7,504 research outputs found
Probing the parton densities of polarized photons at a linear e^+e^- collider
The present theoretical status of spin-dependent parton densities Delta
f^{\gamma}(x,Q^2) of circularly polarized photons is briefly reviewed. It is
then demonstrated that measurements of the deep-inelastic spin asymmetry
A_1^{\gamma}\simeq g_1^{\gamma}/F_1^{\gamma} and of di-jet rapidity
distributions at a future linear e^+e^- collider appear to be particularly
suited for a determination the spin-dependent photonic quark and gluon
densities, respectively. Special emphasis is devoted to a comparison of the
different sources of polarized photons at a linear collider: the equivalent
photon approximation and backscattered laser (Compton) photons. It is shown
that backscattered laser photons are highly favorable, even indispensable, for
decent measurements of the Delta f^{\gamma}(x,Q^2).Comment: 7 pages, LaTeX, 4 eps figures, uses amssymb, espcrc2 (included), and
epsfig styles; Contribution to the Proceedings of the `International
Conference on the Structure and Interactions of the Photon (Photon99)' [Nucl.
Phys. B (Proc. Suppl.)], Freiburg, Germany, May 199
Transcriptional control of the H-NS antagonists LeuO and RcsB-BglJ in Escherichia coli
The bacterial nucleoid-associated protein (NAP) H-NS is involved in the organization and compaction of the bacterial chromatin and acts as a global respressor, mainly of genes that have been acquired by horizontal gene transfer and that are related to stress responses and pathogenicity. Binding of H-NS to the DNA and formation of a nucleoprotein complex at promoter regions leads to repression. This repressor effect of H-NS can be antagonized by gene-specific transcription factors (H-NS antagonists) that activate transcription of H NS-repressed genes by competing with H-NS for binding or by disturbing formation of the nucleoprotein complex. Two examples of such H NS antagonists are the LysR-type transcription factor LeuO and the FixJ/NarL-type transcription factor heterodimer RcsB-BglJ. LeuO is a pleiotropic regulator of stress responses and virulence determinants. RcsB-BglJ activates transcription of the H NS-repressed bgl (aryl-β,D-glucoside) operon.
In this work, novel targets of RcsB-BglJ were identified in Escherichia coli by microarray analyses. The results suggest that heterodimerization of RcsB and BglJ is essential for regulation. Further, in addition to genes related to unknown or predicted function in the membrane the leuO gene was identified as a target gene. Detailed analysis of transcriptional regulation of leuO demonstrated that RcsB-BglJ strongly activates transcription of leuO by binding proximal to a newly mapped leuO promoter. Thus RcsB-BglJ antagonizes repression of leuO by H-NS and the H-NS-like protein StpA. Additional data presented here show that LeuO negatively autoregulates its own expression and inhibits activation of leuO by RcsB-BglJ.
Regulation of leuO by RcsB-BglJ and autoregulation by LeuO, as shown here, as well as activation of bglJ by LeuO, as published previously, indicates a feedback control mechanism of two global transcriptional regulators and H-NS antagonists.This feedback regulation may ensure turn on of their expression in response to specific environmental signals. Screens to search for novel regulators or upstream signals were performed by transposon mutagenesis and by using a genomic expression library. These screens indicate that additional factors may be involved in the regulation of this leuO-bglJ feedback loop
Exploring Nucleon Spin Structure in Longitudinally Polarized Collisions
We review how RHIC is expected to deepen our understanding of the spin
structure of longitudinally polarized nucleons. After briefly outlining the
current status of spin-dependent parton densities and pointing out open
questions, we focus on theoretical calculations and predictions relevant for
the RHIC spin program. Estimates of the expected statistical accuracy for such
measurements are presented, taking into account the acceptance of the RHIC
detectors.Comment: 10 pages, 7 figures, uses aipproc style, invited talk presented at
the 15th International Spin Physics Symposium 'SPIN 2002', Sept. 9-14 2002,
Brookhaven National Laboratory, Upton, N
Spin Physics at RHIC - a Theoretical Overview
We review how RHIC is expected to deepen our understanding of the spin
structure of longitudinally and transversely polarized nucleons. After briefly
outlining the current status of spin-dependent parton densities and pointing
out open questions, we focus on theoretical calculations and predictions
relevant for the RHIC spin program. Estimates of the expected statistical
accuracy for such measurements are presented, taking into account the
acceptance of the RHIC detectors.Comment: 13 pages, 8 figures, uses ws-procs9x6 style, invited talk presented
at the workshop `Testing QCD through Spin Observables in Nuclear Targets',
Charlottesville, VA, April 18-20, 200
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