuprate superconductors, when they are in their normal state, is incoherent in the direction normal to the planes. The interlayer tunneling (ILT) model for high-temperature superconductivity argues that the driving force for forming superconducting electron pairs is the coherent transport brought about by pairing (see the commentary by Leggett, p. 1157). The strength of interlayer Josephson tunneling between layers, especially in one-layer materials such as Tl2Ba2CuO6+δ, is a stringent test of the ILT model. Moler et al. (p. 1193) imaged interlayer Josephson vortices in this material and measured an interlayer penetration depth of ~20 micrometers, which implies that the ILT mechanism can provide only 1/1000 of the condensation energy need to form the superconducting state. Anderson (p. 1196) discusses these results in light of other experimental studies more supportive of the ILT model and suggests that difficulties associated with preparing Tl2Ba2CuO6+δ may mean that is it not a true one-layer material.

  8. Keeping secrets

    One way to encrypt a message is to introduce it into a noisy circuit in such a way that it can be retrieved, with appropriate circuitry, at another location. Such methods have been demonstrated in electronic circuits, but the low bandwidths in these systems make them less practical for high-speed communications. VanWiggeren and Roy (p. 1198; see the commentary by Gauthier, p. 1156) show in a microwave optical system (1.53-micrometer wavelength) that a 10-megahertz message can be buried and retrieved in larger amplitude chaotic carrier. Recovery of the message cannot be achieved by filtering but requires a receiver that replicates the transmitter.

  9. Altered speech

    The planning and execution of motor commands can be perturbed by altering sensory input, for instance, by wearing prism glasses that shift the visual field. Houde and Jordan (p. 1213) used this approach to demonstrate that the production of vowels can be altered by shifting the acoustic feedback, that this alteration is stable for at least tens of minutes, and that this adaptation generalizes to production of the same vowel sounds in different contexts and different vowel sounds in the same contexts. It may thus become feasible to examine the basic units of motor speech production (such as phonemes and syllables) and the relative contributions of internal representations and contextual influence.

  10. Seeing somatic mutations

    During the development of the immune response, B cell immunoglobulin genes acquire mutations, which change the resultant antibody's specificity or affinity for particular antigens. This process of somatic mutation is one of the ways that the immune system has developed to recognize the multitude of viral and bacterial proteins. To determine what genes may be involved in somatic mutation, Cascalho et al. (p. 1207; see the commentary by Shannon, p. 1159) have used a mouse with quasi-monoclonal immunoglobulin genes that lacks the usual hundreds of immunoglobulin gene segments that can be recombined by the cell into a myriad of antibodies. This system enabled them to easily detect any mutations that were the result of somatic mutation. When they crossed these mice with mice defective in their ability to repair mismatches in their DNA, they found that Pms2, a gene required for the repair of mismatches, instead seems to aid in the retention of mutations.

  11. Surviving mutations

    A theoretical model of genetics is developed to show that the effect a mutation can have on the distribution of genotypes within a population is markedly dependent on whether the mutation produces multiple phenotypes, that is, it exerts pleiotropic effects. Waxman and Peck (p. 1210; see the commentary by Wagner, p. 1158) show that when they analyze a mutation that can affect three or more traits, the predicted distribution of genotypes in a natural population of organisms begins to be heavily weighted toward optimal genotypes. Nonpleitropic mutations generate a population with a variety of suboptimal genotypes and very few individuals with the optimal genotype. Thus, even in the presence of a high rate of mutations, optimal genotypes survive and flourish as long as mutations are pleiotropic.

