Structural basis of human mitochondrial transcription initiation and processive elongation

In eukaryotic cells, mitochondria produce the vast majority of ATP, the universal energy currency of life. To do so, they maintain a highly reduced genome as well as the molecular machinery necessary for its expression. Transcription in mitochondria is carried out by a dedicated mitochondrial RNA polymerase (mtRNAP), which is related to single-subunit RNA polymerases (RNAPs) found in bacteriophages. In contrast to these self-sufficient enzymes, however, mtRNAP requires additional protein factors for all steps of transcription, suggesting a complex regulation. Moreover, it also produces the RNA primers necessary to initiate DNA synthesis, placing this enzyme at the heart of mitochondrial gene expression and organelle maintenance. Structures of mtRNAP have provided a first glimpse at the central actor orchestrating these important processes, but the mechanistic principles governing the individual steps of mitochondrial transcription remain poorly understood. In this study, we expand our understanding of these processes by investigating the structural basis of transcription initiation and processive elongation, two steps of regulatory importance. To initiate transcription, mtRNAP associates with the two initiation factors TFAM and TFB2M and promoter DNA to form an initiation complex (IC). Here, I present the structure of human TFB2M at 1.75 Å resolution and of the human initiation complex at 4.5 Å resolution. Together with published structures of mtRNAP and TFAM, this allows for construction of a pseudo- atomic model of the IC. The structures reveal how mtRNAP is recruited to the promoter by TFAM and suggest that TFB2M induces a rearrangement in mtRNAP to facilitate promoter opening. The open complex is further stabilized by interactions between TFB2M and the melted non-template DNA strand. Structural comparisons demonstrate that transition to elongation is accompanied by a profound re-arrangement of the upstream DNA. Following initiation, mtRNAP associates with the elongation factor TEFM for processive transcription elongation. This factor enables mtRNAP to transcribe through a G-quadruplex forming sequence in the mitochondrial genome, which otherwise leads to transcription termination and primer formation for replication. However, the mechanistic basis for this anti- termination activity of TEFM is unknown. Here, I present crystal structures of the human TEFM domains and, in a collaborative effort with the Temiakov Lab, we functionally define their roles in transcription. In addition, I have determined the structure of an anti-termination complex, comprised of the functional domains of TEFM bound to transcribing mtRNAP. These structures demonstrate that TEFM stabilizes the elongation complex by enclosing the downstream DNA in a “sliding clamp” and by interacting with the non-template strand in the transcription bubble. Moreover, these data suggest that TEFM prevents formation of the G-quadruplex in the RNA exit path, thereby mediating the switch between transcription and DNA replication. Taken together, these results greatly advance our understanding of mitochondrial transcription and elucidate the mechanistic basis for the factor dependence of mtRNAP. Furthermore, they provide a framework for future studies aimed at deciphering the regulatory mechanisms of transcription and DNA replication in human mitochondria

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Efficient direct seawater electrolysers using selective alkaline NiFe-LDH as OER catalyst in asymmetric electrolyte feeds

Direct seawater electrolysis faces fundamental catalytic and process engineering challenges. Here we demonstrate a promising seawater electrolyser configuration using asymmetric electrolyte feeds. We further investigated the faradaic O2 efficiency of NiFe-LDH in alkalinized Cl−-containing electrolytes in comparison to commercial IrOx-based catalysts. Other than IrOx, NiFe-LDH prevents the oxidation of Cl− and appears highly selective for the oxygen evolution reaction in alkalinized seawater even at cell potentials beyond 3.0 Vcell.TU Berlin, Open-Access-Mittel - 2020BMWI, 03EIV041F, Verbundvorhaben: MethQuest - MethFuel - Innovative Methanerzeugung auf Basis Erneuerbarer Quellen; Teilvorhaben: MeerwasserelektrolyseDFG, 424873219, Einfluss der Katalysator-Träger-Wechselwirkung auf die Aktivität und Stabilität von Wasserspaltungs-Katalysatore

