Förster Resonance Energy Transfer - from single molecule spectroscopy to imaging

Sammanfattning: During the last fifteen years several methods have been developed for probing biomolecules (DNA, RNA, proteins) one at a time. Among these methods fluorescence spectroscopy and in particular its many implementations for monitoring Förster Resonance Energy Transfer (FRET), have attracted much interest.This thesis deals mainly with high-precision single molecule FRET (smFRET) studies between a donor and an acceptor fluorophore attached to a biomolecule. Methodologies like multi-parameter fluorescence detection (MFD) and Probability Distribution Analysis (PDA) are used. We investigate, how and in which occasions; complex photophysical properties of the acceptor could influence the experimentally obtained FRET efficiency distributions. The value of smFRET experiments in enzymology is exemplified by presenting studies on DNA-related enzymes. Three structural conformations (Open, Closed, and Nucleotide-Binding) of Klentaq1, a DNA polymerase, have been resolved by measurements on freely diffusing molecules. We observe that the levels of occupancy of these conformations and the transitions among them, are dependent on the nature of the incoming dNTP, shedding more light into how conformational selection controls the incorporation cycle. Additionally, smFRET studies on MutS, a protein responsible for the initiation of the DNA mismatch repair machinery, have identified the existence of a preferred orientation of binding of the protein to asymmetric mismatches of DNA strands. Inhibiting MutS from binding in this preferred orientation has negative implications on the efficiency of the initiation of the overall DNA repair process.Shifting from spectroscopy to microscopy, we use FRET imaging for monitoring interactions between the Human Epidermal Growth Receptors, HER1 and HER2, and the Insulin Growth Factor 1 Receptor, IGF1R, in fixed cells obtained from patients with suspect breast cancer lesions. While working on FRET imaging, the need for developing methodologies for the objective evaluation of the sensitivity of confocal laser scanning microscopes (CLSM) was identified. In order to provide figure of merits for the sensitivity of a microscope, we use Fluorescence Correlation Spectroscopy (FCS) and Transient State (TRAST) imaging measurements on aqueous solutions of Rhodamine 110. Our results suggest that TRAST imaging measurements could serve as a fast and easy test for the day-to-day maintenance of a CLSM and could provide reference standards for comparing images obtained by different microscope systems.