634 research outputs found
Model of Transcriptional Activation by MarA in Escherichia coli
We have developed a mathematical model of transcriptional activation by MarA
in Escherichia coli, and used the model to analyze measurements of
MarA-dependent activity of the marRAB, sodA, and micF promoters in mar-rob-
cells. The model rationalizes an unexpected poor correlation between the
mid-point of in vivo promoter activity profiles and in vitro equilibrium
constants for MarA binding to promoter sequences. Analysis of the promoter
activity data using the model yielded the following predictions regarding
activation mechanisms: (1) MarA activation of the marRAB, sodA, and micF
promoters involves a net acceleration of the kinetics of transitions after RNA
polymerase binding, up to and including promoter escape and message elongation;
(2) RNA polymerase binds to these promoters with nearly unit occupancy in the
absence of MarA, making recruitment of polymerase an insignificant factor in
activation of these promoters; and (3) instead of recruitment, activation of
the micF promoter might involve a repulsion of polymerase combined with a large
acceleration of the kinetics of polymerase activity. These predictions are
consistent with published chromatin immunoprecipitation assays of interactions
between polymerase and the E. coli chromosome. A lack of recruitment in
transcriptional activation represents an exception to the textbook description
of activation of bacterial sigma-70 promoters. However, use of accelerated
polymerase kinetics instead of recruitment might confer a competitive advantage
to E. coli by decreasing latency in gene regulation.Comment: 30 pages, 2 figure
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Conceptualizing Cancer Drugs as Classifiers
Cancer and healthy cells have distinct distributions of molecular properties and thus respond differently to drugs. Cancer drugs ideally kill cancer cells while limiting harm to healthy cells. However, the inherent variance among cells in both cancer and healthy cell populations increases the difficulty of selective drug action. Here we formalize a classification framework based on the idea that an ideal cancer drug should maximally discriminate between cancer and healthy cells. More specifically, this discrimination should be performed on the basis of measurable cell markers. We divide the problem into three parts which we explore with examples. First, molecular markers should discriminate cancer cells from healthy cells at the single-cell level. Second, the effects of drugs should be statistically predicted by these molecular markers. Third, drugs should be optimized for classification performance. We find that expression levels of a handful of genes suffice to discriminate well between individual cells in cancer and healthy tissue. We also find that gene expression predicts the efficacy of some cancer drugs, suggesting that these cancer drugs act as suboptimal classifiers using gene profiles. Finally, we formulate a framework that defines an optimal drug, and predicts drug cocktails that may target cancer more accurately than the individual drugs alone. Conceptualizing cancer drugs as solving a discrimination problem in the high-dimensional space of molecular markers promises to inform the design of new cancer drugs and drug cocktails.</p
An Analysis of B_s Decays in the Left-Right-Symmetric Model with Spontaneous CP Violation
Non-leptonic B_s decays into CP eigenstates that are caused by \bar b -> \bar
cc\bar s quark-level transitions, such as B_s -> D_s^+D^-_s, J/psi eta^(') or
J/psi phi, provide a powerful tool to search for ``new physics'', as the
CP-violating effects in these modes are tiny in the Standard Model. We explore
these effects for a particular scenario of new physics, the
left-right-symmetric model with spontaneous CP violation. In our analysis, we
take into account all presently available experimental constraints on the
parameters of this model, i.e. those implied by K- and B-decay observables; we
find that CP asymmetries as large as O(40%) may arise in the B_s channels,
whereas the left-right-symmetric model favours a small CP asymmetry in the
``gold-plated'' mode B_d -> J/psi K_S. Such a pattern would be in favour of
B-physics experiments at hadron machines, where the B_s modes are very
accessible.Comment: 12 pages, 5 figure
An Ex-Ante Method to Verify Commercial U.S. Nuclear Power Plant Decommissioning Cost Estimates
There are billions of dollars at stake in the US nuclear power plant decommissioning market. Approximately 100 nuclear power plants are still operating but will come offline and need to be decommissioned over the next few decades. The US Nuclear Regulatory Commission (NRC) mandates that the operators of these plants set money aside in segregated funds to finance decommissioning work. However, it is hard for external stakeholders to verify the cost estimations, which ultimately determine how much operators are required to save. In this paper, we develop a method to validate the existing cost models and calculate a contingency empirically for these models. We extend Reference Class Forecasting methods using adaptive kernel fitting and the Wilks' formula. Based on this method, and assuming a social tolerance for potential cost overruns of 20%, we calculate a new contingency of 48% of the estimated radiological decommissioning cost. After a "stress test" of the current decommissioning trust funds of operating reactor sites, we find that 48% of reactors we considered have sufficient funding-in many cases substantially more than required-and could therefore finance the potential scale of overrun. However, we find that 28 plants (52%) would fall short on average $211 million. Still, overruns at every plant are not a foregone conclusion because-while overruns are probable, based on past experience-the actual scale and frequency is not known. Nevertheless, our results add further evidence to the mounting call for the NRC to revise its cost models in light of new information
Solar astronomy
An overview is given of modern solar physics. Topics covered include the solar interior, the solar surface, the solar atmosphere, the Large Earth-based Solar Telescope (LEST), the Orbiting Solar Laboratory, the High Energy Solar Physics mission, the Space Exploration Initiative, solar-terrestrial physics, and adaptive optics. Policy and related programmatic recommendations are given for university research and education, facilitating solar research, and integrated support for solar research
Temperature and Emission-Measure Profiles Along Long-Lived Solar Coronal Loops Observed with TRACE
We report an initial study of temperature and emission measure distributions
along four steady loops observed with the Transition Region and Coronal
Explorer (TRACE) at the limb of the Sun. The temperature diagnostic is the
filter ratio of the extreme-ultraviolet 171-angstrom and 195-angstrom
passbands. The emission measure diagnostic is the count rate in the
171-angstrom passband. We find essentially no temperature variation along the
loops. We compare the observed loop structure with theoretical isothermal and
nonisothermal static loop structure.Comment: 10 pages, 3 postscript figures (LaTeX, uses aaspp4.sty). Accepted by
ApJ Letter
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