1,970 research outputs found
The Effect of Ambient Temperature on Cold Start Urban Traffic Emissions for a Real World SI Car
The influence of ambient temperature on exhaust
emissions for an instrumented Euro 1 SI car was
determined. A real world test cycle was used, based on
an urban drive cycle that was similar to the ECE urban
drive cycle. It was based on four laps of a street circuit
and an emissions sample bag was taken for each lap.
The bag for the first lap was for the cold start emissions.
An in-vehicle direct exhaust dual bag sampling
technique was used to simultaneously collect exhaust
samples upstream and downstream of the three-way
catalyst (TWC). The cold start tests were conducted
over a year, with ambient temperatures ranging from –
2°C to 32°C. The exhaust system was instrumented with
thermocouples so that the catalyst light off temperature
could be determined. The results showed that CO
emissions for the cold start were reduced by a factor of
8 downstream of catalyst when ambient temperature
rose from -2°C to 32°C, the corresponding hydrocarbon
emissions were reduced by a factor of 4. There was no
clear relationship between NOx emissions and ambient
temperature. For subsequent laps of the test circuit the
reduction of CO and HC emissions as a function of
ambient temperature was lower. The time for catalyst
light off increased by 50% as the ambient temperature
was reduced. The results show that the vehicle used is
unlikely to meet the new – 7oC cold start CO emission
regulations
Genome-scale reconstruction of Salinispora tropica metabolism; Microbial engineering and its applications in secondary metabolite production
Bacteria within the order Actinomycetales are a well-known source of natural products such as antibiotics and anticancer agents, and the genus Salinispora is no exception. Salinispora tropica is a marine actinomycete that produces diverse secondary metabolites, including many that possess pharmaceutical properties such as Salinosporamide A (NPI-0052), a potent anticancer agent, and sporolides, candidates for antiviral compounds. Here, we present the first manually curated genome-scale metabolic model (iCC908) for Salinispora tropica strain CNB-440. The reconstruction enables characterization of the metabolic capabilities for understanding and modeling the cellular physiology of this newly sequenced genome. The model was based on physiological and biochemical information of primary and secondary metabolism pathways. The reconstructed stoichiometric matrix consists of 1169 biochemical reactions, 204 transport reactions and 1317 metabolites. A total of 908 structural open reading frames (ORFs) were included in the reconstructed network. The number of gene functions included corresponds to 20% of all characterized ORFs in the S. tropica genome. The genome-scale metabolic model was used to study strain-specific capabilities in defined minimal media and to analyze growth capabilities in 41 different minimal growth-supporting environments. These nutrient sources were evaluated experimentally to assess the accuracy of in-silico growth simulations. The model predicted no auxotrophies for essential amino acids, which was corroborated experimentally. The strain is able to use 21 different carbon sources, 8 nitrogen sources and 4 sulfur sources from the nutrient sources tested. Cases where the model was incorrect provided opportunities to gain new insights into the physiology of this specie and generate hypotheses. The incorporation of modifications led to increased accuracy in predicting the outcome of growth/no growth experiments from 76 to 93%. New data, and modifications can be incorporated into the reconstruction to iteratively improve the reconstruction.
Since specialized pathways were included in the reconstruction, growth simulations and in silico gene deletions can be performed by using flux balance analysis (FBA) to dramatically increase secondary metabolites production and yield in Salinispora for possible “gene cluster identification” so specific pathways can be cloned in more efficient strains. For example, iCC908 has been used to define a production medium to improve Salinosporamide A production in a recombinant strain with increases over 20% compared to the wild type. This presentation will describe the main features of the metabolic flux analysis and microbial engineering methodology based on reconstruction of the whole metabolism and its applications in the optimization of secondary metabolite production
Spiders fluoresce variably across many taxa
The evolution of fluorescence is largely unexplored, despite the newfound occurrence of this phenomenon in a variety of organisms. We document that spiders fluoresce under ultraviolet illumination, and find that the expression of this trait varies greatly among taxa in this species-rich group. All spiders we examined possess fluorophores in their haemolymph, but bright fluorescence appears to result when a spider sequesters fluorophores in its setae or cuticle. By sampling widely across spider taxa, we determine that fluorescent expression is labile and has evolved multiple times. Moreover, examination of the excitation and emission properties of extracted fluorophores reveals that spiders possess multiple fluorophores and that these differ among some families, indicating that novel fluorophores have evolved during spider diversification. Because many spiders fluoresce in wavelengths visible to their predators and prey (birds and insects), we propose that natural selection imposed by predator–prey interactions may drive the evolution of fluorescence in spiders
Acquired genetic and epigenetic variation in human pluripotent stem cells
