Articles Alert

The Scientist has asked a group of experts to comment periodically upon recent articles that they have found noteworthy. Their selections, presented herein every issue, are neither endorsements of content nor the result of systematic searching. Rather, the list represents personal choices of articles the columnists believe the scientific community as a whole may also find interesting. Reprints of any articles cited here may be ordered through The Genuine Article, 3501 Market St., Philadelphia, Pa. 19104, or by telephoning (215) 386-4399. Department of Microbiology & Immunology University of Illinois Chicago Effective traffic control of nuclear- and mitochondrial-encoded proteins during assembly requires the participation of "chaperonin" proteins and polypeptide processing. Yeast proteins that end up between the inner and outer membranes are first transported across both membranes and then sent back across the inner membrane. F.-U. Hartl, W. Neupert, "Protein sorting to mitochondria: evolutionary conservations of folding and assembly," Science, 247, 930-8, 23 February 1990. (University of California, Los Angeles; University of Munich) Plants as well as animals respond to physical stimulation. Specific mRNA levels of the flowering plant Arabidopsis increase up to 100-fold within minutes of physical stimulation. The cadmodulin message also increases, implicating calcium's role in responses of the plant to its environment. J. Braam, R.W. Davis, "Rain-, wind-, and touch-induced expression of calmodulin and calmodulin-related genes in Arabidopsis," Cell, 60, 357-64, 9 February 1990. (Stanford University, Calif.) Plant viruses have a harder time getting from cell to cell than animal viruses do, because of the barrier of the plant cell wall. The RNA molecules of the tobacco mosaic virus (TMV) move through the intercellular cytoplasmic channels (the plasmodesmata), using the P30 nucleic acid-binding protein. Overproduction of P30 in E. coli and in vitro experiments show that P30 coats both single-stranded RNA and DNA rather indiscriminately. The nucleic acid-binding determinant of P30 has been localized to a 21-amino acid (out of 283) region. V. Citovsky, D. Knorr, G. Schuster, P. Zambryski, "The P30 movement protein of tobacco mosaic virus is a single-strand nucleic acid binding protein," Cell, 60, 637-47, 23 February 1990. (University of California, Berkeley) We usually hear of the polymerase chain reaction (PCR) in the context of either "DNA fingerprinting" or tests for the HIV viral cDNA. In a chatty "how-I-did-it" account of brainstorming while driving through a dark California night, Mullis relays the conception and development of PCR at Cetus. K.B. Mullis, "The unusual origin of the polymerase chain reaction," Scientific American, 262, 56-65, April 1990. (private consultant, La Jolla, Calif.) Department of Earth Sciences Open University Milton Keynes, U.K. In the search for major craters that might have resulted from the postulated Cretaceous/Tertiary (K/T) asteroid impact, the twin impact structure Kara/Ust-Kara in the USSR has been investigated. Unfortunately, with an age of at least 70 million years, it is too old to be associated with the K/T mass extinctions; however, it may be related to the lesser extinctions at the Campanian-Maastrichtian boundary. C. Koeberl, V.L. Sharpton, A.V. Murali, K. Burke, "Kara and Ust-Kara impact structures (USSR) and their relevance to the K/T boundary event," Geology, 18, 50-53, January 1990. (Lunar and Planetary Institute, Houston; University of Vienna, Austria) (Mg,Fe)SiO3 perovskite is thought to be the most abundant mineral on earth, accounting for about 50 percent of the whole planet and 70 percent to 100 percent of the lower mantle. Yet information about this mineral is sparse. New hardness tests on single crystals of the iron-free end-member, MgSiO3 perovskite, show that this mineral is about twice as hard as typical upper-mantle minerals. Thus, by implication, it is significantly more resistant to plastic flow and, hence, to convective motions. S.