710 research outputs found
On The Weak-Coupling Limit for Bosons and Fermions
In this paper we consider a large system of Bosons or Fermions. We start with
an initial datum which is compatible with the Bose-Einstein, respectively
Fermi-Dirac, statistics. We let the system of interacting particles evolve in a
weak-coupling regime. We show that, in the limit, and up to the second order in
the potential, the perturbative expansion expressing the value of the
one-particle Wigner function at time , agrees with the analogous expansion
for the solution to the Uehling-Uhlenbeck equation. This paper follows in
spirit the companion work [\rcite{BCEP}], where the authors investigated the
weak-coupling limit for particles obeying the Maxwell-Boltzmann statistics:
here, they proved a (much stronger) convergence result towards the solution of
the Boltzmann equation
Role of ATP hydrolysis in the DNA translocase activity of the bovine papillomavirus (BPV-1) E1 helicase
The E1 protein of bovine papillomavirus type-1 is the viral replication initiator protein and replicative helicase. Here we show that the C-terminal âŒ300 amino acids of E1, that share homology with members of helicase superfamily 3 (SF3), can act as an autonomous helicase. E1 is monomeric in the absence of ATP but assembles into hexamers in the presence of ATP, single-stranded DNA (ssDNA) or both. A 16 base sequence is the minimum for efficient hexamerization, although the complex protects âŒ30 bases from nuclease digestion, supporting the notion that the DNA is bound within the protein complex. In the absence of ATP, or in the presence of ADP or the nonâhydrolysable ATP analogue AMPâPNP, the interaction with short ssDNA oligonucleotides is exceptionally tight (T(1/2) > 6 h). However, in the presence of ATP, the interaction with DNA is destabilized (T(1/2) âŒ60 s). These results suggest that during the ATP hydrolysis cycle an internal DNA-binding site oscillates from a high to a low-affinity state, while proteinâprotein interactions switch from low to high affinity. This reciprocal change in proteinâprotein and proteinâDNA affinities could be part of a mechanism for tethering the protein to its substrate while unidirectional movement along DNA proceeds
Common determinants in DNA melting and helicase-catalysed DNA unwinding by papillomavirus replication protein E1
E1 and T-antigen of the tumour viruses bovine papillomavirus (BPV-1) and Simian virus 40 (SV40) are the initiator proteins that recognize and melt their respective origins of replication in the initial phase of DNA replication. These proteins then assemble into processive hexameric helicases upon the single-stranded DNA that they create. In T-antigen, a characteristic loop and hairpin structure (the pre-sensor 1ÎČ hairpin, PS1ÎČH) project into a central cavity generated by protein hexamerization. This channel undergoes large ATP-dependent conformational changes, and the loop/PS1ÎČH is proposed to form a DNA binding site critical for helicase activity. Here, we show that conserved residues in BPV E1 that probably form a similar loop/hairpin structure are required for helicase activity and also origin (ori) DNA melting. We propose that DNA melting requires the cooperation of the E1 helicase domain (E1HD) and the origin binding domain (OBD) tethered to DNA. One possible mechanism is that with the DNA locked in the loop/PS1ÎČH DNA binding site, ATP-dependent conformational changes draw the DNA inwards in a twisting motion to promote unwinding
Experimentally increased group diversity improves disease resistance in an ant species.
A leading hypothesis linking parasites to social evolution is that more genetically diverse social groups better resist parasites. Moreover, group diversity can encompass factors other than genetic variation that may also influence disease resistance. Here, we tested whether group diversity improved disease resistance in an ant species with natural variation in colony queen number. We formed experimental groups of workers and challenged them with the fungal parasite Metarhizium anisopliae. Workers originating from monogynous colonies (headed by a single queen and with low genetic diversity) had higher survival than workers originating from polygynous ones, both in uninfected groups and in groups challenged with M. anisopliae. However, an experimental increase of group diversity by mixing workers originating from monogynous colonies strongly increased the survival of workers challenged with M. anisopliae, whereas it tended to decrease their survival in absence of infection. This experiment suggests that group diversity, be it genetic or environmental, improves the mean resistance of group members to the fungal infection, probably through the sharing of physiological or behavioural defences
Transport and conservation laws
We study the lowest order conservation laws in one-dimensional (1D)
integrable quantum many-body models (IQM) as the Heisenberg spin 1/2 chain, the
Hubbard and t-J model. We show that the energy current is closely related to
the first conservation law in these models and therefore the thermal transport
coefficients are anomalous. Using an inequality on the time decay of current
correlations we show how the existence of conserved quantities implies a finite
charge stiffness (weight of the zero frequency component of the conductivity)
and so ideal conductivity at finite temperatures.Comment: 6 pages, Late
Transport in the XX chain at zero temperature: Emergence of flat magnetization profiles
We study the connection between magnetization transport and magnetization
profiles in zero-temperature XX chains. The time evolution of the transverse
magnetization, m(x,t), is calculated using an inhomogeneous initial state that
is the ground state at fixed magnetization but with m reversed from -m_0 for
x0. In the long-time limit, the magnetization evolves into a
scaling form m(x,t)=P(x/t) and the profile develops a flat part (m=P=0) in the
|x/t|1/2 while it
expands with the maximum velocity, c_0=1, for m_0->0. The states emerging in
the scaling limit are compared to those of a homogeneous system where the same
magnetization current is driven by a bulk field, and we find that the
expectation values of various quantities (energy, occupation number in the
fermionic representation) agree in the two systems.Comment: RevTex, 8 pages, 3 ps figure
Finite temperature Drude weight of the one dimensional spin 1/2 Heisenberg model}
Using the Bethe ansatz method, the zero frequency contribution (Drude weight)
to the spin current correlations is analyzed for the easy plane
antiferromagnetic Heisenberg model. The Drude weight is a monotonically
decreasing function of temperature for all 0<Delta< 1, it approaches the zero
temperature value with a power law and it appears to vanish for all finite
temperatures at the isotropic Delta=1 point.Comment: 5 pages, 2 Postscript figure
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