10,466 research outputs found
Regulation of the \u3cem\u3eEscherichia coli\u3c/em\u3e Tryptophan Operon by Early Reactions in the Aromatic Pathway
7-Methyltryptophan (7MT) or compounds which can be metabolized to 7MT, 3-methylanthranilic acid (3MA) and 7-methylindole, cause derepression of the trp operon through feedback inhibition of anthranilate synthetase. Tyrosine reverses 3MA or 7-methylindole derepression, apparently by increasing the amount of chorismic acid available to the tryptophan pathway. A mutant isolated on the basis of 3MA resistance (MAR 13) was found to excrete small amounts of chorismic acid and to have a feedback-resistant phenylalanine 3-deoxy-d-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase. Genetic evidence indicates that the mutation conferring 3MA resistance and feedback resistance is very closely linked to aroG, the structural gene for the DAHP synthetase (phe). Since feedback inhibition of anthranilate synthetase by l-tryptophan (or 7MT) is competitive with chorismic acid, alterations in growth conditions (added tyrosine) or in a mutant (MAR 13) which increase the amount of chorismic acid available to the tryptophan pathway result in resistance to 7MT derepression. Owing to this competitive nature of tryptophan feedback inhibition of anthranilate synthetase by chorismic acid, the early pathway apparently serves to exert a regulatory influence on tryptophan biosynthesis
Mechanism of 3-Methylanthranilic Acid Derepression of the Tryptophan Operon in \u3cem\u3eEscherichia coli\u3c/em\u3e
3-Methylanthranilic acid (3MA) inhibits growth and causes derepression of the tryptophan biosynthetic enzymes in wild-type strains of Escherichia coli. Previous reports attributed this effect to an inhibition of the conversion of 1-(o-carboxyphenylamino)-1-deoxyribulose 5-phosphate to indole-3-glycerol phosphate and a consequent reduction in the concentration of endogenous tryptophan. Our studies have shown that 3MA-resistant mutants linked to the tryptophan operon have a feedback-resistant anthranilate synthetase; mutants with an altered indole-3-glycerol phosphate synthetase were not found. 3MA or 7-methylindole can be metabolized to 7-methyltryptophan, and 3MA, 7-methylindole, and 7-methyltryptophan lead to derepression of the tryptophan operon. Furthermore, 3MA-resistant mutants are also resistant to 7-methylindole derepression. These results strongly suggest that the primary cause of derepression by 3MA is through its conversion to 7-methyltryptophan, which can inhibit anthranilate synthetase, thereby decreasing the concentration of endogenous tryptophan. Unlike 5- or 6-methyltryptophan, 7-methyltryptophan does not appear to function as an active corepressor
Embedding approach for dynamical mean field theory of strongly correlated heterostructures
We present an embedding approach based on localized basis functions which
permits an efficient application of the dynamical mean field theory (DMFT) to
inhomogeneous correlated materials, such as semi-infinite surfaces and
heterostructures. In this scheme, the semi-infinite substrate leads connected
to both sides of the central region of interest are represented via complex,
energy-dependent embedding potentials that incorporate one-electron as well as
many-body effects within the substrates. As a result, the number of layers
which must be treated explicitly in the layer-coupled DMFT equation is greatly
reduced. To illustrate the usefulness of this approach, we present numerical
results for strongly correlated surfaces, interfaces, and heterostructures of
the single-band Hubbard model.Comment: 8 pages, 4 figures; typos correcte
One-particle irreducible functional approach - a new route to diagrammatic extensions of DMFT
We present an approach which is based on the one-particle irreducible (1PI)
generating functional formalism and includes electronic correlations on all
length-scales beyond the local correlations of dynamical mean field theory
(DMFT). This formalism allows us to unify aspects of the dynamical vertex
approximation (D\GammaA) and the dual fermion (DF) scheme, yielding a
consistent formulation of non-local correlations at the one- and two-particle
level beyond DMFT within the functional integral formalism. In particular, the
considered approach includes one-particle reducible contributions from the
three- and more-particle vertices in the dual fermion approach, as well as some
diagrams not included in the ladder version of D\GammaA. To demonstrate the
applicability and physical content of the 1PI approach, we compare the
diagrammatics of 1PI, DF and D\GammaA, as well as the numerical results of
these approaches for the half-filled Hubbard model in two dimensions.Comment: 36 pages, 12 figures, updated versio
Synthetic aperture radar target simulator
A simulator for simulating the radar return, or echo, from a target seen by a SAR antenna mounted on a platform moving with respect to the target is described. It includes a first-in first-out memory which has digital information clocked in at a rate related to the frequency of a transmitted radar signal and digital information clocked out with a fixed delay defining range between the SAR and the simulated target, and at a rate related to the frequency of the return signal. An RF input signal having a frequency similar to that utilized by a synthetic aperture array radar is mixed with a local oscillator signal to provide a first baseband signal having a frequency considerably lower than that of the RF input signal
GdI_2: A New Ferromagnetic Excitonic Solid?
The two-dimensional, colossal magnetoresistive system GdI_2 develops an
unusual metallic state below its ferromagnetic transition and becomes
insulating at low temperatures. It is argued that this geometrically
frustrated, correlated poor metal is a possible candidate for a ferromagnetic
excitonic liquid. The renormalized Fermi surface supports a further breaking of
symmetry to a charge ordered, excitonic solid ground state at lower
temperatures via order by disorder mechanism. Several experimental predictions
are made to investigate this unique orbitally correlated ground state.Comment: 4 pages, 4 figures, changed Fig. 1 with extended energy scale, added
text and references, author list shortene
Modelling seasonal patterns in longitudinal profiles with correlated circular random walks
Seasonal patterns, as they occur in time series of infectious disease surveillance counts, are frequently modelled using a superposition of sine and cosine functions. However, in some cases this might be too simple. We propose the use of circular second order random walks instead and extend this approach to multivariate time series of counts. A correlated Gaussian Markov random field
approach combines a uniform correlation matrix with a circular random walk to allow the seasonal pattern to be similar across regions, say, but not identical.
Thus, spatially-varying disease onsets may be accounted for. The methodology is applied to weekly number of deaths from in uenza and pneumonia in nine major regions of the USA
Correlated GMRF priors for multivariate age-period-cohort models
Multivariate age-period-cohort models have recently been proposed for the analysis of heterogeneous time trends. For a fully Bayesian analysis, Gaussian Markov random field (GMRF) priors are typically used. However, standard GMRF priors do not account for a potential dependence between outcomes. We present an extended approach based on correlated smoothing priors and corre-lated overdispersion parameters. Algorithmic routines are based on either Markov chain Monte Carlo or integrated nested Laplace approximations. Results are discussed for data on female mortality in Denmark and Norway and compared by means of DIC, proper scoring rules and the marginal likelihood
Diagrammatic routes to nonlocal correlations beyond dynamical mean field theory
Strong electronic correlations pose one of the biggest challenges to solid
state theory. We review recently developed methods that address this problem by
starting with the local, eminently important correlations of dynamical mean
field theory (DMFT). On top of this, non-local correlations on all length
scales are generated through Feynman diagrams, with a local two-particle vertex
instead of the bare Coulomb interaction as a building block. With these
diagrammatic extensions of DMFT long-range charge-, magnetic-, and
superconducting fluctuations as well as (quantum) criticality can be addressed
in strongly correlated electron systems. We provide an overview of the
successes and results achieved---hitherto mainly for model Hamiltonians---and
outline future prospects for realistic material calculations.Comment: 60 pages, 42 figures, replaced by the version to be published in Rev.
Mod. Phys. 201
- …