Fakultät für Chemie und Pharmazie - Digitale Hochschulschriften der LMU - Teil 04/06 Archiv
250 Episoden
1 Minute
Eukaryotic nuclear transcription is carried out by three different Polymerases (Pol), Pol I, Pol II and Pol III. Among these, Pol I is dedicated to transcription of the rRNA, which is the first step of ribosome biogenesis, and cell growth is regulated during Pol I transcription initiation by the conserved factor Rrn3/TIF-IA in yeast/human. A wealth of structural information is available on Pol II and its general transcription factors (GTFs). Recently, also the architectures of Pol I and Pol III have been described by electron microscopy and the additional subunits that are specific to Pol I and Pol III have been identified as orthologs of the Pol II transcription factors TFIIF and TFIIE. Nevertheless, we still lack information about the architecture of the Pol I initiation complex and structural data is missing explaining the regulation of Pol I initiation mediated by its central transcription initiation factor Rrn3. The Rrn3 structure solved in this study reveals a unique HEAT repeat fold and indicates dimerization of Rrn3 in solution. However, the Rrn3-dimer is disrupted upon Pol I binding. The Rrn3 structure further displays a surface serine patch. Phosphorylation of this patch represses human Pol I transcription (Mayer et al, 2005; Mayer et al, 2004), and a phospho-mimetic patch mutation prevents Rrn3 binding to Pol I in vitro, and reduces S. cerevisiae cell growth and Pol I gene occupancy in vivo. This demonstrates a conserved regulation mechanism of the Pol I-Rrn3 interaction. Crosslinking indicates that Rrn3 does not only interact with Pol I subunits A43/14, but the interface further extends past the RNA exit tunnel and dock domain to AC40/19. The corresponding region of Pol II binds the Mediator head (Soutourina et al., 2011) that co-operates with TFIIB (Baek et al, 2006). Consistent with this, the Rrn3 binding partner, core factor subunit Rrn7, is predicted to be a TFIIB homologue. Taken together, our results provide the molecular basis of Rrn3-regulated Pol I initiation and cell growth and indicate a universally conserved architecture of eukaryotic transcription initiation complexes.
![Full Functionalization of the Thieno[3,2-b]thiophene Scaffold. Benzo[b]thiophenes via Intramolecular Carbomagnesiation of Alkynyl(aryl)thioethers. Preparation and Reactions of Solid Organozinc Reagents.](https://cdn.podcastcms.de/images/shows/175/2034623/s/624675964/full-functionalization-of-the-thieno32-bthiophene-scaffold-benzobthiophenes-via-intramolecular-carbomagnesiation-of-alkynylarylthioethers-preparation-and-reactions-of-solid-organozinc-reagents.png)
1 Minute
On the one hand this work intends to present new possibilities on how the combination of characterization methods can be used to gain information not available from the individual techniques. On the other hand discrete tomography - a relatively new method in materials science - is used to image real three-dimensional nano structures with a resolution of only a few nanometers. Visualization not only facilitates the interpretation of scientific results, but also aims at contributing to a better general understanding of nano technology.
1 Minute
In the course of this work the excited state dynamics of individual single-walled carbon nanotubes (SWCNTs) were studied by a combination of confocal PL spectroscopy and time correlated single photon counting (TCSPC) measurements. Nonradiative decay channels dominate the excited state dynamics of SWCNTs leading to low photoluminescence (PL) quantum yields and PL decay times on the picosecond timescale. Knowledge about the microscopic nature of these decay channels is crucial to improve the material properties. The measurements on the single nanotube level revealed large tube-to-tube variations of PL decay times, which could be attributed to different defect densities for different tubes. For the present SWCNT material the PL decay times only depend weakly on the nanotube length. SWCNT material synthesized by using a cobalt-molybdenum catalyst (CoMoCAT) systematically display short monoexponential PL decays, while the PL decay dynamics of SWCNTs produced high pressure decomposition of carbon monoxide process (HiPco) is either mono or biexponential depending on the respective local environment of the nanotube. The transition from a bi- to monoexponential PL decay can be explained by synthesis-dependent differences in defect densities. This defect related nonradiative decay channels reduce the amplitude of one decay component below the experimental detection limit. It is further shown, that photo-induced defects and gold atoms adsorbed to the sidewalls of SWCNTs are shown to alter the PL properties of individual SWCNTs. Additional low-energy PL satellite bands arise in the spectra. Their origin can be attributed to emission from nominally dark excitons which are ”brightened” due to defect facilitated mixing of intrinsic states with different parity/spin. The role of defects in the brightening process was investigated by time-resolved PL measurements and complementary Raman spectroscopy. Based on its energy separation and the unusually slow PL decay dynamics the lowest energy satellite band can be assigned to the radiative recombination of a triplet exciton. In a second project a common-path interference scattering approach (iSCAT) utilizing a conventional inverted laser scanning confocal microscope combined with a photonic crystal fibre as a supercontinuum white light source is successfully tested for its capabilities for elastic scattering imaging and spectroscopy of individual SWCNTs. Finally, it is shown that single layer graphene can selectively be turned luminescent upon exposure to a mild oxygen plasma. The treatment leads to a strong and spatially uniform PL which is characterized by a single, broad PL band extending from the visible to the near infrared spectral region. The analysis of the defect related Raman I(D)/I(G) intensity ratio indicates the formation of nanometer sized islands for which the sp2 conjugated lattice of graphene is still preserved. Emission of quantum confined states within these islands is discussed as a possible origin of the PL.
