This Week in Science

Science  19 Sep 1997:
Vol. 277, Issue 5333, pp. 1737
  1. Origins of sterol diversity

    Isoprenoids are found as, for example, visual pigments, reproductive and mating hormones and steroids, and membrane constituents. Abnormalities in their biosynthesis can lead to heart disease and cancer, whereas isoprenoids such as taxol offer promise as beneficial drugs. The diverse isoprenoids are generated from compounds with 5, 10, 15 and 20 carbon atoms that form substrates for many enzymes, primarily cyclases, as discussed in a Perspective by Sacchettini and Poulter (p. 1788). Lesburg et al. (p. 1820), Starks et al. (p. 1815), and Wendt (p. 1811) describe the structures of three of these cyclases and discuss the mechanism of protonation.

  2. Small steps in solution

    Charging of small structures, such as quantum dots or single molecules, can occur in a series of discrete one-electron (or coulomb) steps. Fan and Bard (p. 1791) show that with sensitive detection methods, such charging steps can also be observed for nanometer-scale electrodes in solutions containing reactants that form an oxidation-reduction couple.

  3. Folding conjugated polymers

    A relative simple polymer made up of identical aromatic hydrocarbon units displays solvent-driven folding behavior. Nelson et al. (p. 1793; see the news story by Pennisi, p. 1764) show that a polymer with a phenylacetylene backbone folds into a compact helical structure, and the transition depends on chain length, solvent, and temperature. Nonspecific forces drive the transition, as there are no hydrogen-bonding interactions.

  4. A light touch

    Certain chalcogenide glasses are known to exhibit photoinduced anisotropy; these materials absorb or refract polarized light differently in various directions. Krecmer et al. (p. 1799; see the Perspective by Tanaka, p. 1786) show that upon absorption of polarized light, the films contract parallel to the electric vector of the polarized light and dilatate orthogonal to it. The effect may be utilized in nanotechnology, where positioning with a precision to within a nanometer is required.

  5. Early mounds

    Early Native Americans built several complex mounds at Watson Brake, Louisiana, marking one of the beginnings of complex society in the Western Hemisphere. Saunders et al. (p. 1796; see the news story by Pringle, p. 1761) obtained many radiocarbon dates from the mounds and show that these were built about 5000 to 5400 years ago, much earlier than was thought. Evidently the mounds were occupied seasonally.

  6. Carbohydrate clues to disease

    A molecular basis for a fatal, neurodegenerative disease, late infantile neuronal ceroid lipofuscinosis, has been found with an approach that should be useful in studying other lysosomal storage diseases. Sleat et al. (p. 1802) show that newly synthesized soluble lysosomal enzymes contain a modified carbohydrate, mannose-6-phosphate (man-6-P) that acts as a targeting signal. The receptor for man-6-P was used to pull out the modified proteins and look for differences between patients and controls. The sequence of the gene resembles proteases, and it is deficient in the brains of patients with the disease.

  7. Genetics of vision loss

    Age-related macular degeneration (AMD) is the primary cause of severe vision loss among the elderly. Allikmets et al. (p. 1805; see the news story by Pennisi, p. 1765) examined 167 unrelated AMD patients and found that 26 of them had mutations in ABCR, a gene that codes for an adenosine triphosphate-binding transporter protein in retinal photoreceptors. Mutations in the same gene had been identified in patients with Stargardt's disease, an inherited form of macular dystrophy. This discovery may lead to presymptomatic testing for AMD and new strategies for prevention and therapy.

  8. All wrapped up

    The structure of the DNA exonuclease from bacteriophage lambda has been determined, and it reveals some of the reasons for its functions. Kovall and Matthews (p. 1824) found that the functional exonuclease would wrap around the DNA. The tunnel formed by the protein accommodates double-stranded DNA at one end but only single-stranded DNA at the other. In between lie the active degradative sites. The assembled structure does not leave a DNA strand until it falls off the end, or its own subunits dissociate; thus, the observed biochemical function is processivity.

  9. Cichlid loss: A murky tale

    The colorful cichlid fish in Lake Victoria, notable for their rapid evolution, are now also notable for their rapid decline: Half of all species have disappeared in recent years. Seehausen et al. provide a likely explanation (p. 1808). Cichlid coloration and species diversity are maintained by color-associative mating, which is correlated with light conditions. Eutrophication of the lake is increasing the turbidity of the water, which is leading to the breakdown of reproductive barriers and loss of diversity. Thus, diversity is being rapidly lost in the absence of classical extinction.

  10. Getting a good grip

    The primary structure of one of the collagens secreted by the foot of the mussel is presented by Coyne et al. (p. 1830; see the Perspective by Engel, p. 1795). Flanking the central structural portion, which forms the hallmark collagen triple helix, are domains with sequences similar to those found in elastin (a chief component of tendons and ligaments) and spider silk. Outside of these domains are zinc-binding sequences that may help bundle individual molecules into the tough but pliable byssal threads that anchor the mussel to rock.

  11. Using vitamin D

    Vitamin D is critical to healthy bones and a responsive immune system. Its absence, or inability to metabolize it, results in rickets. Vitamin D, available from dietary sources, must be converted within the body into the physiologically active form. Takeyama et al. (p. 1827) have identified in mouse the enzyme that manages this step, 1α-hydroxylase that resides in the kidney.