Twin-lattice atom interferometry

Inertial sensors based on cold atoms have great potential for navigation, geodesy, or fundamental physics. Similar to the Sagnac effect, their sensitivity increases with the space-time area enclosed by the interferometer. Here, we introduce twin-lattice atom interferometry exploiting Bose-Einstein condensates. Our method provides symmetric momentum transfer and large areas in palm-sized sensor heads with a performance similar to present meter-scale Sagnac devices

Imaging characteristics of intravascular spherical contrast agents for grating-based x-ray dark-field imaging - effects of concentrations, spherical sizes and applied voltage

This study investigates the x-ray scattering characteristics of microsphere particles in x-ray-grating-based interferometric imaging at different concentrations, bubble sizes and tube voltages (kV). Attenuation (ATI), dark-field (DFI) and phase-contrast (PCI) images were acquired. Signal-to-noise (SNR) and contrast-to-noise ratios with water (CNRw) and air as reference (CNRa) were determined. In all modalities, a linear relationship between SNR and microbubbles concentration, respectively, microsphere size was found. A significant gain of SNR was found when varying kV. SNR was significantly higher in DFI and PCI than ATI. The highest gain of SNR was shown at 60kV for all media in ATI and DFI, at 80kV for PCI. SNR for all media was significantly higher compared to air and was slightly lower compared to water. A linear relationship was found between CNRa, CNRw, concentration and size. With increasing concentration and decreasing size, CNRa and CNRw increased in DFI, but decreased in PCI. Best CNRa and CNRw was found at specific combination of kV and concentration/size. Highest average CNRa and CNRw was found for microspheres in ATI and PCI, for microbubbles in DFI. Microspheres are a promising contrast-media for grating-based-interferometry, if kV, microsphere size and concentration are appropriately combined

Time-bin entanglement at telecom wavelengths from a hybrid photonic integrated circuit

Mass-deployable implementations for quantum communication require compact, reliable, and low-cost hardware solutions for photon generation, control and analysis. We present a fiber-pigtailed hybrid photonic circuit comprising nonlinear waveguides for photon-pair generation and a polymer interposer reaching 68dB of pump suppression and photon separation with >25dB polarization extinction ratio. The optical stability of the hybrid assembly enhances the quality of the entanglement, and the efficient background suppression and photon routing further reduce accidental coincidences. We thus achieve a 96(-8,+3)% concurrence and a 96(-5,+2)% fidelity to a Bell state. The generated telecom-wavelength, time-bin entangled photon pairs are ideally suited for distributing Bell pairs over fiber networks with low dispersion

Twin-lattice atom interferometry

Inertial sensors based on cold atoms have great potential for navigation, geodesy, or fundamental physics. Similar to the Sagnac effect, their sensitivity increases with the space-time area enclosed by the interferometer. Here, we introduce twin-lattice atom interferometry exploiting Bose-Einstein condensates of rubidium-87. Our method provides symmetric momentum transfer and large areas offering a perspective for future palm-sized sensor heads with sensitivities on par with present meter-scale Sagnac devices. Our theoretical model of the impact of beam splitters on the spatial coherence is highly instrumental for designing future sensors

Interplay of thermal and non-thermal effects in x-ray-induced ultrafast melting

X-ray laser-induced structural changes in silicon undergoing femtosecond melting have been investigated by using an x-ray pump-x-ray probe technique. The experimental results for different initial sample temperatures reveal that the onset time and the speed of the atomic disordering are independent of the initial temperature, suggesting that equilibrium atomic motion in the initial state does not play a pivotal role in the x-ray-induced ultrafast melting. By comparing the observed time-dependence of the atomic disordering and the dedicated theoretical simulations, we interpret that the energy transfer from the excited electrons to ions via electron-ion coupling (thermal effect) as well as a strong modification of the interatomic potential due to electron excitations (non-thermal effect) trigger the ultrafast atomic disordering. Our finding of the interplay of thermal and non-thermal effects in the x-ray-induced melting demonstrates that accurate modeling of intense x-ray interactions with matter is essential to ensure a correct interpretation of experiments using intense x-ray laser pulses