1,253 research outputs found
DNA binding induces active site conformational change in the human TREX2 3′-exonuclease
The TREX enzymes process DNA as the major 3′→5′ exonuclease activity in mammalian cells. TREX2 and TREX1 are members of the DnaQ family of exonucleases and utilize a two metal ion catalytic mechanism of hydrolysis. The structure of the dimeric TREX2 enzyme in complex with single-stranded DNA has revealed binding properties that are distinct from the TREX1 protein. The TREX2 protein undergoes a conformational change in the active site upon DNA binding including ordering of active site residues and a shift of an active site helix. Surprisingly, even when a single monomer binds DNA, both monomers in the dimer undergo the structural rearrangement. From this we have proposed a model for DNA binding and 3′ hydrolysis for the TREX2 dimer. The structure also shows how TREX proteins potentially interact with double-stranded DNA and suggest features that might be involved in strand denaturation to provide a single-stranded substrate for the active site
Test of the CLAS12 RICH large scale prototype in the direct proximity focusing configuration
A large area ring-imaging Cherenkov detector has been designed to provide
clean hadron identification capability in the momentum range from 3 GeV/c up to
8 GeV/c for the CLAS12 experiments at the upgraded 12 GeV continuous electron
beam accelerator facility of Jefferson Laboratory. The adopted solution
foresees a novel hybrid optics design based on aerogel radiator, composite
mirrors and high-packed and high-segmented photon detectors. Cherenkov light
will either be imaged directly (forward tracks) or after two mirror reflections
(large angle tracks). We report here the results of the tests of a large scale
prototype of the RICH detector performed with the hadron beam of the CERN T9
experimental hall for the direct detection configuration. The tests
demonstrated that the proposed design provides the required pion-to-kaon
rejection factor of 1:500 in the whole momentum range.Comment: 15 pages, 23 figures, to appear on EPJ
Subthreshold rho^0 photoproduction on 3He
A large reduction of the rho^0 mass in the nuclear medium is reported,
inferred from dipion photoproduction spectra in the 1 GeV region, for the
reaction 3He(gamma,pi+ pi-)X with a 10% duty factor tagged-photon beam and the
TAGX multi-particle spectrometer. The energy range covered (800 < E(gamma) <
1120 MeV) lies mostly below the free rho^0 production threshold, a region which
is believed sensitive to modifications of light vector-meson properties at
nuclear-matter densities. The rho^0 masses extracted from the MC fitting of the
data, m*(rho^0) = 642 +/- 40, 669 +/- 32, and 682 +/- 56 MeV/c^2 for E(gamma)
in the 800-880, 880-960, and 960-1040 MeV regions respectively, are
independently corroborated by a measured, assumption-free, kinematical
observable. This mass shift, far exceeding current mean-field driven
theoretical predictions, may be suggestive of rho^0 decay within the range of
the nucleonic field.Comment: 40 pages, 13 figures, submitted to Phys. Rev.
Investigation of the Exclusive 3He(e,e'pp)n Reaction
Cross sections for the 3He(e,e'pp)n reaction were measured over a wide range
of energy and three- momentum transfer. At a momentum transfer q=375 MeV/c,
data were taken at transferred energies omega ranging from 170 to 290 MeV. At
omega=220 MeV, measurements were performed at three q values (305, 375, and 445
MeV/c). The results are presented as a function of the neutron momentum in the
final-state, as a function of the energy and momentum transfer, and as a
function of the relative momentum of the two-proton system. The data at neutron
momenta below 100 MeV/c, obtained for two values of the momentum transfer at
omega=220 MeV, are well described by the results of continuum-Faddeev
calculations. These calculations indicate that the cross section in this domain
is dominated by direct two-proton emission induced by a one-body hadronic
current. Cross section distributions determined as a function of the relative
momentum of the two protons are fairly well reproduced by continuum-Faddeev
calculations based on various realistic nucleon-nucleon potential models. At
higher neutron momentum and at higher energy transfer, deviations between data
and calculations are observed that may be due to contributions of isobar
currents.Comment: 14 pages, 1 table, 17 figure
The He(e, ed)p Reaction in q-constant Kinematics
The cross section for the He(e, ed)p reaction has been measured as a
function of the missing momentum in q -constant kinematics at
beam energies of 370 and 576 MeV for values of the three-momentum transfer
of 412, 504 and 604 \mevc. The L(+TT), T and LT structure functions have been
separated for = 412 and 504 \mevc. The data are compared to three-body
Faddeev calculations, including meson-exchange currents (MEC), and to
calculations based on a covariant diagrammatic expansion. The influence of
final-state interactions and meson-exchange currents is discussed. The
-dependence of the data is reasonably well described by all calculations.
