PHOTO: NASA
SCIENCE sciencemag.org 4 SEPTEMBER 2020 • VOL 369 ISSUE 6508 1203BIOGEOCHEMISTRY
Terrestrial
biogeochemistry of silicon
Silicon is an important element
in plant tissues and contributes
to structural defenses against
herbivores and other stresses.
However, the terrestrial bio-
geochemical cycling of silicon
is poorly understood, particu-
larly the relative importance of
geochemical and biological
mechanisms in its regulation.
de Tombeur et al. studied this
question in 2-million-year
chronosequences of soil and
vegetation in Western Australia.
Sites became progressively
more weathered and infertile
as they aged, indicating that
the silicon cycle shifts from
geochemical to biological control
as the ecosystem develops (see
the Perspective by Carey). They
found that foliar silicon conce n-
trations increase continuouslyduring ecosystem develop-
ment, even though rock-derived
silicon is depleted in the older
soils. By contrast, other major
rock-derived nutrients showed
decreasing concentrations in
plants. Hence, biological silicon
cycling allows plants to maintain
concentrations even under con-
ditions of extreme soil infertility.
—AMS
Science, this issue p. 1245;
see also p. 1161STRUCTURAL BIOLOGY
Finding the start
Eukaryotic translation involves
many players in a dynamic and
well-orchestrated process. A
43 S preinitiation complex (PIC)
comprises the 40S ribosomal
subunit; initiation factors, includ-
ing the eIF3 complex, which is
known to play a key role; and the
transfer RNA used for translation
initiation. The PIC is recruited tothe cap-binding complex eIF4F
at the 5′ end of messenger RNA
(mRNA) to form a 48S complex
that scans along the mRNA for
a start codon. Brito Querido et
al. determined the structure
of a reconstituted human 48S
complex using cryo–electron
microscopy. They found that
eIF4F binds to eIF3 near the
exit site of the ribosome. This
positioning suggests that down-
stream mRNA is likely pulled
through the 40S subunit to find
the start codon. —VV
Science, this issue p. 1220CORONAVIRUS
A viral block on host
protein synthesis
As the coronavirus disease 2019
(COVID-19) pandemic continues
to cause devastation, scientists
race to increase their under-
standing of the disease-causingsevere acute respiratory syn-
drome coronavirus 2. Once inside
host cells, not only does the
virus hijack the cells’ transla-
tional machinery to make viral
proteins, but the virulence factor
nonstructural protein 1 (Nsp1)
also shuts down translation of
host messenger RNA. Thoms et
al. determined a 2.6-angstrom
resolution cryo–electron micros-
copy structure of a reconstituted
complex of Nsp1 bound to the
human 40S ribosomal subunit
and showed that Nsp1 blocks the
messenger RNA entry tunnel.
A structural inventory of native
Nsp1-ribosome complexes
from human cells confirms this
mechanism. Cellular studies
show that the translational shut-
down almost completely inhibits
the innate immune response. The
binding pocket on the ribosome
may be a target for drugs to treat
COVID-19. —VV
Science, this issue p. 1249RESEARCH
PLANETARY SCIENCELunar hematite holds clues
T
he poles of the Moon may contain a
record of billions of years of oxygen
isotopes of Earth’s atmosphere. The
lunar surface is highly reducing, yet
a recent study using data collected
by the Moon Mineralogy Mapper provides
strong evidence for the presence of hema-
tite, a common iron oxidation product
on Earth, at high lunar latitudes. Li et al.
mapped the hematite deposits, noting that
they are mostly found in association with
east equator–facing sides of topographic
highs on the lunar nearside of the Moon.
They propose that oxygen, delivered from
Earth’s upper atmosphere, could produce
a locally oxidizing environment, allowing
hematite to form. —RK
Sci. Adv. 10.1126/sciadv.aba1940 (2020).Remote observation
reveals the mineral
hematite on steep walls
of lunar craters.IN SCIENCE JOURNALS
Edited by Michael FunkPublished by AAAS