Iron as a Stain for Nucleic Acids in Electron Microscopy

An electron stain that would react with nucleic acids would obviously be a valuable addition to the techniques available for the electron microscopic investigation of cell structure. Information available from light microscopy (1-3) indicates that iron has a marked affinity for acidic residues as well as an excellent capacity for the penetration of biological materials. As a heavy metal, it was expected that iron would introduce sufficient electron density into the specimen to provide for visibility in the electron microscope. The bacteriophage 1"2 of E. cdi is known to consist of a head, containing a deoxyribonucleic acid core surrounded by a protein coat, and a tail, apparently also protein (4). In view of the known and discrete distribution of materials, this bacterial virus was chosen as a test object for investigating the use of iron as a stain for nucleic acids in electron microscopy. The observations were made on thin sections of methacrylate-embedded material, in an RCA EMU-2 electron microscope, equipped with a 25 ~ objective aperture. Samples of purified bacteriophage conmining approximately l0 n particles per ml. were p!petted into 1 per cent 0sO, buffered with veronal to pH 7.4. After 50