In situ distribution of EcoRI methylase and restriction endonuclease in cells of Escherichia coli Bs 5.

Specific IgG antibodies were raised in rabbits against purified EcoRI methylase and restriction endonuclease. Post embedding labeling experiments, using the protein A-gold technique, were made with paraformaldehyde-glutaraldehyde fixed cells, embedded in Lowicryl K4M resin at low temperatures. Labeling with methylase-specific antibodies showed 60-70% of gold particles in the cytoplasm and 30-40% at the cell envelope, whereas the use of restriction enzyme-specific antibodies led to a distribution of 10-30% in the cytoplasm and 70-90% in the cell envelope. The results coincide with the proposed function of the enzymes: in the cytoplasm methylase protects the cells' own DNA from self-destruction, and the restriction endonuclease cuts foreign DNA when entering the cell.