Thomas Kalkbrenner

Internal Dynamics of Supercoiled DNA Molecules

The intramolecular diffusive motion within supercoiled DNA molecules is of central importance for a wide array of gene regulation processes. It has recently been shown, using fluorescence correlation spectroscopy, that plasmid DNA exhibits unexpected acceleration of its internal diffusive motion upon supercoiling to intermediate density. Here, we present an independent study that shows a similar acceleration for fully supercoiled plasmid DNA. We have developed a method that allows fluorescent labeling of a 200-bp region, as well as efficient supercoiling by Escherichia coli gyrase. Compared to plain circular or linear DNA, the submicrosecond motion within the supercoiled molecules appears faster by up to an order of magnitude. The mean-square displacement as a function of time reveals an additional intermediate regime with a lowered scaling exponent compared to that of circular DNA. Although this unexpected behavior is not fully understood, it could be explained by conformational constraints of the DNA strand within the supercoiled topology in combination with an increased apparent persistence length.

Super-resolution microscopy heads towards 3D dynamics

Abstract Resolving fine details of subcellular structures is key to understanding the organization and function of cellular networks. Recent advances in far-field fluorescence microscopy provide the necessary tools to analyze these structures with resolutions well below the classical diffraction limit in all three dimensions. Technical improvements go hand-in-hand with new versions of switchable fluorophores that allow nonlinear optical effects to be more efficiently used to push the resolution limit down further. High contrast combined with the wide spectrum of available colors currently endow these fluorescence-based super-resolution techniques with the power to study the complexity of subcellular organelles and the relation of their constituting components down to the molecular level and under physiological conditions. In this way, they give us a far better understanding of the assembly of macromolecular complexes and their functions within a cell than has been possible before employing conventional imaging methods. In this review, we give an overview of the technical state-of-the art of these technologies, their fundamental and technical trade-offs, and provide typical application examples in this exciting field.

SIM and PALM: high-resolution microscopy methods and their consequences for cell biology

The diffraction limit in traditional fluorescence microscopy (approximately 200 and 600 nanometers in lateral and axial directions, respectively) has restricted the applications in bio-medical research. However, over the last 10 years various techniques have emerged to overcome this limit. Each of these techniques has its own characteristics that influence its application in biology. This paper will show how two of the techniques, Structured Illumination Microscopy (SIM) and PhotoActivated Localization Microscopy (PALM), complement each other in imaging of biological samples beyond the resolution of classical widefield fluorescence microscopy. As a reference the properties of two well known standard imaging techniques in this field, confocal Laser Scanning Microscopy (LSM) and Total Internal Reflection (TIRF) microscopy, are compared to the properties of the two high resolution techniques. Combined SIM/PALM imaging allows the extremely accurate localization of individual molecules within the context of various fluorescent structures already resolved in 3D with a resolution of up to 100nm using SIM. Such a combined system provides the biologist with an unprecedented view of the sub-cellular organization of life.

SIM and PALM as tools to study protein structural organization, numbers, interaction and dynamics

Lately quite a plethora of concepts have been successfully developed, which take resolution beyond the classical limits of a light microscope. Among these structured illumination microscopy (SIM) and photo activated localization microscopy (PALM) hold the promise to provide biologists with unprecedented insights into sub-cellular organizations. A combination of these methods seems particularly attractive as it allows adapting to the required resolution and enables to map single molecules or molecule ensembles in the context of highly resolved structures. SIM achieves two fold resolution enhancements in both lateral and axial directions, so structures can be highly resolved in 3D. Adapting the structuring to the wavelength opens up the avenue for multi-color staining. Hence the distribution of one protein and its associated structure can be viewed in the context of others. Since all common fluorescent dyes can be used sample preparation is straightforward. Besides the classical approach to obtain highly resolved structures with up to 10 times the classical resolution, the power of PALM lies additionally in its ability to count and observe single molecules. As such clustering of molecules can be studied as well as many molecules tracked simultaneously to study their diffusion. New strategies open up the possibility to obtain resolution enhancement in the axial direction as well. These applications start already to have an impact on our view how a cell is organized and how different proteins contribute to its make-up.