167 research outputs found
Identification of basepairs within Tn5 termini that are critical sfor H-NS binding to the transpososome and regulation of Tn5 transposition
<p>Abstract</p> <p>Background</p> <p>The H-NS protein is a global regulator of gene expression in bacteria and can also bind transposition complexes (transpososomes). In Tn5 transposition H-NS promotes transpososome assembly <it>in vitro </it>and disruption of the <it>hns </it>gene causes a modest decrease in Tn5 transposition (three- to five-fold). This is consistent with H-NS acting as a positive regulator of Tn5 transposition. Molecular determinants for H-NS binding to the Tn5 transpososome have not been determined, nor has the strength of the interaction been established. There is also uncertainty as to whether H-NS regulates Tn5 transposition <it>in vivo </it>through an interaction with the transposition machinery as disruption of the <it>hns </it>gene has pleiotropic effects on <it>Escherichia coli</it>, the organism used in this study.</p> <p>Results</p> <p>In the current work we have further examined determinants for H-NS binding to the Tn5 transpososome through both mutational studies on Tn5 termini (or 'transposon ends') and protein-protein cross-linking analysis. We identify mutations in two different segments of the transposon ends that abrogate H-NS binding and characterize the affinity of H-NS for wild type transposon ends in the context of the transpososome. We also show that H-NS forms cross-links with the Tn5 transposase protein specifically in the transpososome, an observation consistent with the two proteins occupying overlapping binding sites in the transposon ends. Finally, we make use of the end mutations to test the idea that H-NS exerts its impact on Tn5 transposition <it>in vivo </it>by binding directly to the transpososome. Consistent with this possibility, we show that two different end mutations reduce the sensitivity of the Tn5 system to H-NS regulation.</p> <p>Conclusions</p> <p>H-NS typically regulates cellular functions through its potent transcriptional repressor function. Work presented here provides support for an alternative mechanism of H-NS-based regulation, and adds to our understanding of how bacterial transposition can be regulated.</p
Quantum Transport in Molecular Rings and Chains
We study charge transport driven by deformations in molecular rings and
chains. Level crossings and the associated Longuet-Higgins phase play a central
role in this theory. In molecular rings a vanishing cycle of shears pinching a
gap closure leads, generically, to diverging charge transport around the ring.
We call such behavior homeopathic. In an infinite chain such a cycle leads to
integral charge transport which is independent of the strength of deformation.
In the Jahn-Teller model of a planar molecular ring there is a distinguished
cycle in the space of uniform shears which keeps the molecule in its manifold
of ground states and pinches level crossing. The charge transport in this cycle
gives information on the derivative of the hopping amplitudes.Comment: Final version. 26 pages, 8 fig
Non-adiabaticity and single-electron transport driven by surface acoustic waves
Single-electron transport driven by surface acoustic waves (SAW) through a
narrow constriction, formed in two-dimensional electron gas, is studied
theoretically. Due to long-range Coulomb interaction, the tunneling coupling
between the electron gas and the moving minimum of the SAW-induced potential
rapidly decays with time. As a result, nonadiabaticiy sets a limit for the
accuracy of the quantization of acoustoelectric current
Consumers’ responses to front-of-pack labels that vary by interpretive content
Previous research has shown that front-of-pack labels (FoPLs) can assist people to make healthier food choices if they are easy to understand and people are motivated to use them. There is some evidence that FoPLs providing an assessment of a food’s health value (evaluative FoPLs) are easier to use than those providing only numerical information on nutrients (reductive FoPLs). Recently, a new evaluative FoPL (the Health Star Rating (HSR)) has been introduced to Australia and New Zealand. The HSR features a summary indicator, differentiating it from many other FoPLs being used around the world. The aim of this study was to understand how consumers of all ages use and make sense of reductive FoPLs and evaluative FoPLs including evaluative FoPLs with and without summary indicators. Ten focus groups were conducted in Perth, Western Australia with adults (n=50) and children aged 10-17 years (n=35) to explore reactions to one reductive FoPL (the Daily Intake Guide), an existing evaluative FoPL (multiple traffic lights), and a new evaluative FoPL (the HSR). Participants preferred the evaluative FoPLs over the reductive FoPL, with the strongest preference being for the FoPL with the summary indicator (HSR). Discussions revealed the cognitive strategies used when interpreting each FoPL (e.g., using cut offs, heuristics, and the process of elimination), which differed according to FoPL format. Most participants reported being motivated to use the evaluative FoPLs (particularly the HSR) to make choices about foods consumed as part of regular daily meals, but not for discretionary foods consumed as snacks or deserts. The findings provide further evidence of the potential utility of evaluative FoPLs in supporting healthy food choices and can assist policy makers in selecting between alternative FoPL formats
Acoustoelectric effect in a finite-length ballistic quantum channel
The dc current induced by a coherent surface acoustic wave (SAW) of wave
vector q in a ballistic channel of length L is calculated. The current contains
two contributions, even and odd in q. The even current exists only in a
asymmetric channel, when the electron reflection coefficients r_1 and r_2 at
both channel ends are different. The direction of the even current does not
depend on the direction of the SAW propagation, but is reversed upon
interchanging r_1 and r_2. The direction of the odd current is correlated with
the direction of the SAW propagation, but is insensitive to the interchange of
r_1 and r_2. It is shown that both contributions to the current are non zero
only when the electron reflection coefficients at the channel ends are energy
dependent. The current exhibits geometric oscillations as function of qL. These
oscillations are the hallmark of the coherence of the SAW and are completely
washed out when the current is induced by a flux of non-coherent phonons. The
results are compared with those obtained previously by different methods and
under different assumptions.Comment: 7 pages, 2 figure
A numerical investigation of a piezoelectric surface acoustic wave interaction with a one-dimensional channel
We investigate the propagation of a piezoelectric surface acoustic wave (SAW)
across a GaAs/AlGaAs heterostructure surface, on which there is
fixed a metallic split-gate. Our method is based on a finite element
formulation of the underlying equations of motion, and is performed in
three-dimensions fully incorporating the geometry and material composition of
the substrate and gates. We demonstrate attenuation of the SAW amplitude as a
result of the presence of both mechanical and electrical gates on the surface.
