Translocation of helicase-like proteins on nucleic acids underlies key cellular functions. However, it is still unclear how translocation can drive removal of DNA bound proteins, and basic properties like the elementary step size remain controversial. Using single molecule fluorescence analysis on a prototypical superfamily 1 helicase, Bacillus stearothermophilus PcrA, we discovered that PcrA preferentially translocates on the DNA lagging strand instead of unwinding the template duplex. PcrA anchors itself to the template duplex using the 2B subdomain and reels in the lagging strand, extruding a single stranded loop. Static disorder limited previous ensemble studies of PcrA stepping mechanism. Here, highly repetitive looping revealed that PcrA translocates in uniform steps of 1 nt. This reeling-in activity requires the open conformation of PcrA and can rapidly dismantle a preformed RecA filament even at low PcrA concentrations suggesting a mode of action for eliminating potentially deleterious recombination intermediates.
Although organic-inorganic hybrid perovskites have been extensively investigated for promising applications in energy-related devices, their mechanical properties, which restrict their practical deployment as flexible and wearable devices, have been largely unexplored at the atomistic level. Toward this level of understanding, we predict the elastic constant matrix and various elastic properties of CHNHPbI (MAPbI) using atomistic simulations. We find that single-crystalline MAPbI is much stiffer and exhibits higher ultimate tensile strength than polycrystalline samples, but the latter exhibit unexpected, greatly enhanced nanoductility and fracture toughness, resulting from the extensive amorphization during the yielding process. More interestingly, polycrystalline MAPbI exhibits inverse Hall-Petch grain-boundary strengthening effect, in which the yield stress is reduced when decreasing the grain size, due to amorphous grain boundaries. By monitoring the centro-symmetry parameter and local stress evolution, we confirm the soft and nanoductile nature of defective MAPbI with a crack. By conducting atomic stress decomposition, we attribute such fracture toughness primarily to the strong electrostatic interactions between the ionic components. The observed limited brittle fracture behavior is attributed to the transformation of partial edge dislocations to disordered atoms (nanovoid formation). A significant plastic deformation region is observed when nanovoids enlarge and coalesce with adjacent ones, which ultimately leads to crack propagations via ionic-chain breaking. After comparing with traditional inorganic energy-related materials, we find that hybrid perovskites are more compressible and can absorb more strain energy before fracture, which makes them well suited for wearable functional devices with high mechanical flexibility and robustness.
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