Binding and unbinding of transcription regulators at operator sites constitute a primary mechanism for gene regulation. While many cellular factors are known to regulate their binding, little is known on how cells can modulate their unbinding for regulation. Using nanometer-precision single-molecule tracking, we study the unbinding kinetics from DNA of two metal-sensing transcription regulators in living Escherichia coli cells. We find that they show unusual concentration-dependent unbinding kinetics from chromosomal recognition sites in both their apo- and holo-forms. Unexpectedly, their unbinding kinetics further varies with the extent of chromosome condensation, and more surprisingly, varies in opposite ways for their apo-repressor vs. holo-activator forms. These findings suggest likely broadly relevant mechanisms for facile switching between transcription activation and deactivation in vivo and in coordinating transcription regulation of resistance genes with the cell cycle.

Enzymes often show catalytic allostery in which reactions occurring at different sites communicate cooperatively over distances of up to a few nanometres. Whether such effects can occur with non-biological nanocatalysts remains unclear, even though these nanocatalysts can undergo restructuring and molecules can diffuse over catalyst surfaces. Here we report that phenomenologically similar, but mechanistically distinct, cooperative effects indeed exist for nanocatalysts. Using spatiotemporally resolved single-molecule catalysis imaging, we find that catalytic reactions on a single Pd or Au nanocatalyst can communicate with each other, probably via hopping of positively charged holes on the catalyst surface, over ~10 nanometres and with a temporal memory of ~10 to 10 seconds, giving rise to positive cooperativity among its surface active sites. Similar communication is also observed between individual nanocatalysts, however it operates via a molecular diffusion mechanism involving negatively charged product molecules, and its communication distance is many micrometres. Generalization of these long-range intra- and interparticle catalytic communication mechanisms may introduce a novel conceptual framework for understanding nanoscale catalysis.

The formation of well-controlled circular patterns on the nanoscale is important for the fabrication of a range of devices such as sensors, memories, lasers, transistors, and quantum devices. Concentric, smooth ring patterns with tunable dimensions have been formed from a cylinder-forming poly(styrene- b-dimethylsiloxane) (PS-PDMS) diblock copolymer under confinement in shallow circular trenches. The high etch selectivity between PS and PDMS facilitates pattern transfer, illustrated by the fabrication of arrays of ferromagnetic cobalt rings with a density of 1.1 x 10 (9)/cm (2). The effects of confinement diameter and commensurability on the diameter and period of the concentric rings are analyzed using a free energy model that includes interfacial, strain, and bending energies. This work provides a simple process for the fabrication of nanoscale circular patterns with very narrow line width using a much coarser-scale template, and may facilitate the miniaturization of a variety of microelectronic devices.

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A series of two dimensional close-packed Co, NiFe, and CoFe/ Cu/ NiFe magnetic particle arrays, in which the particles have mean diameters of 34 nm, thicknesses of 5 -20 nm, and periodicity of 56 nm, were made using a process based on self-assembled polystyrene-b-polyferrocenyldimethylsilane block copolymer templates. Interparticle magnetostatic interactions lead to the thermally assisted collective reversal of small groups of particles. The switching field distribution, whose width decreases as the thickness increases, has been modeled as a result of the distribution of particle size, shape, and microstructure. For multilayered particles, interlayer magnetostatic interactions stabilize flux-closed states with antiparallel alignment of the CoFe and NiFe layers at remanence. The multilayer particles show a greater thermal stability than single-layer particles, and a magnetoresistance comparable to that of the unpatterned film.

Narrow mesoscopic NiFe/Cu/Co elliptical rings exhibit room-temperature giant magnetoresistance with distinct resistance levels corresponding to three different micromagnetic states. The highest and lowest resistance states of the multilayer rings correspond to the Co layer being in a bidomain state, antiparallel or parallel, respectively, to the NiFe, while the intermediate resistance corresponds to the Co layer being in a vortex state. Micromagnetic simulations suggest that the behavior of these rings is dominated by magnetostatic interactions between the domain walls in the Co and NiFe layers. Additional magnetization states in the NiFe at low applied fields can account for the minor loop behavior.

Multicomponent efflux complexes constitute a primary mechanism for Gram-negative bacteria to expel toxic molecules for survival. As these complexes traverse the periplasm and link inner and outer membranes, it remains unclear how they operate efficiently without compromising periplasmic plasticity. Combining single-molecule superresolution imaging and genetic engineering, we study in living Escherichia coli cells the tripartite efflux complex CusCBA of the resistance-nodulation-division family that is essential for bacterial resistance to drugs and toxic metals. We find that CusCBA complexes are dynamic structures and shift toward the assembled form in response to metal stress. Unexpectedly, the periplasmic adaptor protein CusB is a key metal-sensing element that drives the assembly of the efflux complex ahead of the transcription activation of the cus operon for defending against metals. This adaptor protein-mediated dynamic pump assembly allows the bacterial cell for efficient efflux upon cellular demand while still maintaining periplasmic plasticity; this could be broadly relevant to other multicomponent efflux systems.multicomponent efflux complex | substrate-responsive dynamic assembly | periplasmic adaptor protein | metal sensing | single-molecule tracking B acteria are often exposed to harsh environments, including high metal ion concentrations and toxic organic molecules. Efflux of metal ions helps bacteria maintain appropriate intracellular concentrations of essential metals while removing toxic ones (1-6). Efflux of organic molecules, including antibiotics, is a key mechanism for bacterial multidrug resistance (7-14). The tripartite resistance-nodulation-division (RND) family efflux pumps confer major clinically relevant drug resistance in Gram-negative bacteria such as Escherichia coli and the infectious Pseudomonas aeruginosa (7-14). They are composed of a proton-motive-force-driven inner-membrane pump, a periplasmic adaptor protein, and an outer-membrane channel. Once assembled, these pumps traverse the cell periplasm, providing a direct extrusion pathway from the periplasm (and cytoplasm) to the outside of the cell. However, these direct pathways also tightly link the inner and outer membranes, which, if overly stable, would impede the periplasm's plasticity and ability to respond dynamically to external and internal stimuli to buffer the cell from changes in its surroundings (15).How can tripartite efflux pumps operate without compromising the dynamic nature of the periplasm? One possibility is that these efflux complexes are dynamic structures and assemble only in the presence of their substrates. This mechanism has been hypothesized for the E. coli HlyBD-TolC complex (16), in which HlyB is a ATP-binding cassette superfamily efflux pump. However, experimental validation of this mechanism, as well as its relevance to the RND family efflux pumps, remains elusive, partly due to the difficulty in studying two membrane proteins together with a periplasmic protein under physiologically relevant conditi...

We have investigated high-quality MgO tunnel junctions with a range of barrier thickness in order to identify the underlying physical mechanism responsible for dielectric breakdown. Two types of dielectric breakdown (“soft” and “hard”) were observed. Soft breakdown was observed in a few percent of the devices. This breakdown mode is not intrinsic and is attributed to tunnel junction imperfections. The hard breakdown occurs because a critical electric field is reached across the tunnel barrier. Other possible breakdown mechanisms, such as thermally driven mass diffusion or charge trapping, were not consistent with the hard dielectric breakdown data and were ruled out.