Farnesol and Cyclic AMP Signaling Effects on the Hypha-to-Yeast Transition in Candida Albicans

Candida albicans, a fungal pathogen of humans, regulates its morphology in response to many environmental cues and this morphological plasticity contributes to virulence. Farnesol, an autoregulatory molecule produced by C. albicans, inhibits the induction of hyphal growth by inhibiting adenylate cyclase (Cyr1). The role of farnesol and Cyr1 in controlling the maintenance of hyphal growth has been less clear. Here, we demonstrate that preformed hyphae transition to growth as yeast in response to farnesol and that strains with increased cyclic AMP (cAMP) signaling exhibit more resistance to farnesol. Exogenous farnesol did not induce the hypha-to-yeast transition in mutants lacking the Tup1 or Nrg1 transcriptional repressors in embedded conditions. Although body temperature is not required for embedded hyphal growth, we found that the effect of farnesol on the hypha-to-yeast transition varies inversely with temperature. Our model of Cyr1 activity being required for filamentation is also supported by our liquid assay data, which show increased yeast formation when preformed filaments are treated with farnesol. Together, these data suggest that farnesol can modulate morphology in preformed hyphal cells and that the repression of hyphal growth maintenance likely occurs through the inhibition of cAMP signaling

Observation of isotropic giant magnetoresistance in paramagnetic Au80 Fe20

Magnetization and magnetoresistance were measured at room temperature and above on Au80Fe20 platelets and ribbons obtained by solid-state quenching and melt spinning. The as-quenched samples contain a solid solution of Fe in Au and exhibit a paramagnetic (Curie-Weiss) behavior in the considered temperature range; magnetic data indicate very short-ranged magnetic correlation among adjacent spins, enhanced by local composition fluctuations. The solid solution is very stable. Only a very limited fraction (never exceeding 1%) of nanometer-sized, bcc Fe particles appears after long-time isothermal anneals at suitable temperatures. A negative magnetoresistance was observed at room temperature in all examined samples. The observed effect is anhysteretic, isotropic, and quadratically dependent on magnetic field H and magnetization M. The signal scales with M rather than with H, indicating that it depends on the field-induced magnetic order of the Fe moments, as it does for conventional giant magnetoresistance in granular magnetic systems. This effect derives from spin-dependent scattering of conduction electrons from single Fe spins or very small Fe clusters. The scattering centers are almost uncorrelated at a distance of the order of the electronic mean free path (of the order of 1.5 nm, or a few atomic spacings, at RT

Epoxide-Mediated CifR Repression of cif Gene Expression Utilizes Two Binding Sites in Pseudomonas aeruginosa

Pseudomonas aeruginosa secretes an epoxide hydrolase virulence factor that reduces the apical membrane expression of ABC transporters such as the cystic fibrosis transmembrane conductance regulator (CFTR). This virulence factor, named CFTR inhibitory factor (Cif), is regulated by a TetR-family, epoxide-responsive repressor known as CifR via direct binding and repression. We identified two sites of CifR binding in the intergenic space between cifR and morB, the first gene in the operon containing the cif gene. We have mapped these binding sites and found they are 27 bp in length, and they overlap the -10 and +1 sites of both the cifR and morB regulatory region and the start of transcription, respectively. In addition, we found that CifR binds to each repression site with differing affinity. Mutagenesis of these binding sites resulted in a loss of DNA binding in vitro, and mutation of one of these sites in vivo resulted in an increase in transcription of both the cif and cifR genes. We characterized cif and cifR gene expression in sputum and found that, whereas cif gene expression varied relative to an in vitro coculture control, cifR gene expression was consistently higher. Analysis of a longitudinal sample of CF isolates from nine patients revealed that Cif protein was expressed over time, although variably, and these changes could not be linked to mutations in the cifR gene or the promoters of these genes. Finally, we tested CifR responsiveness to other epoxides and showed that CifR can respond to multiple epoxides to various degrees

Magnetic properties and giant magnetoresistance in melt-spun CoCu alloys

Magnetic, structural, and transport properties of as quenched and annealed Co10Cu90 samples have been investigated using x¿ray diffraction and a SQUID magnetometer. The largest value of MR change was observed for the as¿quenched sample annealed at 450°C for 30 min. The magnetic and transport properties closely correlate with the microstructure, mainly with Co magnetic particle size and its distribution. For thermal annealing the as quenched samples below 600°C, the Co particle diameters increase from 4.0 to 6.0 nm with a magnetoresistance (MR) drop from 33.0% to 5.0% at 10 K. Comparison with the theory indicates that the interfacial electron spin¿dependent scattering mechanism correlates with GMR for Co particle diameters up to about 6.0 nm

The role of the alloy structure in the magnetic behavior of granular systems

The effect of grain size, easy magnetization axis and anisotropy constant distributions in the irreversible magnetic behavior of granular alloys is considered. A simulated granular alloy is used to provide a realistic grain structure for the Monte Carlo simulation of the ZFC-FC curves. The effect of annealing and external field is also studied. The simulation curves are in good agreement with the FC and ZFC magnetization curves measured on melt spun Cu-Co ribbons.Comment: 13 pages, 10 figures, submitted to PR

Dipolar interactions and anisotropic magnetoresistance in metallic granular systems

We revisit the theory of magnetoresistance for a system of nanoscopic magnetic granules in metallic matrix. Using a simple model for the spin dependent perturbation potential of the granules, we solve Boltzmann equation for the spin dependent components of the non equilibrium electronic distribution function. For typical values of the geometric parameters in granular systems, we find a peculiar structure of the distribution function of conduction electrons, which is at variance with the two-current model of conduction in inhomogeneous systems. Our treatment explicitly includes the effects of dipolar correlations yielding a magnetoresistance ratio which contains, in addition to the term proportional to the square of uniform magnetization (), a weak anisotropic contribution depending on the angle between electric and magnetic fields, and arising from the anisotropic character of dipolar interactions.Comment: 9 pages, 2 figures, accepted in PR

EFFECTS OF COMBINED MECHANICAL AND THERMAL TREATMENTS ON TRANSITION METAL-METALLOID AMORPHOUS RIBBONS

Des mesures de frottement intérieure et de module dynamique de Young ont été réalisées par la technique de la lame mince vibrante entre la température ambiante et TG sur des rubans amorphes de composition différente. Les variations de Q-1 et de Md avec la temperature ont été étudiées à valeurs differentes de déformation inélastique induite dans les rubans et après des cycles combinés de déformation mécanique et de vieillissement. On en déduit que la déformation plastique entraîne une modification de la structure amorphe, plus évidente dans les échantillons non traités que dans ceux dont la structure a relaxé. Des déformations successives peuvent produire un "mechanical annealing". L'effet du désordre de la structure amorphe et de ses variations sur les propriétés inélastiques des échantillons étudiés est interprété en termes du volume libre et de la distribution des tensions internes dans les verres metalliques.Internal friction and dynamic Young modulus measurements have been performed on amorphous ribbons of different chemical composition by the vibrating reed technique, between room and glass transition temperature. The Q-1 (T) and Md(T) spectra have been measured after different amounts of inelastic deformation and of combined straining and ageing cycles. Plastic deformation seems to induce some structural modifications, particularly evident in the as cast specimens in comparison with those structurally relaxed. Repeated straining can result in mechanical annealing. The role of quenched in disorder and its changes, in terms of free volume and internal stress distribution in determining the inelastic behaviour is discussed with reference to current models of inelastic deformation in metallic glasses