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This Week in SCIENCE, Volume 294, Issue 5547,
dated November 23 2001, is now available at:
sciencemag.org
A plain-text copy of the "This Week in SCIENCE" section has been
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This Week in SCIENCE
November 23 2001, 294 (5547)
THIS WEEK IN SCIENCE
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Truly Global Impact
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The Cretaceous-Tertiary (K-T) extinction event is attributed to a massive
bolide hitting the Yucatan peninsula of Mexico. In North America, there is
evidence of massive deforestation by an impact winter or wildfires. Now,
Vajda et al. (p. 1700; see the Perspective by Flannery) have found evidence
for deforestation in New Zealand associated with an iridium anomaly. Thus,
the impact event was global in its effects and caused significant
terrestrial devastation in the Southern Hemisphere.
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Parallel Assembly
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Designs for artificial bone scaffolds have generally focused on the
remineralization issue--getting hydroxyapatite (HA) to grow at the organic-
inorganic interface. However, bone is a complex mineral assembly with many
higher levels of organization that contribute to its mechanical properties.
Hartgerink et al. (p. 1684; see the news story by Service) designed a
peptide amphiphile (PA) to use as a scaffold material that incorporated
separate units for rigidity, HA adsorption, and cell adhesion. By varying
pH to control the assembly and cross-linking of the PA, the authors were
able to promote mineralization of HA as a series of parallel sheets. The HA
could be redissolved by changing the pH, which weakens the linkages in the
PA assembly.
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A Tale of Two Strategies
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The net CO2 uptake by forests can be estimated in two ways, either by
measuring local changes in the concentration of CO2 of air (a method called
"eddy covariance flux") or by taking inventories of biomass gain or loss.
These approaches have not always agreed in the past, and the differences
between them have resulted in conflicting estimates of regional carbon
budgets, such as for North America. Barford et al. (p. 1688) compared 9
years of eddy flux measurements of carbon uptake in Harvard Forest to
traditional inventory estimates and found close agreement between the
values, an important validation of the eddy flux method. The substantial
uptake at Harvard Forest is mostly due to prior land use. Climatic
variations account for seasonal and interannual variations in CO2 uptake.
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Asteroids Near and Far
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The Lincoln Near-Earth Asteroid Research (LINEAR) project has been
identifying and cataloging near-Earth asteroids (NEAs) over a small portion
of the sky for several years. Stuart (p. 1691) used this data to estimate
the size and shape of the NEA population for the full sky based on
detection probabilities. He finds that the number of 1-kilometer-diameter
asteroids should be about 1200 and that the population is more highly
inclined to the ecliptic than previously thought. Asteroid families are
thought to form by major collisions in the main belt, and their perturbed
members make good candidate sources for NEAs. Michel et al. (p. 1696; see
the cover) show that large family members can be made by gravitational
reaccumulation after the main collision, which can explain the presence of
large and small asteroids in observed families. Bottke et al. (p. 1693)
show that these newly created asteroid families can spread out due to a
thermal force called the Yarkovsky effect. Their simulations match
observations of asteroid family distributions. Interestingly, the
simulations also place some members on the edge of resonances, that is,
regions that can eject main belt asteroid into near-Earth orbits.
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but a little in Mining Chucka says not that bad:
Do Not Disturb Too Much
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What causes the high species richness of trees in tropical rain forests?
Arguments have increasingly focused on the role of disturbance, in the form
of gaps in the canopy created by dead and fallen trees, in helping to
maintain this richness. In a survey of selectively logged and unexploited
forests in French Guiana, Molino et al. (p. 1702) assessed the effects of
larger scale impacts of logging in tropical forests compared to the smaller
disturbances caused by naturally occurring canopy gaps.
Their findings
provide support for the hypothesis that intermediate levels of disturbance
produce maximum species richness.
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Pushing Ahead
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The leading edge of motile eukaryotic cells is pushed forward by a branched
actin filament network. The seven-subunit Arp2/3 complex initiates the
growth of branches on the sides of older filaments but only when activated
by Wiskott-Aldrich syndrome protein (WASp). Robinson et al. (p. 1679; see
the Perspective by Weeds and Yeoh) have determined the crystal structure at
2.0 angstrom resolution of the Arp2/3 complex. The two actin-related
proteins (Arps) are not appropriately oriented to form two subunits of an
actin helix, which explains why the complex is inactive. The authors
propose that nucleation-promoting factors such as WASp, actin filaments,
and adenosine triphosphate act by favoring the active conformation of the
Arp2/3 complex.
CREDIT: ROBINSON ET AL.
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Muscling In on Atrophy
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Muscle atrophy that occurs when physical activity is decreased results from
enhanced breakdown of muscle protein by the ubiquitin proteasome pathway,
but the precise molecular mediators are unknown. Using gene expression
profiling methods and subsequent analyses of knockout mice, Bodine et al.
(p. 1704) identified two muscle-specific ubiquitin ligases (enzymes that
target protein substrates for proteolysis by the proteasome), called MAFbx
and MuRF1, that are required for skeletal muscle atrophy. Identification of
these proteins may lead to new therapies aimed at preventing the loss of
muscle that often accompanies illness and aging.
