Publikationen 2018

Matthias Reisser, Anja Palmer, Achim P. Popp, Christopher Jahn, Gilbert Weidinger & J. Christof M. Gebhardt

Zygotic genome activation (ZGA), the onset of transcription after initial quiescence, is a major developmental step in many species, which occurs after ten cell divisions in zebrafish embryos. How transcription factor (TF)-chromatin interactions evolve during early development to support ZGA is largely unknown. We establish single molecule tracking in live developing zebrafish embryos using reflected light-sheet microscopy to visualize two fluorescently labeled TF species, mEos2-TBP and mEos2-Sox19b. We further develop a data acquisition and analysis scheme to extract quantitative information on binding kinetics and bound fractions during fast cell cycles. The chromatin-bound fraction of both TFs increases during early development, as expected from a physical model of TF-chromatin interactions including a decreasing nuclear volume and increasing DNA accessibility. For Sox19b, data suggests the increase is mainly due to the shrinking nucleus. Our single molecule approach provides quantitative insight into changes of TF-chromatin associations during the developmental period embracing ZGA.
Nature Communications
www.nature.com/articles/s41467-018-07731-8

Björn Hellenkamp, Sonja Schmid, Olga Doroshenko, Oleg Opanasyuk, Ralf Kühnemuth, Soheila Rezaei Adariani, Benjamin Ambrose, Mikayel Aznauryan, Anders Barth, Victoria Birkedal, Mark E. Bowen, Hongtao Chen, Thorben Cordes, Tobias Eilert, Carel Fijen, Christian Gebhardt, Markus Götz, Giorgos Gouridis, Enrico Gratton, Taekjip Ha, Pengyu Hao, Christian A. Hanke, Andreas Hartmann, Jelle Hendrix, Lasse L. Hildebrandt, Verena Hirschfeld, Johannes Hohlbein, Boyang Hua, Christian G. Hübner, Eleni Kallis, Achillefs N. Kapanidis, Jae-Yeol Kim, Georg Krainer, Don C. Lamb, Nam Ki Lee, Edward A. Lemke, Brié Levesque, Marcia Levitus, James J. McCann, Nikolaus Naredi-Rainer, Daniel Nettels, Thuy Ngo, Ruoyi Qiu, Nicole C. Robb, Carlheinz Röcker, Hugo Sanabria, Michael Schlierf, Tim Schröder, Benjamin Schuler, Henning Seidel, Lisa Streit, Johann Thurn, Philip Tinnefeld, Swati Tyagi, Niels Vandenberk, Andrés Manuel Vera, Keith R. Weninger, Bettina Wünsch, Inna S. Yanez-Orozco, Jens Michaelis *corresponding authors, Claus A. M. Seidel * corresponding authors, Timothy D. Craggs * corresponding authors & Thorsten Hugel * corresponding authors

Single-molecule Förster resonance energy transfer (smFRET) is increasingly being used to determine distances, structures, and dynamics of biomolecules in vitro and in vivo. However, generalized protocols and FRET standards to ensure the reproducibility and accuracy of measurements of FRET efficiencies are currently lacking. Here we report the results of a comparative blind study in which 20 labs determined the FRET efficiencies (E) of several dye-labeled DNA duplexes. Using a unified, straightforward method, we obtained FRET efficiencies with s.d. between ±0.02 and ±0.05. We suggest experimental and computational procedures for converting FRET efficiencies into accurate distances, and discuss potential uncertainties in the experiment and the modeling. Our quantitative assessment of the reproducibility of intensity-based smFRET measurements and a unified correction procedure represents an important step toward the validation of distance networks, with the ultimate aim of achieving reliable structural models of biomolecular systems by smFRET-based hybrid methods.
Nature Methods
www.nature.com/articles/s41592-018-0085-0 

Rahel Stefanie Wiehe, Boris Gole, Laurent Chatre, Paul Walther, Enrico Calzia, Anja Palmer, J. Christof M. Gebhardt, Miria Ricchetti and Lisa Wiesmüller

Endonuclease G (EndoG) is a nuclear-encoded endonuclease, mostly localised in mitochondria. In the nucleus EndoG participates in site-specific cleavage during replication stress and genome-wide DNA degradation during apoptosis. However, the impact of EndoG on mitochondrial DNA (mtDNA) metabolism is poorly understood. Here, we investigated whether EndoG is involved in the regulation of mtDNA replication and removal of aberrant copies. We applied the single-cell mitochondrial Transcription and Replication Imaging Protocol (mTRIP) and PCR-based strategies on human cells after knockdown/knockout and re-expression of EndoG. Our analysis revealed that EndoG stimulates both mtDNA replication initiation and mtDNA depletion, the two events being interlinked and dependent on EndoG’s nuclease activity. Stimulation of mtDNA replication by EndoG was independent of 7S DNA processing at the replication origin. Importantly, both mtDNA-directed activities of EndoG were promoted by oxidative stress. Inhibition of base excision repair (BER) that repairs oxidative stress- induced DNA damage unveiled a pronounced effect of EndoG on mtDNA removal, reminiscent of recently discovered links between EndoG and BER in the nucleus. Altogether with the downstream effects on mitochondrial transcription, protein expression, redox status and morphology, this study demonstrates that removal of damaged mtDNA by EndoG and compensatory replication play a critical role in mitochondria homeostasis.
Oncotarget
doi.org/10.18632/oncotarget.24822

J. Hettich J.C.M. Gebhardt

Response time and transcription level are vital parameters of gene regulation. They depend on how fast transcription factors (TFs) find and how efficient they occupy their specific target sites. It is well known that target site search is accelerated by TF binding to and sliding along unspecific DNA and that unspecific associations alter the occupation frequency of a gene. However, whether target site search time and occupation frequency can be optimized simultaneously is mostly unclear. We developed a transparent and intuitively accessible state-based formalism to calculate search times to target sites on and occupation frequencies of promoters of arbitrary state structure. Our formalism is based on dissociation rate constants experimentally accessible in live cell experiments. To demonstrate our approach, we consider promoters activated by a single TF, by two coactivators or in the presence of a competitive inhibitor. We find that target site search time and promoter occupancy differentially vary with the unspecific dissociation rate constant. Both parameters can be harmonized by adjusting the specific dissociation rate constant of the TF. However, while measured DNA residence times of various eukaryotic TFs correspond to a fast search time, the occupation frequencies of target sites are generally low. Cells might tolerate low target site occupancies as they enable timely gene regulation in response to a changing environment.
Journal of Theoretical Biology
doi.org/10.1016/j.jtbi.2018.05.037
 

Marianne Schwarz, Kevin Schall, Eleni Kallis, Sebastian Eustermann, Mara Guariento, Manuela Moldt, Karl-Peter Hopfner, Jens Michaelis

Genome maintenance and integrity requires continuous alterations of the compaction state of the chromatin structure. Chromatin remodelers, among others the INO80 complex, help organize chromatin by repositioning, reshaping, or evicting nucleosomes. We report on INO80 nucleosome remodeling, assayed by single‐molecule Foerster resonance energy transfer on canonical nucleosomes as well as nucleosomes assembled from tailless histones. Nucleosome repositioning by INO80 is a processively catalyzed reaction. During the initiation of remodeling, probed by the INO80 bound state, the nucleosome reveals structurally heterogeneous states for tailless nucleosomes (in contrast to wild‐type nucleosomes). We, therefore, propose an altered energy landscape for the INO80‐mediated nucleosome sliding reaction in the absence of histone tails.
FEBS Letters
Copyright © 2018, Federation of European Biochemical Societies, Wiley.
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