  12. Complex underpinnings

    In dividing cells, separation of sister chromatids is regulated by a proteolytic mechanism under control of the anaphase-promoting complex (APC). The APC is a large complex of at least 12 subunits that mediates ubiquitin-dependent degradation of key proteins that participate in cell cycle control. Zachariae et al. (p. 1216) and Yu et al. (p. 1219) describe cloning and analysis of subunits of the APCs from humans and budding yeast, respectively. One of the common subunits from both organisms, Apc2, is similar to a protein, Cdc53p, that participates in a different complex, the SPC or S phase-promoting complex, that also mediates ubiquitination of proteins but controls initiation of DNA synthesis. Thus, although the APC and SPC are distinct complexes with different modes of regulation, the similarity of Apc2 and Cdc53p may indicate that there are shared mechanisms operating in both complexes.

  13. Tracking viral infection

    Efforts to understand the dynamics of killing and replacement of T lymphocytes have been central to recent advances in analyzing prognosis and therapy of human immunodeficiency virus-type 1 infection and AIDS. To follow the changes in lymphocyte turnover more precisely in vivo, Mohri et al. (p. 1223) have followed the incorporation of bromodeoxyuridine into lymphocytes of rhesus macaques. Infection with simian immunodeficiency virus was associated with increases in the rates of turnover in memory and naïve T cells as well as natural killer and B cell populations, which suggest a general state of activation.

  14. Neurotransmitter release kinetics

    The role of N-ethylmaleimide-sensitive factor (NSF) in membrane fusion events has been the subject of much research. Schweizer et al. (p. 1203) now show that NSF acts to regulate not just the extent but also the kinetics of neurotransmitter release at the synapse.

  15. Sea-salt reactions

    Particles play an important role in chemical reaction cycles in the atmosphere. However, because most techniques measure bulk samples containing a complex mixture of particles, it is difficult to determine exactly which particles are present and which reactions are occurring. Gard et al. have used an aerosol time-of-flight mass spectrometer capable of determining single-particle size and chemical composition, together with an atmospheric transport and reaction model, to show how particle compositions differ depending on the history of the air parcel in which they arrived at the spectrometer. A replacement of chloride by nitrate in sea-salt particles as a result of reaction with nitrous oxide pollutants is observed in air parcels that have spent some time over land, whereas those air parcels that arrive straight from the sea contain mostly unreacted sea-salt particles.

  16. Payoffs for plant defense

    Interaction with herbivores induces a series of chemical cascades in plants. Although the mechanisms underlying some of these induced responses are well characterized, and they are consistent with resistance to herbivore attack, evidence that they truly confer a fitness benefit to the plants has been lacking. Now a field study by Agrawal provides this evidence. Responses were experimentally induced in wild radishes early in the growing season; these plants were subsequently less prone to herbivory and gained substantially in indices of fitness over control plants. [See the cover.]

  17. ABCR Gene and Age-Related Macular Degeneration

    Technical Comment Summaries

    R. Allikmets et al. performed a genetic study (Reports, 19 Sept., p. 1805) of 167 elderly patients with age-related macular degeneration (ADM). They screened the gene ABCR, which “encodes a retinal rod photoreceptor protein and is defective in Stargardt disease, a common hereditary form of macular dystrophy” that arises in young adults. Allikmets et al. found 13 alterations in one allele of ABCR in 26 patients with ADM, and these alterations appeared to be associated with ADM.

    T. P. Dryja et al. “point out methodological deficiencies that call into question” how Allikmets et al. interpreted their results. They describe possible biases in sampling and analysis in the study as well as inconsistencies in “the projection of the incidence of Stargardt-gene mutations in ADM.” C. C. W. Klaver et al. also point to “a flaw in the interpretation of the data” and question aspects of diagnosis of ADM in the study. In response, Allikmets and his colleagues (M. Dean et al.) discuss the criticisms point by point and provide a table showing the “completed typing of all variants [in the ABCR gene] on 220 control [normal] individuals.” They state that their results show that ABCR mutations “may confer an increased risk to ADM” and conclude that they “look forward to the dissemination of data relevant to [their] hypotheses.”

    The full text of these comments can be seen at http://www.sciencemag.org/cgi/content/full/279/5354/1107a