Summary NusG is a conserved regulatory protein interacting with RNA polymerase (RNAP) and other proteins to form multi-component complexes that modulate transcription. The crystal structure of Thermotoga maritima NusG (TmNusG) shows a three-domain architecture, comprising well conserved amino-terminal (NTD) and carboxy-terminal (CTD) domains with an additional, species-specific domain inserted into the NTD. NTD and CTD directly contact each other, occluding a surface of the NTD for binding to RNAP and a surface on the CTD interacting either with transcription termination factor Rho or transcription anti-termination factor NusE. NMR spectroscopy confirmed the intra-molecular NTD-CTD interaction up to the optimal growth temperature of Thermotoga maritima. The domain interaction involves a dynamic equilibrium between open and closed states and contributes significantly to the overall fold stability of the protein. Wild type TmNusG and deletion variants could not replace for endogenous Escherichia coli NusG, suggesting that the NTD-CTD interaction of TmNusG represents an auto-inhibited state.

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To understand the elastic-plastic deformation response of shock-compressed molybdenum (Mo) - a body-centered cubic metal, single crystal samples were shocked along the [100] crystallographic orientation to an elastic impact stress of 12.5 GPa. Elastic-plastic wave profiles, measured at different propagation distances ranging between ∼0.23 to 2.31 mm using laser interferometry, showed a time-dependent material response. Within the experimental scatter, the measured elastic wave amplitudes were nearly constant over the propagation distances examined. These data point to a large and rapid elastic wave attenuation near the impact surface, before reaching a threshold value (elastic limit) of ∼3.6 GPa. Numerical simulations of the measured wave profiles, performed using a dislocation-based continuum model, suggested that {110}⟨111⟩ and/or {112}⟨111⟩ slip systems are operative under shock loading. In contrast to shocked metal single crystals with close-packed structures, the measured wave profiles in Mo single crystals could not be explained in terms of dislocation multiplication alone. A dislocation generation mechanism, operative for shear stresses larger than that at the elastic limit, was required to model the rapid elastic wave attenuation and to provide a good overall match to the measured wave profiles. However, the physical basis for this mechanism was not established for the high-purity single crystal samples used in this study. The numerical simulations also suggested that Mo single crystals do not work harden significantly under shock loading in contrast to the behavior observed under quasi-static loading.

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