A new method for the routine spreading of DNA in protein-free conditions.

Electron microscopic studies of DNA are hampered by difficulties encountered with the spreading of DNA under protein-free conditions. The established and currently popular technique of spreading DNA on carbon membranes treated by glow discharge in an atmosphere of pentylamine is limited with regard to its reproducibility and the proper distribution of spread molecules on the surface of the membranes. A new, reliable, and highly reproducible technique using tri-L-(dimethylaminomethyl)phenol (DMP 30), a promotor factor for spreading circular DNA, linear DNA, and DNA-protein complexes, is described in this paper. Monolayers of DMP 30 are formed at the air-water interface by a microdiffusion procedure on droplets. DNA molecules that diffuse on this monolayer can easily be picked up on hydrophobic carbon membranes. This method, which is easy, reproducible, and fast, will facilitate electron microscopic studies of DNA-protein interactions.