Quantifying hopping and jumping in facilitated diffusion of DNA-binding proteins.

Facilitated diffusion of DNA-binding proteins is known to speed up target site location by combining three dimensional excursions and linear diffusion along the DNA. Here we explicitly calculate the distribution of the relocation lengths of such 3D excursions, and we quantify the short-range correlated excursions, also called hops, and the long-range uncorrelated jumps. Our results substantiate recent single-molecule experiments that reported sliding and 3D excursions of the restriction enzyme EcoRV on elongated DNA molecules. We extend our analysis to the case of anomalous 3D diffusion, likely to occur in a crowded cellular medium.

Controlled three-dimensional immobilization of biomolecules on chemically patterned surfaces.

We used electron-beam lithography to fabricate chemical nanostructures, i.e. amino groups in aromatic self-assembled monolayers (SAMs) on gold surfaces. The amino groups are utilized as reactive species for mild covalent attachment of fluorescently labeled proteins. Since non-radiative energy transfer results in strong quenching of fluorescent dyes in the vicinity of the metal surfaces, different labeling strategies were investigated. Spacers of varying length were introduced between the gold surface and the fluorescently labeled proteins. First, streptavidin was directly coupled to the amino groups of the SAMs via a glutaraldehyde linker and fluorescently labeled biotin (X-Biotin) was added, resulting in a distance of approximately 2 nm between the dyes and the surface. Scanning confocal fluorescence images show that efficient energy transfer from the dye to the surface occurs, which is reflected in poor signal-to-background (S/B) ratios of approximately 1. Coupling of a second streptavidin layer increases the S/B-ratio only slightly to approximately 2. The S/B-ratio of the fluorescence signals could be further increased to approximately 4 by coupling of an additional fluorescently labeled antibody layer. Finally, we introduced tetraethylenepentamine as functional spacer molecule to diminish fluorescence quenching by the surface. We demonstrate that the use of this spacer in combination with multiple antibody layers enables the controlled fabrication of highly fluorescent three-dimensional nanostructures with S/B-ratios of >20. The presented technique might be used advantageously for the controlled three-dimensional immobilization of single protein or DNA molecules and the well-defined assembly of protein complexes.