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Heiss, Gregor (2011): Single-molecule microscopy study of nano-systems: From synthetic photo-switchable nano-devices to the dynamics of naturally occurring transcription factors. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

In this work, techniques were developed and used to study the properties of molecules on a single-molecule level. Single-molecule techniques have the major advantage, that in contrast to ensemble measurements, they allow a detailed insight on the distribution and dynamics of single molecules without averaging over subpopulations. The use of Total Internal Reflection Fluorescence Microscopy (TIRFM) in combination with single-pair Förster Resonance Energy Transfer (spFRET) and Alternating Laser Excitation (ALEX) allows the identification of molecular-states by making quantitative measurements of distances in the Ångström range. The development of highly sensitive photon detectors and the use of versatile labeling techniques with photostable (synthetic or genetically-encoded) fluorophores, extended the application of TIRF microscopy to in vitro and live-cell experiments. Despite reducing the complexity of biological systems down to the single-molecule level, functions of individual molecules and interactions between them can be very sophisticated and challenging to analyze. Using information theory based methods, e.g. HMM, the dynamics extracted from single-molecule data was used to illuminate protein interactions and functions. The highly regulated process of gene transcription plays a central role in living organisms. The TATA-box Binding Protein (TBP) is a Transcription Factor (TF) that mediates the formation of the Pre-Initiation Complex (PIC). The lifetime of TBP at the promoter site is controlled by the Modulator of transcription 1 (Mot1), an essential TBP-associated ATPase involved in repression and in activation of transcription. Based on ensemble measurements, various models for the mechanism of Mot1 have been proposed. However, little is known about how Mot1 liberates TBP from DNA. Using TIRF microscopy, the conformation and interaction of Mot1 with the TBP/DNA complex were monitored by spFRET. In contrast to the current understanding of how Mot1 works, Mot1 bound to the TBP/DNA complex is not able to directly disrupt the TBP/DNA complexes by ATP hydrolysis. Instead, Mot1’s ATPase activity induces a conformational change in the complex. The nature of this changed, "primed", conformation is the change of the bending dynamics of the DNA. The results presented in this work suggest a model in which this primed conformation is a destabilized TBP/DNA complex. The interaction with an additional Mot1 molecule is required in order to liberate TBP from DNA. The effect of Mot1 on the DNA dynamics is TBP binding orientation specific. Mot1 effects on the DNA bending dynamics are strongest for molecules where TBP is bound in the inverted binding orientation. The specificity of Mot1’s regulation of DNA bending dynamics suggests that Mot1 preferably "primes" TBP bound in the inverted binding orientation. The mechanistic insight into the interaction of Mot1 with the TBP/DNA complex serves as a framework for understanding the role of Mot1 in gene up- and down-regulation. In a second project, the same single-molecule techniques were used to fabricate and evaluate self-assembled optically controllable, nanodevices. Based on the specificity of Watson-Crick base pairing, DNA was used as a scaffold to position different fluorophores with nanometer accuracy. The functionality of these nanodevices was expanded by making them optically addressable by incorporation of the switchable fluorescent protein Dronpa. Two functions have been demonstrated: Signal enhancement using Optical Lock-In Detection (OLID) and pH sensing in a live-cell environment.