This Week in Science

Science  07 Nov 1997:
Vol. 278, Issue 5340, pp. 989
  1. Taking the measure of the ribosome

    During translation, transfer RNAs (tRNAs) bind through their anticodon stem-loops to the codons of messenger RNA, but ribosomal RNA (rRNA) itself also plays a role in tRNA binding. Joseph et al. (p. 1093) have used a cleavage method to map out regions of rRNA near the tRNA binding sites. RNA probes were constructed that consisted of stem-loops and 4 to 33 base pairs of RNA; Fe(II) ions attached to the end of RNA helix were used to generate hydroxyl radicals that cleaved nearby rRNA. The cleavage patterns were then used to model the three-dimensional structure of rRNA near the A and P sites.

  2. Lunar time

    The moon has generally been thought to have formed following the impact of a Mars-sized planetesimal into the early Earth. Lee et al. (p. 1098) investigate this early planetary history by measuring tungsten-hafnium isotopes in many lunar samples. This isotope system is useful for assessing early planetary evolution because tungsten-182 has a short half-life (about 9 million years) and tungsten tends to fractionate into a metal core if it forms. The data imply that the moon formed about 4.5 billion years ago.

  3. Good alignment

    The direct measurement of bond distances and angles in macromolecules by nuclear magnetic resonance (NMR) is reported by Tjandra and Bax (p. 1111; see the news story by Service, p. 1015). They dissolved macromolecules in dilute aqueous solutions containing discotic nematic liquid crystals and magnetic particles. When the magnetic field was applied, the liquid crystalline media tended to align the macromolecule (in this case, the protein ubiquitin) so that dipolar couplings between nuclei, instead of canceling to zero, could be measured. The structure obtained is in excellent agreement with the known x-ray structure of ubiquitin, and the method should extend the size range of molecules that can be studied by solution NMR.

  4. Eurasian climate record

    Long-term records of continental climate stretching back beyond even about 1 million years ago have been obtained from Antarctica and the Chinese loess sequences. Williams et al. (p. 1114) now describe a record for Lake Baikal in central Eurasia that extends back about 5 million years. Analysis of the records, primarily of biogenic silica accumulation, implies that the climate response to orbital variations in the interior of the Eurasian continent was similar to that in the oceans throughout the long record.

  5. Selenium in soil

    Selenium is a major contaminant in many watersheds and soils, and its mobility depends on its oxidation state. Many soils and sediments also contain an iron oxide called green rust. Myneni et al. (p. 1106) show that a series of abiotic reactions involving green rust reduce selenium in sediments and thus affect selenium cycling and perhaps the cycling of other trace elements in soil.

  6. Follows convention

    Low-barrier hydrogen bonds (LBHBs) consist of a proton that is shared equally between two atoms. Although LBHBs are known to exist in the gas phase, they also have been proposed to exist within the active sites of enzymes and help stabilize the transition state, thus accelerating the reaction. Ash et al. (p. 1128), using nuclear magnetic resonance spectroscopy, reexamine this issue as it applies to the catalytic triad of a serine protease as well as a small molecule model of the catalytic amino acids. The hydrogen bond formed by the aspartate-histidine dyad appears to be of the conventional kind and not LBHB-like.

  7. Cold tongue

    For endotherms such as whales to flourish in richly productive high-latitude waters, they must have overcome a considerable potential constraint—loss of heat through the tongue. The tongues of baleen whales can be particularly large and, unlike the skin, cannot be insulated by a thick blubber layer. Heyning and Mead (p. 1138) show that the tongues of gray whales contain a large countercurrent heat exchanger. Temperature measurements at the oral cavity of a live gray whale suggest that this heat exchanger can considerably reduce such heat loss when the whale feeds in cold polar and subpolar waters.

  8. Class distinctions

    The enzymes that charge transfer RNAs (tRNAs) with their amino acids (tRNA synthetases, or RSs) have appeared to distribute into two classes in the same way in all organisms. In certain Archaea species (methanogens), however, no lysyl RSs, a class II RS, could be identified. Ibba et al. (p. 1119) have purified this enzyme from one organism and cloned the corresponding gene, which was found to correspond to unassigned open-reading frames in two other methanogens. The sequence suggests that this lysyl RS actually belongs to class I, which also raises questions concerning RS evolution.