However, the most advanced Faddeev calculations, which employ the AV18
nucleon-nucleon interaction and include MEC, overestimate the measured cross
sections, especially the longitudinal part, and at the larger values of .
The diagrammatic approach gives a fair description of the cross section, but
under(over)estimates the longitudinal (transverse) structure function.Comment: 17 pages, 7 figure
Virtual Compton Scattering and the Generalized Polarizabilities of the Proton at Q^2=0.92 and 1.76 GeV^2
Virtual Compton Scattering (VCS) on the proton has been studied at Jefferson
Lab using the exclusive photon electroproduction reaction (e p --> e p gamma).
This paper gives a detailed account of the analysis which has led to the
determination of the structure functions P_LL-P_TT/epsilon and P_LT, and the
electric and magnetic generalized polarizabilities (GPs) alpha_E(Q^2) and
beta_M(Q^2) at values of the four-momentum transfer squared Q^2= 0.92 and 1.76
GeV^2. These data, together with the results of VCS experiments at lower
momenta, help building a coherent picture of the electric and magnetic GPs of
the proton over the full measured Q^2-range, and point to their non-trivial
behavior.Comment: version 2: modified according to PRC Editor's and Referee's
recommendations. Archival paper for the E93-050 experiment at JLab Hall A. 28
pages, 23 figures, 5 cross-section tables. To be submitted to Phys.Rev.
Backward electroproduction of pi0 mesons on protons in the region of nucleon resonances at four momentum transfer squared Q**2 = 1.0 GeV**2
Exclusive electroproduction of pi0 mesons on protons in the backward
hemisphere has been studied at Q**2 = 1.0 GeV**2 by detecting protons in the
forward direction in coincidence with scattered electrons from the 4 GeV
electron beam in Jefferson Lab's Hall A. The data span the range of the total
(gamma* p) center-of-mass energy W from the pion production threshold to W =
2.0 GeV. The differential cross sections sigma_T+epsilon*sigma_L, sigma_TL, and
sigma_TT were separated from the azimuthal distribution and are presented
together with the MAID and SAID parametrizations.Comment: 17 pages, 11 figures, information can be found at
http://hallaweb.jlab.org/experiment/E93-050/vcs.html updated content about
SAID analysis updated MAID results following new reference nucl-th/0310041
updated figure
Effect of Stalling after Mismatches on the Error Catastrophe in Nonenzymatic Nucleic Acid Replication
The frequency of errors during genome replication limits the amount of functionally important information that can be passed on from generation to generation. During the origin of life, mutation rates are thought to have been quite high, raising a classic chicken-and-egg paradox: could nonenzymatic replication propagate sequences accurately enough to allow for the emergence of heritable function? Here we show that the theoretical limit on genomic information content may increase substantially as a consequence of dramatically slowed polymerization after mismatches. As a result of postmismatch stalling, accurate copies of a template tend to be completed more rapidly than mutant copies and the accurate copies can therefore begin a second round of replication more quickly. To quantify this effect, we characterized an experimental model of nonenzymatic, template-directed nucleic acid polymerization. We found that most mismatches decrease the rate of primer extension by more than 2 orders of magnitude relative to a matched (Watson-Crick) control. A chemical replication system with this property would be able to propagate sequences long enough to have function. Our study suggests that the emergence of functional sequences during the origin of life would be possible even in the face of the high intrinsic error rates of chemical replication
Large Momentum Transfer Measurements of the Deuteron Elastic Structure Function A(Q^2) at Jefferson Laboratory
The deuteron elastic structure function A(Q^2) has been extracted in the Q^2
range 0.7 to 6.0 (GeV/c)^2 from cross section measurements of elastic
electron-deuteron scattering in coincidence using the Hall A Facility of
Jefferson Laboratory. The data are compared to theoretical models based on the
impulse approximation with inclusion of meson-exchange currents, and to
predictions of quark dimensional scaling and perturbative quantum
chromodynamicsComment: Submitted to Physical Review Letter
The reaction dynamics of the 16O(e,e'p) cross section at high missing energies
We measured the cross section and response functions (R_L, R_T, and R_LT) for
the 16O(e,e'p) reaction in quasielastic kinematics for missing energies 25 <=
E_miss <= 120 MeV at various missing momenta P_miss <= 340 MeV/c. For 25 <
E_miss < 50 MeV and P_miss \approx 60 MeV/c, the reaction is dominated by
single-nucleon knockout from the 1s1/2-state. At larger P_miss, the
single-particle aspects are increasingly masked by more complicated processes.
For E_miss > 60 MeV and P_miss > 200 MeV/c, the cross section is relatively
constant. Calculations which include contributions from pion exchange currents,
isobar currents and short-range correlations account for the shape and the
transversity but only for half of the magnitude of the measured cross section.Comment: 6 pages, 4 figures, submitted to Phys Rev Lett, formatting error
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