We show that the incorporation of a simple model for the screening by the
two-dimensional electron gas (2DEG), results in a total electric potential
modulation that suggests a mechanism for the capture and release of electrons
by the SAW. Our simulations suggest the absence of any significant turbulence
in the SAW motion which could hamper the operation of SAW based quantum devices
of a more complex geometry.Comment: 8 pages, 8 figure
Giant Oscillations of Acoustoelectric Current in a Quantum Channel
A theory of d.c. electric current induced in a quantum channel by a
propagating surface acoustic wave (acoustoelectric current) is worked out. The
first observation of the acoustoelectric current in such a situation was
reported by J. M. Shilton et al., Journ. Phys. C (to be published). The authors
observed a very specific behavior of the acoustoelectric current in a
quasi-one-dimensional channel defined in a GaAs-AlGaAs heterostructure by a
split-gate depletion -- giant oscillations as a function of the gate voltage.
Such a behavior was qualitatively explained by an interplay between the
energy-momentum conservation law for the electrons in the upper transverse mode
with a finite temperature splitting of the Fermi level. In the present paper, a
more detailed theory is developed, and important limiting cases are considered.Comment: 7 pages, 2 Postscript figures, RevTeX 3.
Quantized charge transport through a static quantum dot using a surface acoustic wave
We present a detailed study of the surface acoustic wave mediated quantized
transport of electrons through a split gate device containing an impurity
potential defined quantum dot within the split gate channel. A new regime of
quantized transport is observed at low RF powers where the surface acoustic
wave amplitude is comparable to the quantum dot charging energy. In this regime
resonant transport through the single-electron dot state occurs which we
interpret as turnstile-like operation in which the traveling wave amplitude
modulates the entrance and exit barriers of the quantum dot in a cyclic fashion
at GHz frequencies. For high RF powers, where the amplitude of the surface
acoustic wave is much larger than the quantum dot energies, the quantized
acoustoelectric current transport shows behavior consistent with previously
reported results. However, in this regime, the number of quantized current
plateaus observed and the plateau widths are determined by the properties of
the quantum dot, demonstrating that the microscopic detail of the potential
landscape in the split gate channel has a profound influence on the quantized
acoustoelectric current transport.Comment: 9 page
Acoustoelectric current and pumping in a ballistic quantum point contact
The acoustoelectric current induced by a surface acoustic wave (SAW) in a
ballistic quantum point contact is considered using a quantum approach. We find
that the current is of the "pumping" type and is not related to drag, i.e. to
the momentum transfer from the wave to the electron gas. At gate voltages
corresponding to the plateaus of the quantized conductance the current is
small. It is peaked at the conductance step voltages. The peak current
oscillates and decays with increasing SAW wavenumber for short wavelengths.
These results contradict previous calculations, based on the classical
Boltzmann equation.Comment: 4 pages, 1 figur
Protocol for a randomized trial assessing consumer evaluations of pre-packaged foods that systematically vary by nutrition information and product attributes
Background: Nutrition information is increasingly provided on pre-packaged foods as a public health measure to help consumers make healthier food choices. Many studies have looked at the independent effects of three main sources of nutrition information: the Nutrition Information Panel (NIP), front-of-pack labels and health claims. However, few studies have considered their interactive effects. A better understanding of how these different sources of nutrition information interact with each other is important given they frequently appear together on food packs. There are also policy implications since many countries specifically mandate the provision of an NIP whenever a health claim is made. Methods: This paper outlines a protocol for an experimental study assessing how nutrition information (FoPLs, health claims and NIP), in combination with food type, price and product healthiness interact to affect consumers’ product evaluations. Consumers’ global impressions, perceptions of healthiness, purchase intentions and assumptions relating to the amount of the product that is appropriate/desirable to consume will be assessed. The nutrition information presented will include NIPs, front-of-pack labels (Daily Intake Guide, Multiple Traffic Light system, Health Star Rating system) and health claims (nutrient content, general level, higher level). A diverse sample of approximately 2000 Australians will be recruited to complete an online survey that will require them to evaluate a range of hypothetical products with varying nutrition and price attributes. All attribute levels will be fully crossed with each other, resulting in a full factorial design. This design has not been used in past studies and offers a higher level of control than achieved previously due to the ability to explore interactions between all attribute levels. Discussion: Study results will indicate (1) the independent and combined effects of each attribute on consumer evaluations, (2) which front-of-pack labels are more effective at helping consumers distinguish between healthier and less healthy foods and (3) how health claims affect perceptions of healthiness. The study will also provide crucial information on the effectiveness of the new Health Star Rating system, for which quantitative research is currently lacking. Trial registration: Australian and New Zealand Clinical Trials Registry ACTRN12616000626460. Retrospectively registered: 16 May, 2016.Zenobia Talati, Simone Pettigrew, Helen Dixon, Bruce Neal, Clare Hughes, Trevor Shilton and Caroline Mille
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