Abstract— In Escherichia coli, fur mutants that constitutively express their native iron chelating agent, enterobactin, are significantly more sensitive to near-UV radiation (NUV) than wild type. An entA mutant, which is incapable of synthesizing enterobactin, is equal to wild type in resistance to NUV irradiation. However, the addition of Fe+3 enterobactin but not Al+1 enterobactin to entA cell suspensions just prior to irradiation results in an increased sensitivity to NUV irradiation. A fes mutant, which is unable to reduce and release iron from enterobactin, is significantly more sensitive to NUV irradiation than wild type. The addition of nontoxic levels of H2O2 (5 μ M ) just prior to irradiation significantly increases sensitivity of both fur and fes mutants. These results suggest that one mechanism by which NUV irradiation leads to cell lethality is by creating a transient iron overload, producing very favorable conditions for the production of highly deleterious free radicals through a variety of mechanisms that lead to oxidative stress and DNA damage including lethal and mutagenic lesions. These results are consistent with the hypothesis that enterobactin is an endogenous chromophore for NUV and contributes to cell lethality via the destruction of its ligand, releasing Fe+2 into the cytoplasm to catalyze the production of highly reactive hydroxyl radicals and other toxic oxygen species via the Haber-Weiss reaction. 相似文献
Spt7 is an integral component of the multi-subunit SAGA complex that is required for the expression of ~10% of yeast genes.
Two forms of Spt7 have been identified, the second of which is truncated at its C-terminus and found in the SAGA-like (SLIK)
complex. 相似文献
We consider numerical methods for a ``quasi-boundary value' regularization of the backward parabolic problem given by
where is positive self-adjoint and unbounded. The regularization, due to Clark and Oppenheimer, perturbs the final value by adding , where is a small parameter. We show how this leads very naturally to a reformulation of the problem as a second-kind Fredholm integral equation, which can be very easily approximated using methods previously developed by Ames and Epperson. Error estimates and examples are provided. We also compare the regularization used here with that from Ames and Epperson.
We consider numerical methods for a ``quasi-boundary value' regularization of the backward parabolic problem given by
where is positive self-adjoint and unbounded. The regularization, due to Clark and Oppenheimer, perturbs the final value by adding , where is a small parameter. We show how this leads very naturally to a reformulation of the problem as a second-kind Fredholm integral equation, which can be very easily approximated using methods previously developed by Ames and Epperson. Error estimates and examples are provided. We also compare the regularization used here with that from Ames and Epperson.
This paper presents an expansion for radial tempered distributions on
in terms of smooth, radial analyzing and synthesizing functions with space-frequency localization properties similar to standard
wavelets. Scales of quasi-norms are defined for the coefficients of the expansion that characterize, via Littlewood-Paley-Stein
theory, when a radial distribution belongs to a Triebel-Lizorkin or Besov space. These spaces include, for example, the
spaces,
Hardy spaces
Sobolev spaces
and Lipschitz
spaces
We also present a smooth radial atomic decomposition and norm estimates for sums of smooth radial molecules. The radial
wavelets, atoms, and molecules that we consider are localized near certain annuli, as opposed to cubes in the usual, nonradial
setting. The radial wavelet expansion is multiscale, where the functions in the different scales are related by dilation.
However, there is no translation structure within a given scale, unlike the situation with standard wavelet systems. 相似文献
Cobalt(II) has been utilized as an external paramagnetic (1)H NMR probe for the study of the structure of dendrimers that possess specifically located metal recognition sites. The hyperfine-shifted (1)H NMR signals of the Co(II) complexes of several 2,6-diamidopyridine-containing dendrimers have been fully assigned by means of 1D and 2D NMR techniques, including NOE difference, EXSY, COSY, and TOCSY. Temperature-dependent T(1) values of the hyperfine-shifted signals were used to conclude that the Co(II)-dendrimer complexes are in the "liquidlike" regime, indicative of a shell-like structure instead of a "dense-core" structure. The presence of sizable cavities within the dendrimers was observed including a loosely packed conformation for the 2,6-diamidopyridino moiety to bind to potential guest molecules. Cooperativity among the dendritic arms in metal binding is also observed, whereby two dendritic arms bind to the metal center at the same time. In the case of dendrimers with the metal binding site located near the surface of the molecule, such binding cooperativity is still observed despite the large degree of freedom of the metal-binding moiety. Cooperativity among the dendritic arms can thus be considered an intrinsic property, which has to be taken into consideration in future design of functional dendrimers for the purpose of specific recognition and catalysis. The hydrodynamic radii of these dendrimers have been determined by means of nuclear Overhouser effect at low temperature. The study offers a method for the study of the dynamics of dendrimers in solution under different conditions and upon ligand binding and recognition. The study also provides a tool for monitoring systematic variation of the metal binding site in different dendrimer frameworks for specific applications, such as catalysis and molecular recognition. 相似文献