4,336 research outputs found
The C Terminus of Ku80 activates the DNA-dependent protein kinase catalytic subunit
Ku is a heterodimeric protein with double-stranded DNA end-binding activity that operates in the process of nonhomologous end joining. Ku is thought to target the DNA-dependent protein kinase (DNA-PK) complex to the DNA and, when DNA bound, can interact and activate the DNA-PK catalytic subunit (DNA-PKcs). We have carried out a 3′ deletion analysis of Ku80, the larger subunit of Ku, and shown that the C-terminal 178 amino acid residues are dispensable for DNA end-binding activity but are required for efficient interaction of Ku with DNA-PKcs. Cells expressing Ku80 proteins that lack the terminal 178 residues have low DNA-PK activity, are radiation sensitive, and can recombine the signal junctions but not the coding junctions during V(D)J recombination. These cells have therefore acquired the phenotype of mouse SCID cells despite expressing DNA-PKcs protein, suggesting that an interaction between DNA-PKcs and Ku, involving the C-terminal region of Ku80, is required for DNA double-strand break rejoining and coding but not signal joint formation. To gain further insight into important domains in Ku80, we report a point mutational change in Ku80 in the defective xrs-2 cell line. This residue is conserved among species and lies outside of the previously reported Ku70-Ku80 interaction domain. The mutational change nonetheless abrogates the Ku70-Ku80 interaction and DNA end-binding activity
Exact Schwarzschild-Like Solution for Yang-Mills Theories
Drawing on the parallel between general relativity and Yang-Mills theory we
obtain an exact Schwarzschild-like solution for SU(2) gauge fields coupled to a
massless scalar field. Pushing the analogy further we speculate that this
classical solution to the Yang-Mills equations shows confinement in the same
way that particles become confined once they pass the event horizon of the
Schwarzschild solution. Two special cases of the solution are considered.Comment: 11 pages LaTe
Opening of DNA double strands by helicases. Active versus passive opening
Helicase opening of double-stranded nucleic acids may be "active" (the
helicase directly destabilizes the dsNA to promote opening) or "passive" (the
helicase binds ssNA available due to a thermal fluctuation which opens part of
the dsNA). We describe helicase opening of dsNA, based on helicases which bind
single NA strands and move towards the double-stranded region, using a discrete
``hopping'' model. The interaction between the helicase and the junction where
the double strand opens is characterized by an interaction potential. The form
of the potential determines whether the opening is active or passive. We
calculate the rate of passive opening for the helicase PcrA, and show that the
rate increases when the opening is active. Finally, we examine how to choose
the interaction potential to optimize the rate of strand separation. One
important result is our finding that active opening can increase the unwinding
rate by 7 fold compared to passive opening.Comment: 13 pages, 3 figure
Complex Lagrangians and phantom cosmology
Motivated by the generalization of quantum theory for the case of
non-Hermitian Hamiltonians with PT symmetry, we show how a classical
cosmological model describes a smooth transition from ordinary dark energy to
the phantom one. The model is based on a classical complex Lagrangian of a
scalar field. Specific symmetry properties analogous to PT in non-Hermitian
quantum mechanics lead to purely real equation of motion.Comment: 11 pages, to be published in J.Phys.A, refs. adde
On the physical meaning of the Unruh effect
We present simple arguments that detectors moving with constant acceleration
(even acceleration for a finite time) should detect particles. The effect is
seen to be universal. Moreover, detectors undergoing linear acceleration and
uniform, circular motion both detect particles for the same physical reason. We
show that if one uses a circularly orbiting electron in a constant external
magnetic field as the Unruh--DeWitt detector, then the Unruh effect physically
coincides with the experimentally verified Sokolov--Ternov effect.Comment: 7 pages, 0 figures references added, small changes in text. To be
published JETP Lett
On the relation between Unruh and Sokolov--Ternov effects
We show that the Sokolov--Ternov effect -- the depolarization of particles in
storage rings coming from synchrotron radiation due to spin flip transitions --
is physically equivalent to the Unruh effect for circular acceleration if one
uses a spin 1/2 particle as the Unruh--DeWitt detector. It is shown that for
the electron, with gyromagnetic number , the exponential
contribution to the polarization, which usually characterizes the Unruh effect,
is "hidden" in the standard Sokolov-Ternov effect making it hard to observe.
Thus, our conclusions are different in detail from previous work.Comment: 23 pages, no figure
Brane in 6D with increasing gravitational trapping potential
A new solution to Einstein equations in (1+5)-spacetime with an embedded
(1+3) brane is given. This solution localizes the zero modes of all kinds of
matter fields and 4-gravity on the (1+3) brane by an increasing, transverse
gravitational potential. This localization occurs despite the fact that the
gravitational potential is not a decreasing exponential, and asymptotically
approaches a finite value rather than zero.Comment: Revised paper. 6 pages, revtex 4. to be published in PR
A de Haas-van Alphen study of the filled skutterudite compounds PrOsAs and LaOsAs
Comprehensive magnetic-field-orientation dependent studies of the
susceptibility and de Haas-van Alphen effect have been carried out on single
crystals of the filled skutterudites PrOsAs and LaOsAs
using magnetic fields of up to 40~T. Several peaks are observed in the
low-field susceptibility of PrOsAs, corresponding to cascades of
metamagnetic transitions separating the low-field antiferromagnetic and
high-field paramagnetic metal (PMM) phases. The de Haas-van Alphen experiments
show that the Fermi-surface topologies of PrOsAs in its PMM phase
and LaOsAs are very similar. In addition, they are in reasonable
agreement with the predictions of bandstructure calculations for
LaOsAs on the PrOsAs lattice. Both observations suggest
that the Pr 4 electrons contribute little to the number of itinerant
quasiparticles in the PMM phase. However, whilst the properties of
LaOsAs suggest a conventional nonmagnetic Fermi liquid, the effects
of direct exchange and electron correlations are detected in the PMM phase of
PrOsAs. For example, the quasiparticle effective masses in
PrOsAs are found to decrease with increasing field, probably
reflecting the gradual suppression of magnetic fluctuations associated with
proximity to the low-temperature, low-field antiferromagnetic state
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