# This Week in Science

Science  08 Sep 2000:
Vol. 289, Issue 5485, pp. 1649
1. # The Big-Ocean Chill

How Pacific sea surface temperatures (SSTs) have differed between glacial and interglacial periods is especially important for understanding these climate changes not only because of the Pacific's size, but because its equatorial region dominates the global tropical maritime region. Lea et al. (p. 1719; see the Perspective by Nürnberg) present planktonic foraminiferal magnesium-calcium and oxygen isotopic records for the past 450,000 years and show that Pacific SSTs were 3° to 5°C cooler during the glacial periods. The surface of the Pacific cooled along with Antarctic air but ahead of continental ice-sheet melting.

2. # Thin Acoustic Shields

Periodic modulations in sound velocity can be used to shield sound within a certain frequency range in a manner analogous to the blocking of light by photonic band gap materials. However, the spatial modulation must be of the same order of magnitude as the sonic wavelength, which would make such structures impracticably large. Liu et al. (p. 1734) show that composites with locally resonant structural units can shield sound with lattice units two orders of magnitude smaller than the relevant sonic wavelength. By varying the size and geometry of the structural units, frequency ranges over which the material acts as a sonic shield can be varied. Thus, relatively thin layers of material can be used as selective sound shields.

3. # Uprooted at the End

The end of the Permian (the Permian to Triassic or P/T boundary, about 250 million years ago) recorded the largest number of marine and vertebrate species extinctions. The cause of this massive extinction is unknown. Ward et al. (p. 1740; see the news story by Kerr) offer new evidence for the rapid die-off of rooted plant life in the Karoo basin of South Africa. Analysis of the change in morphology in seven stratigraphic sections indicates that the fluvial system changed from meandering rivers to braided rivers at the P/T boundary. These fluvial changes were caused by increased sedimentation related to enhanced erosion after the disappearance of rooted plants. Similar fluvial changes have been suggested at other P/T boundary sites around the world and indicate that an event such as an impact caused global-scale changes on the continents.

4. # Skating on Thinner Ice

Records of the freezing and thawing of rivers and lakes have been recorded in some cases for hundreds of years and should provide another perspective on recent climate changes. Magnuson et al. (p. 1743) compile these records from throughout the Northern Hemisphere. Lakes and rivers consistently froze later and became ice-free earlier during the past 150 years. This pattern is consistent with a global temperature increase of about 1.2°C during the past century.

5. # Channeling Heat in One Dimension

Potential applications in micro-electro-mechanical devices have made the issues of electron-phonon coupling and heat dissipation in carbon single-wall nanotubes increasingly important. Specific heat measurements by Hone et al. (p. 1730; see the Perspective by de Heer) indicate that the thermal vibrations (phonons) in the tubes are confined to one dimension. Moreover, they find that there is little coupling between the tubes in nanotube bundles, which suggests that bundles may not be so strong.

6. # Anomalous Earthquake Damage

The magnitude 6.7 Northridge earthquake in 1994 caused an anomalous concentration of damage to buildings and infrastructure in Santa Monica, located about 20 kilometers south of the epicenter. Davis et al. (p. 1746) simulated the event and conclude that faults bounding the northwestern edge of the Los Angeles basin created acoustic lenses that focused the seismic energy to the surface at Santa Monica. Such modeling and continued simulations of complex subsurface seismic wave propagation may help in assessments of seismic hazards, particularly in tectonically active urban areas.

7. # Purple in Tooth and Claw

Photosynthesis relies on chlorophyll, a light-absorbing molecule based on the porphyrin ring with a variety of modifications and additions. Xiong et al. (p. 1724) have sequenced genes encoding biosynthetic enzymes for bacteriochlorophyll from green sulfur and green nonsulfur bacteria. They then combined these data with similar data from other photosynthetic bacterial lineages to construct a molecular phylogeny of photosynthesis (which, of course, need not precisely replicate the phylogeny of photosynthetic organisms). They conclude that photosynthesis appeared first in the purple bacteria and relatively late in the cyanobacteria, the likely ancestor of present-day chloroplasts in plants. In a Perspective, et al. (p. Des Marais (see the cover) places these findings in a geochemical context.

8. # Split Response to Steroids

In mammals, the receptor for the steroid reproductive hormone progesterone is encoded by a single gene, but the protein exists in two forms—progesterone receptor-A (PR-A) and PR-B—that result from alternative starting points for transcription and translation. Mulac-Jericevic et al. (p. 1751) describe mice in which a mutation selectively prevents expression of PR-A. Acting alone, PR-B regulated only a subset of known progesterone-responsive genes in the uterus. Progesterone normally antagonizes estrogen-induced proliferation in the uterine epithelium, yet interactions with PR-B promoted proliferation. The separation of distinct physiological functions for PR-A and PR-B raises the possibility that selective modulators of the PR isoforms could provide more specific therapeutic effects.

9. # C'mon Back!

Useful as the oligodendrocyte precursor cells are to the central nervous system, given their ability to generate a steady supply of the cells that insulate neurons, it seems they can do even better. Kondo and Raff (p. 1754; see the news story by Vogel) now show that these limited-potential precursor cells can, with treatment by a series of external growth factors, be made to “back up” through the developmental cascade. The result is a source of stem cells with greater potential than the original that can form a variety of differentiated neuronal cell types.