  9. Skin-deep response

    Control of limb movement involves a complicated interchange between sensory and motor neurons and muscles. Viana Di Prisco et al. (p. 1122; see the Perspective by Grillner, p. 1087) examined how sensory inputs from the skin translated into tail movements in lamprey. At a certain threshold of stimulation, inputs into reticulospinal neurons are transformed into motor commands.

  10. Evading attack

    Pathogenic bacteria secrete virulence proteins that help them escape killing by the defenses of the host. Attempts to identify an amino acid sequence or other physical properties of such proteins that might target them to the appropriate secretion system have been unsuccessful. Anderson and Schneewind (p. 1140; see the Perspective by Silhavy, p. 1085) report that in Yersinia enterocolitica, a bacterium that infects animals and humans, translation and secretion of virulence factors appear to be closely coupled. Furthermore, frameshift mutations that completely alter the amino acid sequence in the critical region still allowed appropriate secretion. Thus, an intrinsic property of the messenger RNA during translation appears to provide the signal for secretion.

  11. Ins and outs of sea ice

    The extent of sea ice has been recognized to have been decreasing in the Arctic, but patterns for the Antarctic have been more uncertain. Cavalieri et al. analyzed patterns around both poles from 1978 to 1996 using satellite data. The results confirm that the extent of Arctic sea ice has decreased, but show that sea ice has expanded slightly in the Antarctic Ocean. This pattern is consistent with the results of some global climate models of the transient increase in atmospheric carbon dioxide levels.

  12. Transforming ruby with a little heat

    Ruby (chromium-doped aluminum oxide) is used as a pressure calibrant in diamond-anvil cells. These experiments continue to achieve higher and higher pressures (up to hundreds of gigapascals), so it is important to ensure that the structure of the ruby is well defined under such conditions. Recent theoretical work has indicated that there is a structural transformation between about 70 to 90 gigapascals. Funamori and Jeanloz have experimentally confirmed a structural transformation of ruby, probably to the Rh2O3 (II) structure, upon heating in a diamond-anvil cell between 70 to 100 gigapascals. Heating the sample may have helped initiate an otherwise kinetically inhibited phase change, and thus more experiments will be required to understand the high-pressure and high-temperature structural changes of this routinely used pressure calibrant.

  13. Loss begets loss

    What effect does the partial clearance of the rain forest have on the functioning of the remaining forest fragments? Laurance et al. report that fragmented forests in the Amazon are losing considerable aboveground biomass—up to one-third in some fragments. Sharply elevated tree mortality at the fragment edges is likely caused by microclimatic changes and wind disturbance. This long-term study indicates that the losses are not offset by the recruitment of new trees. [See the news story by Williams.

  14. Protein patching

    Protein-protein interfaces, such as those between a protein and its receptor, often include critical amino acids that, when mutated or deleted, disrupt binding. Atwell et al. show one way that proteins can recover from such disruptions. They deleted a critical tryptophan residue from the receptor for human growth hormone, and then constructed a library and screened for mutants that recovered binding activity. They found a protein with five new mutations, and an x-ray crystal structure revealed how this mutation filled the large cavity produced when the bulky tryptophan residue was deleted.

  15. Turned on by lipids

    Many cell surface receptors send signals into the cell through interaction with heterotrimeric guanine nucleotide binding proteins (G proteins). The G proteins are reversibly palmitoylated, but the functional consequences of such lipid modification were unknown. Tu et al. report that palmitoylation of the a subunit of the G protein Gz may prolong signaling by that protein. The active form of the G protein has guanosine triphosphate (GTP) bound, and hydrolysis of the GTP to GDP inactivates the signal. Palmitoylation reduced the response of Gαz to Gz GAP or GAIP, proteins that normally enhance GTPase activity of Gaz. Thus, the main function of palmitoylation appears to be control of the sensitivity of G proteins to inactivation by GTPase activating proteins.

  16. Tracking visual information

    The visual processing streams that mediate spatial recognition and object identity travel from the primary visual cortex, at the back of the brain, to the prefrontal cortex via dorsal and ventral pathways. In parallel with proposals that these pathways begin to merge within this brain area, Ó Scalaidhe et al. provide further evidence in support of the view that these aspects of visual information, commonly known as “where” and “what,” remain largely segregated. They map the locations of neurons in the monkey prefrontal cortex that are responsive to faces, as an exemplar of object identity function, and find them restricted to the inferior convexity, suggesting that the remixing of sensory input and the integration of what and where occurs elsewhere.