SCIENCE sciencemag.org 18 JANUARY 2019 • VOL 363 ISSUE 6424 243-CCELL DIFFERENTIATION
Reversing chromatin
dynamics for development
Compacted chromatin regions,
marked by trimethylation of
histone H3 at position lysine 9
(H3K9me3), occur at highly
repeated DNA sequences, help-
ing to suppress recombination
and gene expression. Because
pluripotent cells contain low
levels of H3K9me3 heterochro-
matin relative to differentiated
cells, it has been thought that
an increase in such hetero-
chromatin helps to define cell
differentiation. Nicetto et al. used
two independent methods to
examine compacted heterochro-
matic domains and found that
H3K9me3 compaction increased
at protein-coding genes during
early mouse organogenesis.
During differentiation, these
domains open up to allow
cell-specific expression. Loss
of heterochromatin by genetic
inactivation of the H3K9me3
methyltransferases caused
ectopic expression of cell-
inappropriate genes and tissue
pathology. —BAP
Science, this issue p. 294
MICROBIOLOGY
Dual roles of skin
microbiota
Like the gut, the skin is colo-
nized by microbes that can
be protective to help maintain
homeostasis or be pathogenic.
In a Perspective, Stacy and
Belkaid discuss the dual roles of
Staphylococcus epidermidis on
skin. This microbe exemplifies
how skin microbiota can pro-
mote immune and antimicrobial
responses against pathogens
but, in certain contexts, aggra-
vate pathologies such as eczema
and possibly skin cancer. Skin is
often termed the largest human
organ, so it is important to
understand how skin microbiota
contribute to health. —GKA
Science, this issue p. 227
METABOLISM
An anti-ammonia
probiotic
Hyperammonemia, or excess
blood ammonia, is a serious
condition that can result in
brain damage and death. Kurtz
et al. modified the metabolism
of a probiotic Escherichia coli
strain to overproduce arginine,
thereby sequestering some of
the ammonia produced by gut
bacteria into the amino acid mol-
ecules. The engineered strain,
called SYNB1020, lowered blood
ammonia and increased survival
in mouse hyperammonemia
models and showed repeat-
dose tolerability in nonhuman
primates. A phase 1 dose-esca-
lation study in healthy human
volunteers resulted in no serious
adverse events and indicated
that the bacterium was meta-
bolically active in vivo. —CAC
Sci. Transl. Med. 11 , eaau7975 (2019).ECOLOGY
Standardizing science for
conservation
Assessments of biodiversity and
human impacts on the envi-
ronment often rely on species
distribution models that make
data-supported predictions
of species survival in chang-
ing environments. However, no
agreed-upon standards for eval-
uating these models exist, and
their application to conservation
policy is inconsistent. Araújo et
al. analyzed 400 model-based
studies over the past 20 years
to assess the adequacy of the
models and their impacts on
scientific interpretation and pro-
jection. They make the case for
the development of best-prac-
tice standards and guidelines
for the evaluation of data and
models used. Such standard-
ized practices should ensure
more transparent and consistent
translation of scientific results
into policy. —PJB
Sci. Adv. 10.1126/sciadv.aat4858
(2019).Published by AAAS