762 research outputs found
Crystal Structures of the structure-selective nuclease Mus81-Eme1 bound to flap DNA substrates
The Mus81-Eme1 complex is a structure-selective endonuclease with a critical role in the resolution of recombination intermediates during DNA repair after interstrand cross-links, replication fork collapse, or double-strand breaks. To explain the molecular basis of 3 ' flap substrate recognition and cleavage mechanism by Mus81-Eme1, we determined crystal structures of human Mus81-Eme1 bound to various flap DNA substrates. Mus81-Eme1 undergoes gross substrate-induced conformational changes that reveal two key features: (i) a hydrophobic wedge of Mus81 that separates pre- and post-nick duplex DNA and (ii) a 5 ' end binding pocket that hosts the 5 ' nicked end of post-nick DNA. These features are crucial for comprehensive protein-DNA interaction, sharp bending of the 3 ' flap DNA substrate, and incision strand placement at the active site. While Mus81-Eme1 unexpectedly shares several common features with members of the 5 ' flap nuclease family, the combined structural, biochemical, and biophysical analyses explain why Mus81-Eme1 preferentially cleaves 3 ' flap DNA substrates with 5 ' nicked ends.X11119Ysciescopu
Three Millisecond Pulsars in FERMI LAT Unassociated Bright Sources
We searched for radio pulsars in 25 of the non-variable, unassociated sources
in the Fermi LAT Bright Source List with the Green Bank Telescope at 820 MHz.
We report the discovery of three radio and gamma-ray millisecond pulsars (MSPs)
from a high Galactic latitude subset of these sources. All of the pulsars are
in binary systems, which would have made them virtually impossible to detect in
blind gamma-ray pulsation searches. They seem to be relatively normal, nearby
(<=2 kpc) millisecond pulsars. These observations, in combination with the
Fermi detection of gamma-rays from other known radio MSPs, imply that most, if
not all, radio MSPs are efficient gamma-ray producers. The gamma-ray spectra of
the pulsars are power-law in nature with exponential cutoffs at a few GeV, as
has been found with most other pulsars. The MSPs have all been detected as
X-ray point sources. Their soft X-ray luminosities of ~10^{30-31} erg/s are
typical of the rare radio MSPs seen in X-rays.Comment: Accepted for publication in ApJ Letter
Eight gamma-ray pulsars discovered in blind frequency searches of Fermi LAT data
We report the discovery of eight gamma-ray pulsars in blind frequency
searches using the LAT, onboard the Fermi Gamma-ray Space Telescope. Five of
the eight pulsars are young (tau_c10^36 erg/s), and
located within the Galactic plane (|b|<3 deg). The remaining three are older,
less energetic, and located off the plane. Five pulsars are associated with
sources included in the LAT bright gamma-ray source list, but only one, PSR
J1413-6205, is clearly associated with an EGRET source. PSR J1023-5746 has the
smallest characteristic age (tau_c=4.6 kyr) and is the most energetic
(Edot=1.1E37 erg/s) of all gamma-ray pulsars discovered so far in blind
searches. PSRs J1957+5033 and J2055+25 have the largest characteristic ages
(tau_c~1 Myr) and are the least energetic (Edot~5E33 erg/s) of the
newly-discovered pulsars. We present the timing models, light curves, and
detailed spectral parameters of the new pulsars. We used recent XMM
observations to identify the counterpart of PSR J2055+25 as XMMU
J205549.4+253959. In addition, publicly available archival Chandra X-ray data
allowed us to identify the likely counterpart of PSR J1023-5746 as a faint,