Human pluripotent stem cells (hPSCs) can acquire non-random genomic variation during culture. Some of these changes are common in tumours and confer a selective growth advantage in culture. Additionally, there is evidence that reprogramming of human induced pluripotent stem cells (hiPSCs) introduces mutations. This poses a challenge to both the safety of clinical applications and the reliability of basic research using hPSCs carrying genomic variation. A number of methods are available for monitoring the genomic integrity of hPSCs, and a balance between practicality and sensitivity must be considered in choosing the appropriate methods for each use of hPSCs. Adjusting protocols by which hPSCs are derived and cultured is an evolving process that is important in minimising acquired genomic variation. Assessing genetic variation for its potential impact is becoming increasingly important as techniques to detect genome-wide variation improve
Area distribution of two-dimensional random walks on a square lattice
The algebraic area probability distribution of closed planar random walks of
length N on a square lattice is considered. The generating function for the
distribution satisfies a recurrence relation in which the combinatorics is
encoded. A particular case generalizes the q-binomial theorem to the case of
three addends. The distribution fits the L\'evy probability distribution for
Brownian curves with its first-order 1/N correction quite well, even for N
rather small.Comment: 8 pages, LaTeX 2e. Reformulated in terms of q-commutator
A Gaussian distribution for refined DT invariants and 3D partitions
We show that the refined Donaldson-Thomas invariants of C3, suitably
normalized, have a Gaussian distribution as limit law. Combinatorially these
numbers are given by weighted counts of 3D partitions. Our technique is to use
the Hardy-Littlewood circle method to analyze the bivariate asymptotics of a
q-deformation of MacMahon's function. The proof is based on that of E.M. Wright
who explored the single variable case.Comment: 11 pages and 3 figure
Apoptosis and failure of checkpoint kinase 1 activation in human induced pluripotent stem cells under replication stress
Background: Human induced pluripotent stem (hiPS) cells have the ability to undergo self-renewal and
differentiation similarly to human embryonic stem (hES) cells. We have recently shown that hES cells under
replication stress fail to activate checkpoint kinase 1 (CHK1). They instead commit to apoptosis, which appears to be
a primary defense mechanism against genomic instability. It is not known whether the failure of CHK1 activation
and activation of apoptosis under replication stress is solely a feature of hES cells, or if it is a feature that can be
extended to hiPS cells.
Methods: Here we generated integration-free hiPS cell lines by mRNA transfection, and characterised the cell lines. To
investigate the mechanism of S phase checkpoint activation, we have induced replication stress by adding excess
thymidine to the cell culture medium, and performed DNA content analysis, apoptosis assays and immunoblottings.
Results: We are showing that hiPS cells similarly to hES cells, fail to activate CHK1 when exposed to DNA replication
inhibitors and commit to apoptosis instead. Our findings also suggest the Ataxia Telangiectasia Mutated pathway
might be responding to DNA replication stress, resulting in apoptosis.
Conclusion: Together, these data suggest that the apoptotic response was properly restored during reprogramming
with mRNA, and that apoptosis is an important mechanism shared by hiPS and hES cells to maintain their genomic
integrity when a replication stress occurs
Interference of a Tonks-Girardeau Gas on a Ring
We study the quantum dynamics of a one-dimensional gas of impenetrable bosons
on a ring, and investigate the interference that results when an initially
trapped gas localized on one side of the ring is released, split via an
optical-dipole grating, and recombined on the other side of the ring. Large
visibility interference fringes arise when the wavevector of the optical dipole
grating is larger than the effective Fermi wavevector of the initial gas.Comment: 7 pages, 3 figure
Some flows in shape optimization
Geometric flows related to shape optimization problems of Bernoulli type are
investigated. The evolution law is the sum of a curvature term and a nonlocal
term of Hele-Shaw type. We introduce generalized set solutions, the definition
of which is widely inspired by viscosity solutions. The main result is an
inclusion preservation principle for generalized solutions. As a consequence,
we obtain existence, uniqueness and stability of solutions. Asymptotic behavior
for the flow is discussed: we prove that the solutions converge to a
generalized Bernoulli exterior free boundary problem
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Nucleosides rescue replication-mediated genome instability of human pluripotent stem cells
Human pluripotent stem cells (PSCs) are subject to the appearance of recurrent genetic variants on prolonged culture. We have now found that, compared with isogenic differentiated cells, PSCs exhibit evidence of considerably more DNA damage during the S phase of the cell cycle, apparently as a consequence of DNA replication stress marked by slower progression of DNA replication, activation of latent origins of replication, and collapse of replication forks. As in many cancers, which, like PSCs, exhibit a shortened G1 phase and DNA replication stress, the resulting DNA damage may underlie the higher incidence of abnormal and abortive mitoses in PSCs, resulting in chromosomal non-dysjunction or cell death. However, we have found that the extent of DNA replication stress, DNA damage, and consequent aberrant mitoses can be substantially reduced by culturing PSCs in the presence of exogenous nucleosides, resulting in improved survival, clonogenicity, and population growth
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