An electron microscopic method for the mapping of proteins attached to nucleic acids.

An electron microscopic method for demonstrating the presence of and mapping the positions of proteins specifically bound to nucleic acids is described. The nucleic acid-protein complex is treated with dinitrofluorobenzene under conditions such that dinitrophenyl (DNP) groups are attached to nucleophilic groups on the protein, with only a low level of random attachment to the nuclei acid. This product is treated with rabbit anti-DNP IgG. The position of the protein-(DNP)n(IgG)m complex on the nucleic acid strand can be observed by electron microscopy by protein free spreading methods and, in many cases, by cytochrome-c spreading. If necessary for visualization by the latter method, the size of the labeled region can be increased by treatment with goat anti-rabbit IgG. High efficiency of electron microscopic labeling is achieved. Examples studied are: the adenovirus-2 DNA terminal protein, a protein covalently bound to SV40 DNA, DNA polymerase I bound to DNA, E. coli RNA polymerase bound to T7 DNA, and proteins UV crosslinked to avian sarcoma virus RNA.