-i. Karato, K. Fujino, E. Ito, "Plasticity of MgSiO3 perovskite: the results of microhardness tests on single crystals," Geophysical Research Letters, 17, 13-16, January 1990. (University of Minnesota, Twin Cities; Ehime University, Japan; Okayama University, Japan) A new analysis of the past intensity of the earth's magnetic field using only the most reliable data suggests that the strength of the geomagnetic dipole has been more or less constant since the late Cretaceous. During the Mesozoic, however, it had only about a third of its present value. M. Pr‚vot, M. E.-M. Derder, M. McWilliams, J. Thompson, "Intensity of the Earth's magnetic field: evidence for a Mesozoic dipole low," Earth and Planetary Science Letters, 97, 129-39, February 1990. (CNRS, Montpelier and Paris; Universit‚ des Sciences et Techniques du Languedoc, Montpelier, France; CRAAG, Bouzareah, Algeria; Stanford University, Calif.) Stacked images of long-period seismic data indicate major discontinuities in the upper mantle at depths of about 400, 520, and 670 kilometers. The first and last of these have long been widely accepted. The 520-kilometer discontinuity appears in few models and thus gains new credence here. However, the images show no sign of the 220-kilometer discontinuity found in numerous other models (for example, the Preliminary Reference Earth Model). P.M. Shearer, "Seismic imaging of upper-mantle structure with new evidence for a 520-km discontinuity," Nature, 344, 121-6, 8 March 1990. (Scripps Institution of Oceanography, La Jolla, Calif.) The records collected over 25 years by the United Kingdom's International Seismological Centre (ISC) form one of the world's major seismological databases. It would appear, however, that they contain nonrandom errors in travel times, with low-gain seismographs systematically reporting late seismic-wave arrivals. This does not mean that all work based on ISC travel times is suspect. It does suggest, however, that care should be exercised in studies in which travel-time differences of one second or less are crucial. S.P. Grand, "A possible station bias in travel time measurements reported to ISC," Geophysical Research Letters, 17, 17-20, January 1990. (University of Texas, Austin) Department of Chemistry University of Texas Austin A Langmuir lecture describes ways in which translational activation, collision-induced activiation, and collision-induced desorption can be invoked to explain the different chemistries observed at high and low reactant pressures. Now, the requirement for high pressure can be bypassed through the use of molecular beams as synthetic tools. S.T. Ceyer, "Translational and collision-induced activation of CH4 on Ni(111): phenomena connecting ultra-high-vacuum surface science to high-pressure heterogeneous catalysis," Langmuir, 6, 82-7, January 1990. (Massachusetts Institute of Technology, Cambridge) As part of an entire issue on materials, one article in Science explores the chemistry of electrides - the alkali metal cations complexed with crown ethers or cryptands in which a trapped electron acts as the counterion. Depending on the degree of localization of the electron, either high or low conductivity can be observed and correlated with crystallographically determined structures. J.L. Dye, "Electrides: ionic salts with electrons as anions," Science, 247, 663-9, 9 February 1990. (Michigan State University, East Lansing) As a contribution to a Harry Drickamer Festschrift (honoring his 70th birthday), Drickamer's colleagues have provided a concise summary of how pressure alters conformational substate populations and changes both static and dynamic properties of protein ensembles. Pressure jumps reveal several relaxation modes and can control small molecule (CO and O2) binding through activation volume effects and population redistributions. The paper that is presented is the summary of an initial study and 11 years' subsequent exploration into its results. H. Frauenfelder, N.A. Alberding, A. Ansari, D. Braunstein, et al., "Proteins and pressure," Journal of Physical Chemistry, 94, 1024-37, 8 February 1990. (University of Illinois, Urbana) Chemical intermediates are often highly reactive and unstable. Therefore, new explorations in studying their chemistry by neutralizing their more stable ionic counterparts are welcome. Reionization to the same ionic structure after microsecond delays, via mass spectroscopic methods, allows for characterization of both stability and unimolecular chemistry of these species. F.W. McLafferty, "Studies