The use of pronase for electron microscopy of protein-complexed DNA

A method is described which demonstrates the posibility of visualizing protein-complexed DNA by using pronase for both deproteinization and spreading of DNA for electron microscopy. The results show that proteolytic digestion is complete even under conditions of short formaldehyde fixation and pronase is an excellent substitute for cytochrome c for spreading of DNA. The pronase method is successfully applied to virions, native and partially denatured chromatin. The procedure is highly advantageous because it does not require preliminary isolation of DNA, can be applied to microamounts of material and permits the visualization of partial denaturation of DNA in chromatin.