excerpted SCIENCE WEEK

Eugene Leitl (Eugene.Leitl@lrz.uni-muenchen.de)
Thu, 19 Jun 1997 21:08:49 +0200 (MET DST)

This week came the interesting news of a 5 km asteroid whose
orbit is dynamically locked into the orbit of the Earth, making
it, aside from the Moon, Earth's only known natural companion.
An explanation of the quite special orbital geometry is difficult
without diagrams, but one can attempt it. The story is that
more than two hundred years ago, the French mathematician
Lagrange investigated the dynamics of the gravitational three-
body problem, and discovered that if there are two bodies, A and
B, and the mass of B is less than 0.04 the mass of A, then as B
orbits around A, there are two equilibrium points in B's orbit,
one leading B and one trailing B, and that at these equilibrium
points a third smaller body (or bodies) can enter the two-body
system and there can be sustained a dynamic equilibrated
arrangement. Such arrangements have in fact been observed for
asteroids associated with the Jupiter and Mars orbits. This new
asteroid, called asteroid 3753, is associated with Earth's orbit,
but in a quite special way: the orbit of 3753 encompasses both
equilibrium points and is a "horseshoe" orbit, and it is only
because the asteroid's orbit is highly inclined with respect to
the plane of the Earth's orbit that a collision is unlikely. The
analysis and numerical solutions of the relevant dynamical
equations were presented by Paul A. Wiegert et al (York
University, CA; University of Turku, FI). (Nature 12 June)

Most chemical reactions involving two or more reactants occur in
the presence of a solvent, a significant presence, since the
solvent molecules are always interacting with the reactants
themselves in some important manner. To fully understand what
occurs between the main reactants, one must understand the
participation of the solvent in the reaction sequence. The
problem has been that these solvent-reactant events are of
extremely short duration, on the order of femtoseconds
[10exp(-15) sec], and it is only recently, with the availability
of laser pulse techniques, that physical chemists can observe
chemical bonds break and reform in real time. B. Jefferys
Greenblatt et al (University of California Berkeley, US) have now
described the use of such techniques to examine the influence of
a solvent on the photodissociation of a molecular cluster ion
(Iodine-Argon). Apparently, in a single experiment, they have
presented "a comprehensive picture of the recombination,
relaxation, and evaporation processes that follow photodissocia-
tion of a diatomic molecular ion in a cluster." (Science 13 June)

Single-molecule optical information storage would make possible
information storage densities enormously greater than any that
can now be achieved. This is one reason why physicists and
physical chemists have recently been much interested in single-
molecule doped systems in which the single molecule is forced to
switch energy states. The problem has been that so far no one has
found a way to produce accurately controlled switching. Now F.
Kulzer et al (University of Munich DE) have reported accurately
controlled switching of energy states of a single molecule of
terrylene resident in a p-terphenyl crystal, and show switching
of "single-molecule absorption lines in a controlled way between
well-defined frequency positions." The experiments involved
flipping the same single molecule switch for 22 days. Single-
molecule optical information storage devices are not yet a
reality, but this is certainly a significant step forward.
(Nature 12 June)

There is a consensus that chemical systems in biological
organisms came into being through evolutionary processes. Some
hold that natural selection is the only evolutionary mechanism
involved, while others consider the possibility of additional
mechanisms. In any case, the extant chemical systems evolved, and
the question of how they evolved is important for our
understanding of many vital chemical systems such as the immune
system in humans. Enzymes, for example, are proteins that are
capable of exquisite selectivity and enormous catalytic power,
and we would like to know how any particular set of enzymes in an
organism came to exist. Gary J. Wedemayer et al (University of
California Berkeley US, Lawrence Livermore National Laboratory,
Berkeley US) this week presented the results of x-ray
crystallographic studies of a particular antibody enzyme (Fab
germline fragment, which matures into 48G7), and correlated the
data with the antibody's biophysical, kinetic, and structural
properties. They suggest that a limited number of protein
frameworks, each capable of evolving multiple configurations, is
the origin of the extraordinary diversity present in the
antibody-catalyst domain. They further suggest that their studies
provide important insights into the molecular basis of the immune
response, and may also be of significance for the understanding
of other biochemical systems and functions. (Science 13 June)

A transposon is a limited DNA sequence that under the proper
dynamic conditions can effectively translocate from one DNA
system to another, either in the same cell, or between cells, or
between cells of different organisms of the same or different
species, and remain functional. Until now, transposons were
thought to be loosely limited to same or neighboring species
translocations. But this week Frederico J. Gueiros-Filho and
Steven M. Beverley (Harvard Medical School, Boston US) have
demonstrated that a transposon of what is known as the "mariner"
family can be experimentally translocated cross-kingdom, in this
case from the insect Drosophila mauritiana (a mosquito) to the
protozoan Leishmania major (the pathogen responsible for
malaria). This development has caused excitement among micro-
biologists because it makes possible new genetic techniques
not only in the study of the malaria parasite, but also in
studies of other parasitic pathogens. The authors point out that
the reported mariner translocation involves the crossing of an
evolutionary divide of one billion years. (Science 13 June)

After a systematic computerized search of a particular entire
yeast genome, Kenneth H. Wolfe and Denis C. Shields (University
of Dublin, IE) report that statistical analysis of duplicate
segments suggests the entire genome underwent duplication
approximately 100 million years ago. Duplication of single genes
is considered a significant source of biological novelty, but
this is the first evidence of the possibility of the duplication
of an entire genome. The authors suggest that duplication to
tetraploidy may have led to the ability of this species, S.
cerevisiae, to anaerobically ferment sugar and produce ethanol.
(Nature 12 June)

There is a growing consensus that at least in bacteria mutation
rates within a species can be highly variable, and that this
variability is itself subjected to Darwinian selection. Two
related reports appeared this week, one involving mathematical
modeling [F. Taddei et al (University of Paris, FR; University of
Sussex, UK], and the other experimental observations of
populations of the bacterium E. coli [P. D. Sniegowski et al
(University of Pennsylvania, US; Michigan State University US)].
Both groups provide support for the idea that high mutation rates
may play an important role in adaptive evolution. In an essay
reviewing this work, E. Richard Moxon (University of Oxford, UK)
and David S. Thaler (Rockefeller University, US) claim a broader
implication, and state, "The generation of variation is itself
under genetic control, allowing a reflectivity -- an informat-
ional feedback -- on the mechanisms by which diversity is
generated in biological evolution." (Nature 12 June)

Hantavirus is a serious pulmonary infectious disease with a high
mortality rate (approximately 50%). It is caused by a family of
viruses (Hantavirus or Hantaan virus) named after the Hantaan
River in Korea, where the first strain was discovered in the
1950s. For the past 40 years it has been thought that the virus
spreads only via the inhalation of microscopic bits of contamin-
ated rodent urine or feces. Now after an investigation into a
hantavirus outbreak in Argentina last year, it has been concluded
that the first known instance of person-to-person transmission of
the hantavirus has occurred. The epidemiology of this pathogen
will need to be revised. (New York Times 17 June)

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