This Week in SCIENCE
May 23 2003, 300 (5623)

THIS WEEK IN SCIENCE
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Electronics Made Transparent
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The development of transparent electronics, which could find use in
multilevel displays, relies on the use of wide band-gap semiconductors.
However, previous efforts to produce high-quality devices, specifically
transistors, have generally failed because of the poor electronic
properties of the materials. Nomura et al. (p. 1269; see the Perspective by
Wager) have developed a high-temperature technique for preparing
high-quality transparent oxide semiconductors. They use this material with
a rare-earth oxide as the gate dielectric to fabricate transparent
transistors that are insensitive to visible light. The devices exhibit
on-off ratios exceeding 106 and electron mobilities of ~80 square
centimeters per volt per second.


CREDIT: NOMURA ET AL.

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Binary Black Holes
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Models of galactic evolution suggest that black-hole binary systems should
be found in quasars and radio galaxies and that they are created during the
merging of galaxies.Sudou et al. (p. 1263) measured elliptical motion in
the core of radio galaxy 3C 66B and infer that this motion is caused by a
supermassive black-hole binary system. These high-resolution results are
consistent with the presumed merger history of giant elliptical galaxies
like 3C 66B.

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Dipoles in Cold Metal Clusters
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A number of bulk ionic materials are ferroelectric--an applied electric
field shifts atoms in the lattice and creates a permanent dipole moment.
Metals generally cannot be polarized in this way because their conduction
electrons neutralize the induced fields. Moro et al. (p. 1265) now report
that certain metals like niobium, in the form of small atomic clusters (3
to 100 atoms), show a normal response at room temperature but a substantial
ferroelectric response under cryogenic conditions (~20 K) that corresponds
to dipole moments up to several Debye. They account for this effect in
terms of a quantum mechanical model in which the charge carriers adopt an
asymmetrical distribution that cannot fully relax.

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Metals Up Close
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The effect of defects and faults on the structure and mechanical behavior
of metals is the subject of two reports. Defects can affect the mechanical
behavior of metals through their control of crystallite size. Lucadamo and
Medlin (p. 1272) now show that defects also control the local packing of
the atoms. High-resolution electron microscopy revealed that atoms around
defects in gold, a face-centered cubic metal, are hexagonally close packed.
The authors use dislocation theory to extend their analysis to a number of
grain-boundary orientations. Nanocrystalline metals can be much harder than
coarser-grain conventional metals, and their deformation mechanisms can
change from that of dislocation slip to one in which there is deformation
twinning that forms through stacking faults. This behavior has been seen in
simulations of nanocrystalline aluminum, despite the high energy required
to form stacking faults in this metal. Chen et al. (p. 1275; see the
Perspective by Bilde-Sorensen and Schiotz) now confirm these results and
observed twinning in transmission electron micrographs when the aluminum
grain size was sufficiently small.


CREDIT: LUCADAMO AND MEDLIN

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Question of Degrees
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The only reconstruction that has been performed on satellite-based data of
temperature changes in the middle to upper troposphere during the past 30
years showed minimal temperature change over that interval. Santer et al.
(p. 1280) present a new tropospheric and stratospheric temperature
reconstruction, based on the same data as the earlier version, to show that
substantial warming of the troposphere has occurred from 1979 to 2001. They
identify a "fingerprint" of combined anthropogenic and natural factors that
are consistent with the new analysis showing tropospheric warming but not
with the old one, which shows virtually no overall warming. They discuss
the implications of observational uncertainty for climate-change detection
studies.

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>From Stem Cells to Oocytes
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Mouse embryonic stem cells can differentiate into cells of all three
embryonic germ layers--endoderm, mesoderm, and ectoderm. However, they had
not been observed to give rise to germ cells in vitro. Hubner et al. (p.
1251; see the cover and the 2 May news story by Vogel) isolated a
population of cells that express a germ line-specific gene and followed the
fate of these cells in cultures to establish conditions whereby they
differentiate toward a germ-cell lineage. These in vitro differentiating
germ cells go through meiosis to form mature oocytes and subsequently
develop into blastocyst-like structures. Hence, these cells are totipotent
rather than pluripotent, as was previously suggested.

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Male-Fertility Factor
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During meiosis, the number of chromosomes in gametes is cut in half so that
subsequent fusion of egg and sperm can reconstitute the full chromosomal
complement. Crackower et al. (p. 1291) show that when the factor Fkbp6 is
inactivated in mice, fertility and meiosis of females is normal, but males
are infertile. Similarly, a spontaneous mutation of Fkbp6 in rats produces
a male-sterile phenotype. This work shows that Fkbp6 functions in
chromosomal pairing and is crucial for sex-specific fertility.

