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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. Author: SIMON SILVER Department of Microbiology & Immunology University of Illinois Chicago Understanding generally brings a more complicated, but also more rational, picture of any given study. Developments with Neurospora show that small 3.6-kilobase plasmids that occur in mitochondria as double-stranded DNA replicate via an RNA intermediate, using a 710-amino-acid DNARNA transcriptase encoded by the plasmid. M.T.R. Kuiper, J.R. Sabourin, A.M. Lambowitz, "Identification of the reverse transcriptase encoded by the Mauriceville and Varkud mitochondrial plasmids of Neurospora," Journal of Biological Chemistry, 265, 6936-43, 25 April 1990. (Ohio State University, Columbus) Christian Gram couldn't have known the questions his reaction mechanism would bring about over these past 100 years. The Gram stain continues to be useful, even though its basis is poorly understood. Gram negatives have an outer membrane, but what about Gram-variable bacteria? An adaptation of the Gram stain to electron microscopy increases our understanding. T.J. Beveridge, "Mechanism of Gram variability in select bacteria," Journal of Bacteriology, 172, 1609-20, March 1990. (University of Guelph, Ontario) Oxygen, although vital, is also toxic. Oxygen-protecting processes in bacteria are turned on by the product of the oxyR regulatory gene, which binds to a series of DNA promoters constituting a "regulon" for stress response. G. Storz, L.A. Tartaglia, B.N. Ames, "Transcriptional regulator of oxidative stress-inducible genes: direct activation by oxidation," Science, 248, 189-94, 13 April 1990. (University of California, Berkeley) The world hardly needs another journal, but the American Association for Cancer Research wanted one. The first issue of this oncojournal includes an article describing a protooncogene that functions in the oocyte and early egg stages during Xenopus differentiation. This protooncogene can be turned on artificially in a kidney cell line. M.S. Kindy, I.M. Verma, "Developmental expression of the Xenopus laevis fos protooncogene," Cell Growth & Differentiation, 1, 27-37, January 1990. (Salk Institute, San Diego) Cantor will be both conductor and a first-chair player in the human genome project. Therefore, one would prefer that his recent paper focus more on the scientific developments that will make the project feasible, rather than committee structures. C.R. Cantor, "Orchestrating the human genome project," Science, 248, 49-51, 6 April 1990. (Lawrence Berkeley Laboratory, Berkeley, Calif.) Genetic rearrangements that involve translocatable elements are found throughout living forms--in plants, bacteria, and animals. Sometimes these have common features. K. Sakaguchi, "Invertrons, a class of structurally and functionally related genetic elements that include linear DNA plasmids, transposable elements, and genomes of adeno-type viruses," Microbiological Reviews, 54, 66-74, March 1990. (Nihon Shokuhin Kako Laboratories, Tokyo) Author: BRUCE G. BUCHANAN Department of Computer Science University of Pittsburgh Pittsburgh Remote procedure calls (RPCs) allow a program running on one computer in a network to invoke a program on another machine. The design and implementation of parallel RPCs in the Andrew distributed computing environment at Carnegie Mellon University (CMU) are reported, and some of the advantages of this design are analyzed. M. Satyanarayanan, E.H. Siegel, "Parallel communication in a large distributed environment," IEEE Transactions on Computers, 39, 328-48, March 1990. (CMU, Pittsburgh) Error-free computation is impossible to guarantee, for reasons of failures in either hardware or software. A recent survey paper reviews the effects of software on reliability and describes a model of "resourceful systems." These systems can determine whether or not goals have been achieved, and, if not, will carry out alternative plans. R.J. Abbott, "Resourceful systems for fault tolerance, reliability, and safety," ACM Computing Surveys, 22, 35-68, March 1990. (Aerospace Corp., Los Angeles) Circumscription is a technique for formalizing some aspects of common sense reasoning that introduces a requirement of minimality: If a formula, P, is satisfied, then no proper subset of P is. Finding ways to express circumscription without introducing second-order formulas has been an active research area. Results of this research are extended, and the computational complexity of model checking for circumscription is analyzed. P.G. Kolaitis, C.H. Papadimitriou, "Some computational aspects of circumscription," Journal of the Association for Computing Machinery, 37, 1-14, January 1990. (University of California, Santa Cruz and San Diego) Explanation-based learning is a relatively new technique for abstracting a solution to a specific problem. The solution will then be used on later problems, based on an explanation of why the solution works. Research is reported that extends the technique to generalize over iterative or recursive processes. Results of the BAGGER2 learning algorithm are presented. J.W. Shavlik, "Acquiring recursive and iterative concepts with explanation-based learning," Machine Learning, 5, 39-70, March 1990. (University of Wisconsin, Madison) Computer chess has become an area of intense interest. Better performance results in recent years are due to deeper searches that are allowed by faster and larger machines. An argument is made that increasing the amount of knowledge will also be necessary for World Championship programs. Several other articles about chess programs appear within the April issue of Artificial Intelligence (AI). H. Berliner, G. Goetsch, M.S. Campbell, C. Ebeling, "Measuring the performance potential of chess programs," AI, 43, 7-20, April 1990. (CMU, Pittsburgh; University of Washington, Seattle) Author: PETER J. SMITH Department of Earth Sciences Open University Milton Keynes, U.K. The Airborne Arctic Stratospheric Expedition (AASE), conducted from Stavanger, Norway, in