596,986 research outputs found
The Forced van der Pol Equation II: Canards in the reduced system
This is the second in a series of papers about the dynamics of the forced van der Pol oscillator [J. Guckenheimer, K. Hoffman, and W. Weckesser, SIAM J. Appl. Dyn. Syst., 2 (2003), pp. 1–35].
The first paper described the reduced system, a two dimensional flow with jumps that reflect fast trajectory segments in this vector field with two time scales. This paper extends the reduced system to account for canards, trajectory segments that follow the unstable portion of the slow manifold in the forced van der Pol oscillator. This extension of the reduced system serves as a template for approximating the full nonwandering set of the forced van der Pol oscillator for large sets of parameter values, including parameters for which the system is chaotic. We analyze some bifurcations in the extension of the reduced system, building upon our previous work in [J. Guckenheimer, K. Hoffman, and W. Weckesser, SIAM J. Appl. Dyn. Syst., 2 (2003), pp. 1–35]. We conclude with computations of return maps and periodic orbits in the full three dimensional flow that are compared with the computations and analysis of the reduced system. These comparisons demonstrate numerically the validity of results we derive from the study of canards in the reduced system
Transcription start site scanning and the requirement for ATP during transcription initiation by RNA polymerase II
Saccharomyces cerevisiae RNA polymerase (Pol) II locates transcription start sites (TSS) at TATA-containing promoters by scanning sequences downstream from the site of preinitiation complex formation, a process that involves the translocation of downstream promoter DNA toward Pol II. To investigate a potential role of yeast Pol II transcription in TSS scanning, HIS4 promoter derivatives were generated that limited transcripts in the 30-bp scanned region to two nucleotides in length. Although we found that TSS scanning does not require RNA synthesis, our results revealed that transcription in the purified yeast basal system is largely ATP-independent despite a requirement for the TFIIH DNA translocase subunit Ssl2. This result is rationalized by our finding that, although they are poorer substrates, UTP and GTP can also be utilized by Ssl2. ATPγS is a strong inhibitor of rNTP-fueled translocation, and high concentrations of ATPγS make transcription completely dependent on added dATP. Limiting Pol II function with low ATP concentrations shifted the TSS position downstream. Combined with prior work, our results show that Pol II transcription plays an important role in TSS selection but is not required for the scanning reaction
The balloon-borne large-aperture submillimeter telescope for polarimetry: BLAST-Pol
The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry
(BLAST-Pol) is a suborbital mapping experiment designed to study the role
played by magnetic fields in the star formation process. BLAST-Pol is the
reconstructed BLAST telescope, with the addition of linear polarization
capability. Using a 1.8 m Cassegrain telescope, BLAST-Pol images the sky onto a
focal plane that consists of 280 bolometric detectors in three arrays,
observing simultaneously at 250, 350, and 500 um. The diffraction-limited
optical system provides a resolution of 30'' at 250 um. The polarimeter
consists of photolithographic polarizing grids mounted in front of each
bolometer/detector array. A rotating 4 K achromatic half-wave plate provides
additional polarization modulation. With its unprecedented mapping speed and
resolution, BLAST-Pol will produce three-color polarization maps for a large
number of molecular clouds. The instrument provides a much needed bridge in
spatial coverage between larger-scale, coarse resolution surveys and narrow
field of view, and high resolution observations of substructure within
molecular cloud cores. The first science flight will be from McMurdo Station,
Antarctica in December 2010.Comment: 14 pages, 9 figures Submitted to SPIE Astronomical Telescopes and
Instrumentation Conference 201
THE EUROPEAN COIIMUNITY AND EAST-WEST COOPERATION. International Herald Tribune Conference, "The Soviet Union in the 1990s" Moscow, 8 June 1989
As the world's largest and most open trading partner, the
European Community has a crucial role to play In the far
reaching changes now occurring In the International system .
This role has gained In Importance with the community's
progress towards a sing le market without Internal frontiers,
paving the way to Increasing economic and pol ltlcal unity.
The great European market of 1992 means a giant step towards
trade I lberal lsatlon, putting the Conmunlty In the front I lne
of the battle against protectionism worldwide. In this
battle, a~d In Its external re I at Ion, In general, the
Community's Instruments of pol Icy are essentially pol ltlcal
and economic, making I~ an attractive partner for al I who
seek economic progress and peaceful change. This attraction
Is particularly evident In the opening up of new
opportunities for east-west cooperation
Structural visualization of key steps in human transcription initiation.
Eukaryotic transcription initiation requires the assembly of general transcription factors into a pre-initiation complex that ensures the accurate loading of RNA polymerase II (Pol II) at the transcription start site. The molecular mechanism and function of this assembly have remained elusive due to lack of structural information. Here we have used an in vitro reconstituted system to study the stepwise assembly of human TBP, TFIIA, TFIIB, Pol II, TFIIF, TFIIE and TFIIH onto promoter DNA using cryo-electron microscopy. Our structural analyses provide pseudo-atomic models at various stages of transcription initiation that illuminate critical molecular interactions, including how TFIIF engages Pol II and promoter DNA to stabilize both the closed pre-initiation complex and the open-promoter complex, and to regulate start--initiation complexes, combined with the localization of the TFIIH helicases XPD and XPB, support a DNA translocation model of XPB and explain its essential role in promoter opening
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