Escherichia coli robustly expresses ATP synthase at growth rate-maximizing concentrations

Fitness-enhancing adaptations of protein expression and its regulation are an important aspect of bacterial evolution. A key question is whether evolution has led to optimal protein expression that maximizes immediate growth rate (short-term fitness) in a robust manner (consistently across diverse conditions). Alternatively, they could display suboptimal short-term fitness, because they cannot do better or because they instead strive for long-term fitness maximization by, for instance, preparing for future conditions. To address this question, we focus on the ATP-producing enzyme F1F0 H+-ATPase, which is an abundant enzyme and ubiquitously expressed across conditions. Its expression is highly regulated and dependent on growth rate and nutrient conditions. For instance, during growth on sugars, when metabolism is overflowing acetate, glycolysis supplies most ATP, while H+-ATPase is the main source of ATP synthesis during growth on acetate. We tested the optimality of H+-ATPase expression in Escherichia coli across different nutrient conditions. In all tested conditions, wild-type E. coli expresses its H+-ATPase remarkably close (within a few per cent) to optimal concentrations that maximize immediate growth rate. This work indicates that bacteria can indeed achieve robust optimal protein expression for immediate growth-rate maximization

The onset of full coupling in multi-filament superconducting tapes exposed to an alternating external magnetic field

At high amplitudes and frequencies of the external magnetic field, the superconducting filaments in a composite conductor are fully coupled. This causes a high magnetisation loss. Methods to decouple the filaments are well known for the low-Tc superconductors and have recently been applied also to high-Tc composites. In this paper, expressions are derived for the magnetic-field amplitude and frequency where full coupling sets in, as well as for the twist length required for decoupling the filaments in a general rectangular composite. A comparison is made with other expressions found in the literature. The filaments in a typical Bi-2223/Ag tape in parallel magnetic fields are decoupled just by twisting. In perpendicular fields a high transverse resistivity is required to decouple the filaments and decrease the magnetisation loss

Magnetisation loss in stacks of high-Tc superconducting tapes in a perpendicular magnetic field

Magnetisation loss is an important factor in the design of superconducting transformers and motors. In these devices the tapes are usually placed face-to-face. Then the magnetisation loss is influenced by the mutual magnetic shielding between adjacent tapes. The shielding is investigated by measuring the magnetisation loss in stacks with various numbers of Bi-2223 tapes, exposed to a 48-Hz perpendicular magnetic field at 77 K. In a stack the penetration field is increased and the magnetisation loss below penetration is greatly decreased, compared to the behaviour of a single tape. The loss at high magnetic-field amplitudes is unaffected. The measured loss is compared to the loss calculated with two different models. The effect of shielding is qualitatively well described with an analytical model. However, predictions made with a numerical model display a better quantitative agreement with the measurement results

Selection for Cell Yield Does Not Reduce Overflow Metabolism in <i>Escherichia coli</i>

Overflow metabolism is ubiquitous in nature, and it is often considered inefficient because it leads to a relatively low biomass yield per consumed carbon. This metabolic strategy has been described as advantageous because it supports high growth rates during nutrient competition. Here, we experimentally evolved bacteria without nutrient competition by repeatedly growing and mixing millions of parallel batch cultures of Escherichia coli. Each culture originated from a water-in-oil emulsion droplet seeded with a single cell. Unexpectedly we found that overflow metabolism (acetate production) did not change. Instead, the numerical cell yield during the consumption of the accumulated acetate increased as a consequence of a reduction in cell size. Our experiments and a mathematical model show that fast growth and overflow metabolism, followed by the consumption of the overflow metabolite, can lead to a higher numerical cell yield and therefore a higher fitness compared with full respiration of the substrate. This provides an evolutionary scenario where overflow metabolism can be favorable even in the absence of nutrient competition

Longitudinal magnetization loss in twisted multifilamentary Bi2223 tape

Multifilamentary Bi2223 tapes are exposed to the longitudinal magnetic field as well as the transverse one in some electrical power apparatuses such as multilayer power transmission cables. Here, we define the longitudinal and transverse magnetic fields as the field components parallel and perpendicular to the tape axis, respectively. If the filament-bundle is twisted, it can couple to the AC longitudinal magnetic field to generate the longitudinal magnetization loss. Furthermore, the AC transport current flowing spirally in the twisted filament-bundle possibly influences the longitudinal magnetization. The longitudinal magnetization loss was measured in a twisted multifilamentary Bi2223 tape exposed to longitudinal magnetic field and carrying the transport current. The measured longitudinal magnetization loss in the twisted tape exposed to the longitudinal magnetic field is larger than that in another untwisted tape. Supplying the AC transport current changes the longitudinal magnetization loss in the twisted tape exposed to the AC longitudinal magnetic field. The influence of the transport current depends on the phase relation between the longitudinal magnetic field and the transport current. If their phase difference is 0°, the longitudinal magnetization loss decreases remarkably with increasing amplitude of the transport current. It means that the change in the current distribution due to the transport current results in the decrease in the power flow from the magnet power supply. But, a preliminary measurement of the transport loss shows that the total loss increases with increasing transport current.\ud \u