the winter of 1988-89 established that the same physical and chemical processes that create an ozone hole over Antarctica also occur in the Arctic polar stratosphere. However, the effects are less marked in the Arctic because of differences in chemical state. Measurements made during the AASE suggest that during the 39-day period of flights into the Arctic vortex, the loss of ozone was about 12 percent at levels with potential temperatures of 440øK to 470øK. There are 63 articles about the mission within a special supplement to the March issue of Geophysical Research Letters, sharing new information about the polar winter stratosphere and global ozone depletion. Geophysical Research Letters, 17, 313-564, March 1990 (supplement). The possible importance of carbonatites as an influence on upper mantle behavior has only recently been appreciated, but hitherto there have been no rheological data on molten carbonatites. Such data are now available from the remote Oldoinyo Lengai volcano (in the northern Tanzania sector of the Gregory Rift Valley), which is the world's only active producer of carbonatites. The carbonatite lava temperatures (575°C to 593°C) are lower than those of any recorded silicate flows and the viscosities of gas-free samples (1 to 5 Paús) are lower than any ever recorded for magma. J.B. Dawson, H. Pinkerton, G.E. Norton, D.M. Pyle, "Physiochemical properties of alkali carbonatite lavas: data from the 1988 eruption of Oldoinyo Lengai, Tanzania," Geology, 18, 260-3, March 1990. (University of Edinburgh, Scotland; University of Lancaster, England; University of Cambridge, England) Acanthostega, one of the earth's oldest known tetrapods, has been studied in its sedimentological context for the first time. The evidence indicates that the tetrapod was primarily aquatic and inhabited active fluvial channels in fluvial-dominated environments. S.E. Bendix-Almgreen, J.A. Clack, H. Olsen, "Upper Devonian tetrapod paleoecology in the light of new discoveries in East Greenland," Terra Nova, 2, 131-7, 1990. (Geologisk Museum, Copenhagen, Denmark; University Museum of Zoology, Cambridge, England; Geological Survey of Greenland, Copenhagen, Denmark) At 9°30'N, the fast-spreading East Pacific Rise with its broad topography implies an abundant magma supply and a large magma chamber (tens of kilometers wide). However, new seismic reflection data suggest the presence only of a horizontal lens of magma no more than 800 to 1,200 meters wide and 10 to 50 meters thick. G.M. Kent, A.J. Harding, J.A. Orcutt, "Evidence for a smaller magma chamber beneath the East Pacific Rise at 9°30'N," Nature, 344, 650-3, 12 April 1990. (Scripps Institution of Oceanography, La Jolla, Calif.) Dikes - sheetlike intrusions cutting across the bedding or foliation of country rock at high angles - are common geological features. But why do they so often form in isotropic country rock in preference to stocks or batholiths? Theoretical analysis suggests that dike formation is favored by the relatively low viscosity of basaltic and most granitic magmas. Stocks and batholiths are generally created only from less common, high-viscosity granitic magma. S.H. Emerman, R. Marrett, "Why dikes?" Geology, 18, 231-3, March 1990. (Cornell University, Ithaca, N.Y.) Author: MARYE ANNE FOX Department of Chemistry University of Texas Austin Despite the obvious importance of hydrogen bonding in controlling associative and structural phenomena in water, hydrogen bonding preferences among multiply intertwined patterns sometimes remain obscure. Empirical rules are developed within a recent paper for predicting preferential hydrogen bonding in solid-state structures, which are used in turn as indicators for parallel effects in solution. M.C. Etter, "Encoding and decoding hydrogen-bond patterns of organic compounds," Accounts of Chemical Research, 23, 120-6, April 1990. (University of Minnesota, Minneapolis) A plenary lecture at the Fifth International Symposium on Organometallic Chemistry Directed Toward Organic Synthesis summarizes several new approaches to functionalized chromium carbene complexes. Many of the included complexes contain optically active auxiliaries, and activation by photolysis produces species that exhibit ketene or metal-bound ketene character. These species can be trapped to form chiral amino acids or alpha-lactams in high chemical and optical yields. L.S. Hegedus, "Chromium carbene complexes in the synthesis of molecules of biological interest," Pure and Applied Chemistry, 62, 691-8, April 1990. (Colorado State University, Fort Collins) Although steady-state current voltage experiments have led to the suggestion that charge transfer between semiconductors and outer sphere redox couples may be extremely fast, simple theory predicts much longer carrier lifetimes. Microwave and radio-frequency conductivity measurements reported detect remarkably long excess carrier lifetimes (>0.2 ms), underscoring the usefulness of real-time techniques in elucidating charge-transfer kinetics. M.D.E. Forbes, N.S. Lewis, "Real-time measurements of interfacial charge transfer rates at silicon/liquid junctions," Journal of the American Chemical Society, 112, 3682-3, 25 April 1990. (California Institute of Technology, Pasadena) Highly ordered, three-dimensional oligomers formed by reiterative binding of monomers allows for the preparation of discretely staged macromolecules, or "generations." The design features of these dendrimers (size, shape, surface) are controlled by the chemical reaction and the choice of monomer. Possible applications in mimicking biological functions, such as drug carriers or immunogens, or in device fabrication are discussed. D.A. Tomalia, A.M. Naylor, W.A. Goddard III, "Starburst dendrimers: molecular-level control of size, shape, surface chemistry, topology, and flexibility from atoms to macroscopic matter," Angewandte Chemie, International Edition, 2, 138-75, February 1990. (Dow Chemical Co. and Michigan Molecular Institute, Midland; et al.)

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