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Lesser Extent of the vCJD Epidemic
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Predicting the magnitude of the variant Creuztfeldt-Jakob disease (vCJD)
epidemic in the United Kingdom is difficult because we do not know how many
people are infected nor the mean incubation period; differing estimates of
these quantities will lead to different predictions (see the Perspective by
Medley). Nonetheless, two groups using statistical methods predict that
incubation times are long and that total cases of the disease will at most
be several thousands. Huillard d'Aignaux et al. (p. 1729; see the 26
October news story by Balter) used statistical back-calculation models,
introduced for estimating HIV cases, to estimate the number of individuals
infected with vCJD. The authors report that even if a very large number of
persons are infected, the mean incubation period must be greater than 70
years. Valleron et al. (p. 1726) noted the low mean age for cases of vCJD
(28 years) and present a model in which susceptibility for the disease is
high through adolescence and then drops exponentially after age 15, and
that all infections occurred between 1980 and 1989. They predict an
incubation period of about 17 years.
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Free-Ranging Fibroblasts
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The fibroblast has been the prototype cell for studying a range of
phenomena, including cell proliferation, morphology, adhesion, and
movement. Yet, our current understanding of fibroblast biology is based
predominantly on observing these cells in two-dimensional (2D) culture.
Cukierman et al. (p. 1708; see the Perspective by Geiger) have designed a
3D matrix system to culture fibroblasts and report some key biological
differences compared to the traditional 2D culture system. A comprehensive
description of cell-matrix adhesions reveals striking structural and
morphological differences that may justify a reevaluation of current
models.
CREDIT: CUKIERMAN ET AL.
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A Second Polymerase for Prokaryotes
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Eukaryotes have independent DNA polymerase enzymes, d and e, for copying
the two different strands (leading and lagging) of DNA. The quintessential
prokaryote Escherichia coli has only one essential DNA polymerase that
copies both the leading and lagging DNA strands. Dervyn et al. (p. 1716)
now find that a second polymerase gene in Bacillus subtilis is present in
the replication complex and seems to play a role in second (lagging)-strand
synthesis. Homologs of this polymerase are found in many other prokaryotes,
suggesting that prokaryotes, like eukaryotes, generally have two DNA
polymerases, one for each strand.
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More Similar Than We Thought
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A single nucleotide polymorphism (SNP) occurs roughly every 600 base pairs
within the human genome and represents a mutational event that occurred
once in human history. The more ancient the mutation, the more common will
be the SNP in a population. Groups of SNPs descended from a single
ancestral chromosome define patterns of variation called haplotypes within
the human genome. Patil et al. (p. 1719; see the Perspective by Kwok) have
used high-density oligonucleotide arrays and somatic cell assays to
estimate the extent of human diversity, and have found fewer haplotypes
than anyone suspected. For chromosome 21, only 8% of the haplotypes were
population-specific and 80% of a globally diverse set of chromosomes could
be characterized by three common haplotypes.
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Exposing Culprit Genes in MS
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Multiple sclerosis (MS) results from immune-mediated loss of the myelin
sheath that surrounds nerves. Using large-scale sequence analysis of
complementary DNA libraries constructed from brain tissue of MS patients,
Chabas et al. (p. 1731) identified a number of genes that were
substantially increased in their expression in MS. Among these was
osteopontin (OPN), a known factor that influences inflammatory T cell
responses. Expression analysis revealed up-regulation of OPN adjacent to
lesions observed in brain tissue of MS patients, as well as in rodents that
develop an experimental form of the disease. Induction of disease and
expression of inflammatory cytokines by T cells were greatly reduced in
mice lacking the OPN gene.
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Coordinated Effort Against DNA Damage
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The protein kinase ATR is implicated in the signaling pathway that causes
cells to arrest the cell division cycle when damage to DNA is detected.
Cortez et al. (p. 1713) describe a new human protein that interacts with
ATR and is a substrate for the kinase. ATRIP (for ATR-interacting protein)
has sequence similarity to the yeast proteins that interact with the yeast
homologs of ATR, but unlike the yeast proteins, ATRIP and ATR regulate each
other's expression in human cells. Such tight coordination of expression is
consistent with the authors' demonstration that both proteins are required
for proper responses to DNA damage and that ATR is required for viability
of human somatic cells.
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Mapping Cardiovascular Genes and Function
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The usefulness of genomics at the level of physiology requires connecting
genes with phenotypes. Stoll et al. (p. 1723) measured more than 200
cardiovascular and renal phenotypes in male rats derived from an F2
intercross and mapped 81 determinants likely to be involved in regulating
blood pressure. The authors also introduce "physiological profiling," in
which the measured attributes are grouped according to the Guyton model of
blood pressure control. For example, physiologic parameters that
cosegregate with an allele on chromosome 10 that impacts responsiveness to
acetylcholine can be inferred simply by scanning a correlation matrix.
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Intracellular Bacterial Vaccines
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In contrast to the situation with viruses, killed or inactivated
intracellular bacteria make poor vaccines because they produce inadequate T
cell memory compared with live bacteria. Lauvau et al. (p. 1735) studied
the differences in CD8 T cell response induced by inoculation of mice with
live versus dead preparations of the intracellular bacteria Listeria
monocytogenes (LM). Although dead bacteria generated memory cells that
could readily expand when confronted with live LM, these T cells could not
protect mice against this infection. This deficiency correlated with the
inability of CD8+ T cells from dead LM-immunized mice to produce interferon
-g or generate cytotoxicity, the two main arms of the anti-LM response.
Thus, important qualitative, rather than quantitative, differences may
exist in immune priming by dead and live bacteria.
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