1,128 research outputs found
Metal-to-semiconductor transition in squashed armchair carbon nanotubes
We investigate electronic transport properties of the squashed armchair
carbon nanotubes, using tight-binding molecular dynamics and Green's function
method. We demonstrate a metal-to-semiconductor transistion while squashing the
nanotubes and a general mechanism for such transistion. It is the distinction
of the two sublattices in the nanotube that opens an energy gap near the Fermi
energy. We show that the transition has to be achieved by a combined effect of
breaking of mirror symmetry and bond formation between the flattened faces in
the squashed nanotubes.Comment: 4 papges, 4 figures, to appear in Phys. Rev. Let
Dynamic Fano Resonance of Quasienergy Excitons in Superlattices
The dynamic Fano resonance (DFR) between discrete quasienergy excitons and
sidebands of their ionization continua is predicted and investigated in dc- and
ac-driven semiconductor superlattices. This DFR, well controlled by the ac
field, delocalizes the excitons and opens an intrinsic decay channel in
nonlinear four-wave mixing signals.Comment: 4pages, 4figure
BRCA1-induced large-scale chromatin unfolding and allele-specific effects of cancer-predisposing mutations
The breast cancer susceptibility gene BRCA1 encodes a protein that has been implicated in multiple nuclear functions, including transcription and DNA repair. The multifunctional nature of BRCA1 has raised the possibility that the polypeptide may regulate various nuclear processes via a common underlying mechanism such as chromatin remodeling. However, to date, no direct evidence exists in mammalian cells for BRCA1-mediated changes in either local or large-scale chromatin structure. Here we show that targeting BRCA1 to an amplified, lac operator–containing chromosome region in the mammalian genome results in large-scale chromatin decondensation. This unfolding activity is independently conferred by three subdomains within the transactivation domain of BRCA1, namely activation domain 1, and the two BRCA1 COOH terminus (BRCT) repeats. In addition, we demonstrate a similar chromatin unfolding activity associated with the transactivation domains of E2F1 and tumor suppressor p53. However, unlike E2F1 and p53, BRCT-mediated chromatin unfolding is not accompanied by histone hyperacetylation. Cancer-predisposing mutations of BRCA1 display an allele-specific effect on chromatin unfolding: 5′ mutations that result in gross truncation of the protein abolish the chromatin unfolding activity, whereas those in the 3′ region of the gene markedly enhance this activity. A novel cofactor of BRCA1 (COBRA1) is recruited to the chromosome site by the first BRCT repeat of BRCA1, and is itself sufficient to induce chromatin unfolding. BRCA1 mutations that enhance chromatin unfolding also increase its affinity for, and recruitment of, COBRA1. These results indicate that reorganization of higher levels of chromatin structure is an important regulated step in BRCA1-mediated nuclear functions
Pathological phenotypes and in vivo DNA cleavage by unrestrained activity of a phosphorothioate-based restriction system in Salmonella
Prokaryotes protect their genomes from foreign DNA with a diversity of defence mechanisms, including a widespread restriction–modification (R–M) system involving phosphorothioate (PT) modification of the DNA backbone. Unlike classical R–M systems, highly partial PT modification of consensus motifs in bacterial genomes suggests an unusual mechanism of PT-dependent restriction. In Salmonella enterica, PT modification is mediated by four genes dptB–E, while restriction involves additional three genes dptF–H. Here, we performed a series of studies to characterize the PT-dependent restriction, and found that it presented several features distinct with traditional R–M systems. The presence of restriction genes in a PT-deficient mutant was not lethal, but instead resulted in several pathological phenotypes. Subsequent transcriptional profiling revealed the expression of > 600 genes was affected by restriction enzymes in cells lacking PT, including induction of bacteriophage, SOS response and DNA repair-related genes. These transcriptional responses are consistent with the observation that restriction enzymes caused extensive DNA cleavage in the absence of PT modifications in vivo. However, overexpression of restriction genes was lethal to the host in spite of the presence PT modifications. These results point to an unusual mechanism of PT-dependent DNA cleavage by restriction enzymes in the face of partial PT modification.National Natural Science Foundation (China) (Grant 31170085)National Natural Science Foundation (China) (Grant 31070058)Ministry of Science and Technology of the People's Republic of China (973 and 863 Programs)China Scholarship CouncilNational Science Foundation (U.S.) (Grant CHE-1019990)Shanghai Municipal Council of Science and Technology. Shanghai Pujiang Program (Grant 12PJD021