highly absorbed source, CXOU J102302.8-574606. The large X-ray absorption
indicates that this could be among the most distant gamma-ray pulsars detected
so far. PSR J1023-5746 is positionally coincident with the TeV source HESS
J1023-575, located near the young stellar cluster Westerlund 2, while PSR
J1954+2836 is coincident with a 4.3 sigma excess reported by Milagro at a
median energy of 35 TeV. Deep radio follow-up observations of the eight pulsars
resulted in no detections of pulsations and upper limits comparable to the
faintest known radio pulsars, indicating that these can be included among the
growing population of radio-quiet pulsars in our Galaxy being uncovered by the
LAT, and currently numbering more than 20.Comment: Submitted to Ap
Branching Fractions of tau Leptons to Three Charged Hadrons
From electron-positron collision data collected with the CLEO detector
operating at CESR near \sqrt{s}=10.6 GeV, improved measurements of the
branching fractions for tau decays into three explicitly identified hadrons and
a neutrino are presented as {\cal
B}(\tau^-\to\pi^-\pi^+\pi^-\nu_\tau)=(9.13\pm0.05\pm0.46)%, {\cal B}(\tau^-\to
K^-\pi^+\pi^-\nu_\tau)=(3.84\pm0.14\pm0.38)\times10^{-3}, {\cal B}(\tau^-\to
K^-K^+\pi^-\nu_\tau)=(1.55\pm0.06\pm0.09)\times10^{-3}, and {\cal B}(\tau^-\to
K^-K^+K^-\nu_\tau)<3.7\times10^{-5} at 90% C.L., where the uncertainties are
statistical and systematic, respectively.Comment: 10 pages postscript, also available through
http://w4.lns.cornell.edu/public/CLNS, to appear in Phys. Rev. Let
Observation of the Hadronic Transitions Chi_{b 1,2}(2P) -> omega Upsilon(1S)
The CLEO Collaboration has observed the first hadronic transition among
bottomonium (b bbar) states other than the dipion transitions among vector
states, Upsilon(nS) -> pi pi Upsilon(mS). In our study of Upsilon(3S) decays,
we find a significant signal for Upsilon(3S) -> gamma omega Upsilon(1S) that is
consistent with radiative decays Upsilon(3S) -> gamma chi_{b 1,2}(2P), followed
by chi_{b 1,2} -> omega Upsilon(1S). The branching ratios we obtain are
Br(chi_{b1} -> omega Upsilon(1S) = 1.63 (+0.35 -0.31) (+0.16 -0.15) % and
Br(chi_{b2} -> omega Upsilon(1S) = 1.10 (+0.32 -0.28) (+0.11 - 0.10)%, in which
the first error is statistical and the second is systematic.Comment: submitted to XXI Intern'l Symp on Lepton and Photon Interact'ns at
High Energies, August 2003, Fermila
Improved Measurement of the Form Factors in the Decay Lambda_c^+ --> Lambda e^+ nu_e
Using the CLEO detector at the Cornell Electron Storage Ring, we have studied
the distribution of kinematic variables in the decay Lambda_c^+ -> Lambda e^+
nu_e. By performing a four-dimensional maximum likelihood fit, we determine the
form factor ratio, R = f_2/f_1 = -0.31 +/- 0.05(stat) +/- 0.04(syst), the pole
mass, M_{pole} = (2.21 +/- 0.08(stat) +/- 0.14(syst)) GeV/c^2, and the decay
asymmetry parameter of the Lambda_c, alpha_{Lambda_c} = -0.86 +/- 0.03(stat)
+/- 0.02(syst), for = 0.67 (GeV/c^2)^2. We compare the angular
distributions of the Lambda_c^+ and Lambda_c^- and find no evidence for
CP-violation: A_{Lambda_c} = (alpha_{Lambda_c^+} + alpha_{Lambda_c^-})/
(alpha_{Lambda_c^+} - alpha_{Lambda_c^-}) = 0.00 +/- 0.03(stat) +/- 0.01(syst)
+/- 0.02, where the third error is from the uncertainty in the world average of
the CP-violating parameter, A_{Lambda}, for Lambda -> p pi^-.Comment: 8 pages postscript,also available through
http://www.lns.cornell.edu/public/CLNS/2004/, submitted to PR
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