ATOMIC FORCE MICROSCOPY OF BIOCHEMICALLY TAGGED DNA

Small fragments of DNA of known length were made with the polymerase c hain reaction. These fragments had biotin molecules covalently attache d at their ends. They were subsequently labeled with a chimeric protei n fusion between streptavidin and two immunoglobulin G-binding domains of staphyloccocal protein A. This tetrameric species was expected to bind up to four DNA molecules via their attached biotin moieties. The DNA-protein complex was deposited on mica and imaged with an atomic fo rce microscope. The images revealed the protein chimera at the expecte d location at the ends of the strands of DNA as well as the expected d imers, trimers, and tetramers of DNA bound to a single protein.