8,975 research outputs found
Illinois Waterfowl Harvest, Hunter Activity, and Attitudes Toward September Canada Goose Season, Canada Goose Harvest Monitoring System, and Dates for Hunting Seasons, 1997-98
Conserved Amino Acids in Each Subunit of the Heteroligomeric tRNA m\u3csup\u3e1\u3c/sup\u3eA58 Mtase from \u3cem\u3eSaccharomyces cerevisiae\u3c/em\u3e Contribute to tRNA Binding
In Saccharomyces cerevisiae, a two-subunit methyltransferase (Mtase) encoded by the essential genes TRM6 and TRM61 is responsible for the formation of 1-methyladenosine, a modified nucleoside found at position 58 in tRNA that is critical for the stability of . The crystal structure of the homotetrameric m1A58 tRNA Mtase from Mycobacterium tuberculosis, TrmI, has been solved and was used as a template to build a model of the yeast m1A58 tRNA Mtase heterotetramer. We altered amino acids in TRM6 and TRM61 that were predicted to be important for the stability of the heteroligomer based on this model. Yeast strains expressing trm6 and trm61 mutants exhibited growth phenotypes indicative of reduced m1A formation. In addition, recombinant mutant enzymes had reduced in vitro Mtase activity. We demonstrate that the mutations introduced do not prevent heteroligomer formation and do not disrupt binding of the cofactor S-adenosyl-l-methionine. Instead, amino acid substitutions in either Trm6p or Trm61p destroy the ability of the yeast m1A58 tRNA Mtase to bind , indicating that each subunit contributes to tRNA binding and suggesting a structural alteration of the substrate-binding pocket occurs when these mutations are present
A Compact Microchip-Based Atomic Clock Based on Ultracold Trapped Rb Atoms
We propose a compact atomic clock based on ultracold Rb atoms that are
magnetically trapped near the surface of an atom microchip. An interrogation
scheme that combines electromagnetically-induced transparency (EIT) with
Ramsey's method of separated oscillatory fields can achieve atomic shot-noise
level performance of 10^{-13}/sqrt(tau) for 10^6 atoms. The EIT signal can be
detected with a heterodyne technique that provides noiseless gain; with this
technique the optical phase shift of a 100 pW probe beam can be detected at the
photon shot-noise level. Numerical calculations of the density matrix equations
are used to identify realistic operating parameters at which AC Stark shifts
are eliminated. By considering fluctuations in these parameters, we estimate
that AC Stark shifts can be canceled to a level better than 2*10^{-14}. An
overview of the apparatus is presented with estimates of duty cycle and power
consumption.Comment: 15 pages, 11 figures, 5 table
Combined visible and near-infrared OPA for wavelength scaling experiments in strong-field physics
We report the operation of an optical parametric amplifier (OPA) capable of
producing gigawatt peak-power laser pulses with tunable wavelength in either
the visible or near-infrared spectrum. The OPA has two distinct operation modes
(i) generation of >350 uJ, sub 100 fs pulses, tunable between 1250 - 1550 nm;
(ii) generation of >190 uJ, sub 150 fs pulses tunable between 490 - 530 nm. We
have recorded high-order harmonic spectra over a wide range of driving
wavelengths. This flexible source of femtosecond pulses presents a useful tool
for exploring the wavelength-dependence of strong-field phenomena, in both the
multi-photon and tunnel ionization regimes.Comment: 14 pages, 9 figures, This paper was published in Proceedings of SPIE
10088, Nonlinear Frequency Generation and Conversion: Materials and Devices
XVI, doi 10.1117/12.225077
Efficient Immunization Strategies for Computer Networks and Populations
We present an effective immunization strategy for computer networks and
populations with broad and, in particular, scale-free degree distributions. The
proposed strategy, acquaintance immunization, calls for the immunization of
random acquaintances of random nodes (individuals). The strategy requires no
knowledge of the node degrees or any other global knowledge, as do targeted
immunization strategies. We study analytically the critical threshold for
complete immunization. We also study the strategy with respect to the
susceptible-infected-removed epidemiological model. We show that the
immunization threshold is dramatically reduced with the suggested strategy, for
all studied cases.Comment: Revtex, 5 pages, 4 ps fig
A Mesoscopic Resonating Valence Bond system on a triple dot
We introduce a mesoscopic pendulum from a triple dot. The pendulum is
fastened through a singly-occupied dot (spin qubit). Two other strongly
capacitively islands form a double-dot charge qubit with one electron in excess
oscillating between the two low-energy charge states (1,0) and (0,1); this
embodies the weight of the pendulum. The triple dot is placed between two
superconducting leads as shown in Fig. 1. Under well-defined conditions, the
main proximity effect stems from the injection of resonating singlet (valence)
bonds on the triple dot. This gives rise to a Josephson current that is charge-
and spin-dependent. Consequences in a SQUID-geometry are carefully
investigated.Comment: final version to appear in PR
A new approach for performing contamination control bakeouts in JPL thermal vacuum test chambers
Contamination control requirements for the Wide Field/Planetary Camera II (WF/PC II) are necessarily stringent to protect against post-launch contamination of the sensitive optical surfaces, particularly the cold charge coupled device (CCD) imaging surfaces. Typically, thermal vacuum test chambers have employed a liquid nitrogen (LN2) cold trap to collect outgassed contaminants. This approach has the disadvantage of risking recontamination of the test article from shroud offgassing during post-test warmup of the chamber or from any shroud warming of even a few degrees during the bakeout process. By using an enclave, essentially a chamber within a chamber, configured concentrically and internally within an LN2 shroud, a method was developed, based on a design concept by Taylor, for preventing recontamination of test articles during bakeouts and subsequent post-test warmup of the vacuum chamber. Enclaves for testing WF/PC II components were designed and fabricated, then installed in three of JPL's Environmental Test Lab chambers. The design concepts, operating procedures, and test results of this development are discussed
Spreading of Persistent Infections in Heterogeneous Populations
Up to now, the effects of having heterogeneous networks of contacts have been
studied mostly for diseases which are not persistent in time, i.e., for
diseases where the infectious period can be considered very small compared to
the lifetime of an individual. Moreover, all these previous results have been
obtained for closed populations, where the number of individuals does not
change during the whole duration of the epidemics. Here, we go one step further
and analyze, both analytically and numerically, a radically different kind of
diseases: those that are persistent and can last for an individual's lifetime.
To be more specific, we particularize to the case of Tuberculosis' (TB)
infection dynamics, where the infection remains latent for a period of time
before showing up and spreading to other individuals. We introduce an
epidemiological model for TB-like persistent infections taking into account the
heterogeneity inherent to the population structure. This sort of dynamics
introduces new analytical and numerical challenges that we are able to sort
out. Our results show that also for persistent diseases the epidemic threshold
depends on the ratio of the first two moments of the degree distribution so
that it goes to zero in a class of scale-free networks when the system
approaches the thermodynamic limit.Comment: 12 pages and 2 figures. Revtex format. Submitted for publication
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