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Keeping the Signals Fresh
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For cells to remain responsive to stimuli such as light or odor, activated
G protein-coupled receptors (GPCRs) must be rapidly desensitized. G
protein-coupled receptor kinases (GRKs) play a role in regulating G protein
signaling by interacting both with GPCRs and with heterotrimeric G
proteins. Lodowski et al. (p. 1256) have determined the structure of GRK2
complexed to G proteinsubunits (G). The three signaling domains of GRK2 act
together to recruit GRK2 to the membrane and orient it to facilitate
phosphorylation of GPCRs. This orientation would allow GRK2 to
simultaneously bind the GPCR and Gand Gsubunits and provide a mechanism for
efficient attenuation of heterotrimeric G protein signaling.

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Putting DNA Sequences Back to Work
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Repetitive Alu sequences are part of the nonprotein coding "junk" DNA, but
mutations in these sequences can form coding exons. Although this process
is associated with disease in some cases, it also is a way for the genome
to evolve. Lev-Maor et al. (p. 1288; see the Perspective by Makalowski)
have assembled a database of "exonized" Alu elements and identified the
particular sequences and positions that can allow the crucial splicing to
occur. They tested their proposed mechanism in a mini-gene construct and
found that a single mutation can result in exonization.

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Infectious Synapse
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Dendritic cells (DCs) may play a role in host-pathogen interactions. For
example, certain proteins expressed on DCs, such as DC-SIGN, can stimulate
the efficiency of infection of target cells by the human immunodeficiency
virus (HIV). McDonald et al. (p. 1295) visualized individual particles of
HIV in living cells and found that DC-associated HIV was recruited to the
site of contact with target cells such as T cells. Simultaneously, the HIV
receptor and coreceptor were recruited to the contact site. This
recruitment effectively concentrates HIV, its receptor, and its coreceptor
and presumably facilitates infection.

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Avoiding Protein Pile-Ups
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The glomerular basement membrane (GBM) is a filtration barrier in the
kidney that permits the loss of excess water while preventing the loss of
valuable proteins. Several human renal diseases are characterized by an
accumulation of immune complexes in the GBM, but Kim et al. (p. 1298)
suggest that this condition may not be caused by immune dysfunction
associated with disease pathology. Rather, the authors observed abnormal
protein accumulation in the GBM of mice deficient in CD2AP, an adaptor
protein expressed in podocytes that comprise the GBM. CD2AP-deficient
podocytes were defective in degrading endocytosed material. Susceptibility
to human renal disease may be determined in part by the intrinsic capacity
of the GBM to clear proteins that the kidney normally encounters.


CREDIT: KIM ET AL.

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Getting Around a Block in the Opposing Lane
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A method for the unambiguous analysis of DNA replication on the leading and
the lagging strands in Escherichia coli has been developed by Pages and
Fuchs (p. 1300). They find that when a specific lesion that transiently
arrests DNA replication is placed on one strand, it does not inhibit
replication on the other. The normally coordinated leading and lagging
strand replicase system in vivo is uncoupled, and replication proceeds
through specialized "repair" polymerases.

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Time-Traveling Butterflies
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Monarch butterflies fly enormous distances during their seasonal migrations
by using the Sun for navigation. How can the butterflies compensate for the
Sun's change in position throughout the day? Froy et al. (p. 1303; see the
news story by Pennisi) show that the butterfly's circadian clock supplies a
"time-of-day" input to the navigation system so that the correct direction
can be maintained. Without a functioning clock, the butterflies become
disoriented. Ultraviolet light drives the Sun-position system and visible
light drives the time-of-day system.

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Jump-Starting Nanoparticle Conductivity
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In order to take advantage of the atomic-like quantum levels of
semiconducting nanoparticles (SC-NPs), it is necessary to inject charge
into them. However, because the SC-NPs are often coated with
organic-protecting layers, films can be poorly conducting, and only surface
charging occurs. Yu et al. (p. 1277) now show that if cross-linkers such as
1,7-heptanediamine are added to films of organically capped CdSe NPs,
conductivity jumps (which can reach up to 10-2 Siemens per square
centimeter) can be seen when these films are charged electrochemically.
Spectroscopic studies show that these jumps correspond to the filling of
the nanoparticles' 1S and 1P states.

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Hot Sites
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Capsaicin, a natural compound in chili peppers that produces a burning
sensation in the mouth, acts through a pain receptor in the mammalian
peripheral nervous system, the TRPV1 ion channel. Neurons expressing TRPV1
also perceive pain associated with stimuli such as high temperature and
acidity. TRPV1 is activated by the hydrolysis of a membrane phospholipid
called phosphatidylinositol-4,5-bisphosphate (PIP2). Prescott et al. (p.
1284) have identified a direct binding site on TRPV1 for PIP2 and show that
the strength of this interaction determines the activation threshold of the
channel.