exhaled CO2 is in equal proportion with the other
five glucose carbons released as CO2 in muscle
mitochondria (Fig. 2A). When the moth is back
at rest (red arrows in Fig. 2), RQ increases again
as a result of a reduction in the use of glycolysis
and the tricarboxylic acid cycle and an increase
in the PPP activity by the moth. With 13C2 (second
glucose carbon labeled), RQ dropped after muscle
activity because of an increase in muscle metabolism as described above, but d13C2 increased (Fig.
2B). The d13C2 increase reflects the decrease in the
proportion of nonlabeled C1 exhaled as CO2 from
PPP activity and an increase in all of the other five
carbons (including 13C2) released in the mitochondria. Together, these results support our hypothesis that the “extra” CO2 that causes an RQ >> 1
originates from glucose carbon atom 1, as only C1
from glucose leaves through the PPP as CO2 (fig. S1).
It has been argued that endogenous antiox-
idant defense is costly for animals to produce
(26). This has led to the suggestion that migrating
birds must consume exogenous antioxidants (e.g.,
anthocyanins and carotenoids) in their food, such
as berries, to meet this need (26, 27). However, it
is known that insectivorous songbirds also con-
sume sugary fruits and flower nectar during stops
along the migratory route (28). Long-distance
(>1000 km) migrating Lepidoptera, such as the
monarch butterfly, as well as migrating hum-
mingbirds, refuel only with carbohydrates (29).
During these refueling stops, shunting nectar
glucose through the PPP may provide these ani-
mals with the endogenous antioxidant potential
needed during the intense exercise of migration.
The energetic cost of antioxidant generation by
the PPP is low, as adenosine 5′-triphosphate is
not required for any of its enzymatic reactions.
The two branches of the PPP also make it adap-
table to the organism’s needs; when in need of
immediate energy, three ribulose-5P molecules
arising from the oxidative branch of the PPP can
be redirected into glycolysis by converting them
into two molecules of fructose-6P and one mol-
ecule of glyceraldehyde-3P, and still produce
antioxidant potential in the form of NADPH (18).
We propose that in flying animals, consuming
carbohydrate-rich diets and the use of the PPP
during rest are causally linked with the ability to
reduce oxidative damage caused by bouts of in-
tense exercise. We suggest that this causal linkage
has enabled the evolution of physiological traits
that can sustain nectarivores’ metabolically demand-
ing modes of locomotion such as hovering and long-
distance migration while feeding only on nectar.
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This study was supported by NSF grant IOS-1053318 to G.D. We thank
H. Costa for maintaining the moth colony and R. Tzach and L. Braulke
for fruitful discussion. The authors declare no conflicts of interest.
Materials and Methods
2 July 2016; accepted 13 January 2017
Host cell attachment elicits
posttranscriptional regulation in
infecting enteropathogenic bacteria
Naama Katsowich,1 Netanel Elbaz,1 Ritesh Ranjan Pal,1 Erez Mills,1 Simi Kobi,1
Tamar Kahan,2 Ilan Rosenshine1†
The mechanisms by which pathogens sense the host and respond by remodeling gene
expression are poorly understood. Enteropathogenic Escherichia coli (EPEC), the cause of
severe intestinal infection, employs a type III secretion system (T3SS) to inject effector
proteins into intestinal epithelial cells. These effectors subvert host cell processes to
promote bacterial colonization. We show that the T3SS also functions to sense the host
cell and to trigger in response posttranscriptional remodeling of gene expression in the
bacteria. We further show that upon effector injection, the effector-bound chaperone
(CesT), which remains in the EPEC cytoplasm, antagonizes the posttranscriptional
regulator CsrA. The Ces T-CsrA interaction provokes reprogramming of expression of
virulence and metabolic genes. This regulation is likely required for the pathogen’s
adaptation to life on the epithelium surface.
Whereas most Escherichia coli strains are commensal, some clones evolve into pathogens through the acquisition of vir- ulence genes carried on pathogenicity islands, prophages, and plasmids. These
pathogens are classified into different virotypes,
each of which causes a distinct disease (1). Two
of these virotypes, enteropathogenic and entero-hemorrhagic E. coli (EPEC and EHEC), as well
as the murine-specific pathogen Citrobacter
rodentium (CR), share a common chromosomal
region of ~35,000 base pairs (bp), termed the
Fig. 2. Change in RQ and d13C over time.
(A) Change in RQ and d13C for C1-labeled glucose-fed moth. (B) Change in RQ and d13C for C2-labeled
glucose-fed moth. Black arrows indicate when the moth
was disturbed and muscle activity started. Red arrows
indicate when movement and muscle activity stop.