Presentation of viral antigen controlled by a gene in the major histocompatibility complex

We describe a mutant human cell line (LBL 721.174) that has lost a function required for presentation of intracellular viral antigens with class I molecules of the major histocompatibility complex (MHC), but retains the capacity to present defined epitopes as extracellular peptides. The cell also has a defect in the assembly and expression of class I MHC molecules, which we show can be restored by exposure of the cells to a peptide epitope. This phenotype suggests a defect in the association of intracellular antigen with class I molecules similar to that described for the murine mutant RMA-S (ref. 5), but in the present case the genetic defect can be mapped within the MHC locus on human chromosome 6.     

Thermodynamics of the B-Z transition in superhelical DNA

One of the most exciting events in recent years in molecular biology was the discovery of the left-handed Z form of the DNA double helix. Originally found in linear self-complementary d(GC)x . d(GC)x polymers and oligomers in non-physiological conditions (a rather high salt concentration), it was recently shown to be easily enough adopted in physiological conditions when purine-pyrimidine sequences are inserted into superhelical DNA. From such a system, superhelical DNA carrying an artificial purine-pyrimidine insert, we can obtain data allowing the determination of the energy of the junction between the B and Z stretches, Fj, and the free energy change delta FBZ per base pair (bp). We present here a simple thermodynamic consideration of the B-Z transition in such a system. By applying the results to experimental data we have shown that the thermodynamic parameters for both sequences studied so far (d(GC)x . d(GC)x and d(GT)x . d(AC)x) are similar and equal to Fj = 4-5 kcal per mol per junction and delta FBZ = 0.5 divided by 0.7 kcal per mol per bp.     

Quantum dynamics of a single vortex

Vortices occur naturally in a wide range of gases and fluids, from macroscopic to microscopic scales. In Bose-Einstein condensates of dilute atomic gases, superfluid helium and superconductors, the existence of vortices is a consequence of the quantum nature of the system. Quantized vortices of supercurrent are generated by magnetic flux penetrating the material, and play a key role in determining the material properties and the performance of superconductor-based devices. At high temperatures the dynamics of such vortices are essentially classical, while at low temperatures previous experiments have suggested collective quantum dynamics. However, the question of whether vortex tunnelling occurs at low temperatures has been addressed only for large collections of vortices. Here we study the quantum dynamics of an individual vortex in a superconducting Josephson junction. By measuring the statistics of the vortex escape from a controllable pinning potential, we demonstrate the existence of quantized levels of the vortex energy within the trapping potential well and quantum tunnelling of the vortex through the pinning barrier.     

Mean-field cluster model for the critical behaviour of ferromagnets

Two separate theories are often used to characterize the paramagnetic properties of ferromagnetic materials. At temperatures T well above the Curie temperature, Tc (where the transition from paramagnetic to ferromagnetic behaviour occurs), classical mean-field theory yields the Curie-Weiss law for the magnetic susceptibility: X(T) infinity 1/(T - Weiss constant), where Weiss constant is the Weiss constant. Close to Tc, however, the standard mean-field approach breaks down so that better agreement with experimental data is provided by critical scaling theory: X(T) infinity 1/(T - Tc)gamma, where gamma is a scaling exponent. But there is no known model capable of predicting the measured values of gamma nor its variation among different substances. Here I use a mean-field cluster model based on finite-size thermostatistics to extend the range of mean-field theory, thereby eliminating the need for a separate scaling regime. The mean-field approximation is justified by using a kinetic-energy term to maintain the microcanonical ensembles. The model reproduces the Curie-Weiss law at high temperatures, but the classical Weiss transition at Tc = Weiss constant is suppressed by finite-size effects. Instead, the fraction of clusters with a specific amount of order diverges at Tc, yielding a transition that is mathematically similar to Bose-Einstein condensation. At all temperatures above Tc, the model matches the measured magnetic susceptibilities of crystalline EuO, Gd, Co and Ni, thus providing a unified picture for both the critical-scaling and Curie-Weiss regimes.     

Performance monitoring by the supplementary eye field

Intelligent behaviour requires self-control based on the consequences of actions. The countermanding task is designed to study self-control; it requires subjects to withhold planned movements in response to an imperative stop signal, which they can do with varying success. In humans, the medial frontal cortex has been implicated in the supervisory control of action. In monkeys, the supplementary eye field in the dorsomedial frontal cortex is involved in producing eye movements, but its precise function has not been clarified. To investigate the role of the supplementary eye field in the control of eye movements, we recorded neural activity in macaque monkeys trained to perform an eye movement countermanding task. Distinct groups of neurons were active after errors, after successful withholding of a partially prepared movement, or in association with reinforcement. These three forms of activation could not be explained by sensory or motor factors. Our results lead us to put forward the hypothesis that the supplementary eye field contributes to monitoring the context and consequences of eye movements.     

The evolutionary inheritance of elemental stoichiometry in marine phytoplankton

Phytoplankton is a nineteenth century ecological construct for a biologically diverse group of pelagic photoautotrophs that share common metabolic functions but not evolutionary histories. In contrast to terrestrial plants, a major schism occurred in the evolution of the eukaryotic phytoplankton that gave rise to two major plastid superfamilies. The green superfamily appropriated chlorophyll b, whereas the red superfamily uses chlorophyll c as an accessory photosynthetic pigment. Fossil evidence suggests that the green superfamily dominated Palaeozoic oceans. However, after the end-Permian extinction, members of the red superfamily rose to ecological prominence. The processes responsible for this shift are obscure. Here we present an analysis of major nutrients and trace elements in 15 species of marine phytoplankton from the two superfamilies. Our results indicate that there are systematic phylogenetic differences in the two plastid types where macronutrient (carbon:nitrogen:phosphorus) stoichiometries primarily reflect ancestral pre-symbiotic host cell phenotypes, but trace element composition reflects differences in the acquired plastids. The compositional differences between the two plastid superfamilies suggest that changes in ocean redox state strongly influenced the evolution and selection of eukaryotic phytoplankton since the Proterozoic era.