Principal inpatient diagnostic cost group model for Medicare risk adjustment
The Balanced Budget Act (BBA) of 1997 required HCFA to implement health-status-based risk adjustment for Medicare capitation payments for managed care plans by January 1, 2000. In support of this mandate, HCFA has been collecting inpatient encounter data from health plans since 1997. These data include diagnoses and other information that can be used to identify chronic medical problems that contribute to higher costs, so that health plans can be paid more when they care for sicker patients. In this article, the authors describe the risk-adjustment model HCFA is implementing in the year 2000, known as the Principal Inpatient Diagnostic Cost Group (PIPDCG) model
Calculations on the Size Effects of Raman Intensities of Silicon Quantum Dots
Raman intensities of Si quantum dots (QDs) with up to 11,489 atoms (about 7.6
nm in diameter) for different scattering configurations are calculated. First,
phonon modes in these QDs, including all vibration frequencies and vibration
amplitudes, are calculated directly from the lattice dynamic matrix by using a
microscopic valence force field model combined with the group theory. Then the
Raman intensities of these quantum dots are calculated by using a
bond-polarizability approximation. The size effects of the Raman intensity in
these QDs are discussed in detail based on these calculations. The calculations
are compared with the available experimental observation. We are expecting that
our calculations can further stimulate more experimental measurements.Comment: 21 pages, 7 figure
Recommended from our members
Differential Radiosensitivity Phenotypes of DNA-PKcs Mutations Affecting NHEJ and HRR Systems following Irradiation with Gamma-Rays or Very Low Fluences of Alpha Particles
We have examined cell-cycle dependence of chromosomal aberration induction and cell killing after high or low dose-rate γ irradiation in cells bearing DNA-PKcs mutations in the S2056 cluster, the T2609 cluster, or the kinase domain. We also compared sister chromatid exchanges (SCE) production by very low fluences of α-particles in DNA-PKcs mutant cells, and in homologous recombination repair (HRR) mutant cells including Rad51C, Rad51D, and Fancg/xrcc9. Generally, chromosomal aberrations and cell killing by γ-rays were similarly affected by mutations in DNA-PKcs, and these mutant cells were more sensitive in G1 than in S/G2 phase. In G1-irradiated DNA-PKcs mutant cells, both chromosome- and chromatid-type breaks and exchanges were in excess than wild-type cells. For cells irradiated in late S/G2 phase, mutant cells showed very high yields of chromatid breaks compared to wild-type cells. Few exchanges were seen in DNA-PKcs-null, Ku80-null, or DNA-PKcs kinase dead mutants, but exchanges in excess were detected in the S2506 or T2609 cluster mutants. SCE induction by very low doses of α-particles is resulted from bystander effects in cells not traversed by α-particles. SCE seen in wild-type cells was completely abolished in Rad51C- or Rad51D-deficient cells, but near normal in Fancg/xrcc9 cells. In marked contrast, very high levels of SCEs were observed in DNA-PKcs-null, DNA-PKcs kinase-dead and Ku80-null mutants. SCE induction was also abolished in T2609 cluster mutant cells, but was only slightly reduced in the S2056 cluster mutant cells. Since both non-homologous end-joining (NHEJ) and HRR systems utilize initial DNA lesions as a substrate, these results suggest the possibility of a competitive interference phenomenon operating between NHEJ and at least the Rad51C/D components of HRR; the level of interaction between damaged DNA and a particular DNA-PK component may determine the level of interaction of such DNA with a relevant HRR component
Expected Performance of the ATLAS Experiment - Detector, Trigger and Physics
A detailed study is presented of the expected performance of the ATLAS
detector. The reconstruction of tracks, leptons, photons, missing energy and
jets is investigated, together with the performance of b-tagging and the
trigger. The physics potential for a variety of interesting physics processes,
within the Standard Model and beyond, is examined. The study comprises a series
of notes based on simulations of the detector and physics processes, with
particular emphasis given to the data expected from the first years of
operation of the LHC at CERN
- …