Gene-specific labeling of chromatin for electron microscopy.

In restriction enzyme digested Xenopus laevis erythrocyte chromatin, the 5S oocyte genes were specifically labeled with colloidal gold so they could be identified in an electron microscopic study. Nuclei were digested first with HindIII restriction endonuclease followed by T7 exonuclease to leave single-stranded 3' tails on the fragments. A biotinylated 15-mer oligonucleotide complementary to the tail on the 3' end of the 5S gene was annealed to the chromatin, and streptavidin complexed colloidal gold was bound to it. In a spread for electron microscopy the labeled fragments were readily recognized.