10. # Probing Proteins in Packed Arrays

Many assays of protein function (such as protein-protein binding) would benefit from dense arrays of correctly folded proteins on a solid substrate. MacBeath and Schreiber (p. 1760; see the news story by Service). have used a contact-printing robot to create dense arrays of immobilized proteins on glass slides. Covalent attachment to aldehyde-derivatized glass can occur through a Schiff's base at several protein surface positions. Excess reactivity is quenched with bovine serum albumin (BSA), which also helps reduce nonspecific binding (peptide and small protein arrays were made on activated BSA monolayers). In a protein-binding assay, a single protein spot in 10,800 on a slide could be detected. Other applications demonstrated included a protein phosphorylation assay and a study of protein targets for small molecules.

11. # Making the Right Tag

DNA detection by hybridization to an array of fragments has wide applications in gene analysis and medicine. Taton et al. (p. 1757) have found that if they use probes labeled with gold particles rather than fluorescent tags, the melting profile of hybrids between oligonucleotides and DNA fragments immobilized on a glass chip had a much steeper slope, which resulted in increased sensitivity and selectivity. With the addition of a silver deposition step, the response was sufficient to allow stain density quantification with a standard flatbed scanner.

12. # Climate and Disease

The alarm has been raised that global warming will inevitably bring an increased threat of vector-borne diseases, such as malaria, to higher latitudes. In contrast, the multivariate approach used by Rogers and Randolph (p. 1763; see the Perspective by Dye and Reiter), invoking both temperature and rainfall predictions from the Hadley Centre Global Climate Model, together with knowledge of the ecology of the parasite, suggests that the gains in malaria distribution in 2050 will, in fact, be rather modest, even under extreme conditions of change. Predicted gains are into the southern United States, westward in China, southward in Brazil, and some expansion in central Asia and into Turkey; Europe would remain largely unaffected. Climate has already been thought to play an important role in cholera outbreaks. Pascual et al. (p. 1766) now resolve some of the complexities in cholera dynamics in Bangladesh and show that while climate plays an important role, it is not the entire story. The authors examined an 18-year disease record and found that outbreaks could be related to previous disease levels and local temperature.

13. # The Dangers of Expansion

Myotonic dystrophy (DM), the most common form of muscular dystrophy in adults, is caused by the expansion of CTG repeats in the 3′ untranslated region of the DMPK protein kinase gene. DMPKtranscripts containing expanded CUG repeats are retained in the nucleus, and it has been hypothesized that nuclear accumulation of these aberrant transcripts, rather than functional alterations of the DMPK protein, causes pathology. Mankodi et al. (p. 1769; see the Perspective by Tapscott) provide strong evidence for this hypothesis: They inserted CUG repeats into a gene unrelated to DM and found that transgenic mice expressing expanded CUG-containing transcripts derived from this artificial gene developed many phenotypic features of DM, whereas control mice expressing transcripts with short CUG repeats were normal. These results represent a long-sought animal model for DM.

14. # Jovian Hot Spots Live On

When Jupiter's atmosphere is imaged in the infrared, persistent hot spots that emit strongly at a wavelength of 5 micrometers can be seen. Depleted in clouds and water vapor, these hot spots are confined to a narrow band just above the equator in the northern hemisphere and propagate westward within the normal eastward flow of the atmosphere, thus creating significant vertical wind shear. Showman and Dowling (p. 1737) simulated the development and dynamics of these hot spots with a relatively simple fluid model and show that they could persist for extended periods. Their model may help explain observations made by the Galileo probe, which measured the properties of the atmosphere within one of these unusual hot spots.

15. # Take It or Leave It

A friend tells you how to make a quick $300—all you have to do is agree to let him receive$700 as his split of a prize—but if you don't agree, you both get nothing. Psychological tests tell us that you are as likely to reject this offer as take it because it is unfair—even though you lose $300. This “Ultimatum Game” appears to deny reason over fairness, and game theory suggests that your friend could get away with offering you even$50. Nowak et al. (p. 1773) describe an evolutionary model for the Ultimatum Game, played over several generations in which persons play both roles, which suggests that your odds of getting a fair offer are tied to your reputation. If you are known to take a low offer (or have no history), you will likely get a low offer. However, if fellow proposers know you will reject a low offer, your odds improve of getting a fair offer near \$500.

16. # HSV Latency-Associated Transcript and Neuronal Apoptosis

Studying the latency-associated transcript (LAT) gene of herpes simplex virus-type 1 (HSV-1), Perng et al. (Reports, 25 February, p. 1500) concluded that viruses containing LAT block apoptosis in rabbit neurons and, thus, that LAT promotes survival of infected neurons after HSV-1 infection. Their experiments used a commercial antibody to test for cleaved poly (ADP-ribose) polymerase (p85 PARP), the presence of which is one indicator of apoptosis. In a comment, Thompson and Sawtell report results of tests that suggest that the antibody used by Perng et al. is not reactive against cleaved PARP in rabbits. Further, Thompson and Sawtell argue that in another test for apoptosis employed by Perng et al. to detect actual DNA fragmentation, “the signal … appears to be from the cytoplasm, an unlikely localization for signal originating from fragmented DNA,” which they suggest should be limited to the nucleus. Thus, conclude Thompson and Sawtell, the Perng et al. study “provides no evidence that the HSV-1 LAT gene blocks virus-induced apoptosis in neurons.”

Wechsler et al. respond with their own Western blot analysis, showing that the antibody used by Perng et al. “clearly…recognizes the rabbit-cleaved PARP p85 protein.” They also present “a number of lines of evidence” to suggest that fragmented DNA can be found in the cytoplasm as well as the nucleus in apoptotic cells.

The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/289/5485/1651a