of unusual simple molecules by neutralization-reionization mass spectrometry," Science, 247, 925-9, 23 February 1990. (Cornell University, Ithaca, N.Y.) Base pairing, the same feature thought to be relevant for self-replication of nucleic acids, can also enhance acyl transfer. This, in turn, can induce autocatalysis in a template-arranged coupling of an amino adensoine to a pentafluorophenyl ester. A link has been established between the information of nucleic acids and the synthesis of amide bonds. T. Tjivikua, P. Ballester, J. Rebek, Jr., "A self-replicating system," Journal of the American Chemical Society, 112, 1249-50, 31 January 1990. (Massachusetts Institute of Technology, Cambridge) Many excited-state processes that involve transition metal complexes include formal 19-electron intermediates. One study in a special issue on the photochemistry and photophysics of coordination compounds, edited by P.C. Ford and R.J. Watts establishes the existence of and describes the synthesis and spectroscopic properties of a stable 19e complex: Cp'Mo(CO)2(L2- P,P'). D.R. Tyler, F. Mao, "Mechanistic studies of nineteen-electron organometallic complexes: synthesis of stable nineteen-electron complexes," Coordination Chemistry Reviews, 97, 119-40, January 1990. (University of Oregon, Eugene) School of Natural Sciences Institute for Advanced Study Princeton, N.J. Precision measurements of the Z boson lifetime has generated considerable excitement recently. The Z boson lifetime, which gives information about the number of different neutrino species, is a value of great interest in cosmology. A complete survey of the situation prior to these measurements provides a self-contained, elementary but masterly, account of the relevant cosmology. D. Denegri, B. Sadoulet, M. Spiro, "The number of neutrino species," Reviews of Modern Physics, 62, 1-42, January 1990. (CERN, Geneva; Centre d'Etudes Nucl‚aires de Saclay, Gif-sur-Yvette, France; University of California, Berkeley) Strange and wonderful things are happening at the center of our galaxy. There is impressive evidence that a massive (greater than 106 solar masses) black hole lives there; bathed in an inrushing whirlpool of inpouring gas, exerting huge gravitational fields on nearby objects. Observations, and some of the ingenious techniques used, are described. C. Townes, R. Genzel, "What is happening at the center of our galaxy?" Scientific American, 262, 46-55, April 1990. (University of California, Berkeley; Max Planck Institute, Mainz, W.Ger.) A classic (and continually refined) chapter of mathematical physics deals with proof of the stability of matter: why it doesn't all collapse in a heap (not obvious, because there are singular attractive forces!). A masterly and quite accessible recent survey of this deep and delicate subject, by one of the main contributors to the field, is given. E.H. Lieb, "The stability of matter: from atoms to stars," Bulletin of the American Mathematical Society, 22, 1-49, January 1990. (Princeton University, N.J.) Fundamental physical theory is dominated at present by ideas of symmetry and gauge invariance. Roughly speaking, gauge invariance means that the laws of physics are most simply expressed (specifically, are local) in terms of variables that form a highly redundant description of the physical world. Most stated equations, therefore, consist of the relationships between arbitrary names. It is a real challenge, then, to extract their physical meaning. In this context, it often becomes difficult even for experts to relay their findings. A profound analysis is presented. M. Henneaux, C. Teitelboim, J. Zanelli, "Gauge invariance and degree of freedom count," Nuclear Physics, B332, 169-88, 26 February 1990. (Centro de Estudios Cientificos de Santiago, Chile; Universit‚ Libre de Bruxelles, Belgium; University of Texas, Austin; Universidad de Chile, Santiago) Two leaders in theoretical physics have presented an accessible, up-to-date summary of the current state of turbulence - quite an outstanding challenge in description alone. U. Frisch, S.A. Orszag, "Turbulence: challenges for theory and experiment," Physics Today, 43, 24-32, January 1990. (Observatoire de Nice, France; Princeton University, N.J.)

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