Immunoelectron microscopic localization of the restriction endonuclease EcoRI in Escherichia coli BS 5

Purified restriction endonuclease EcoRI isolated from Escherichia coli BS 5 was used for the production of enzyme-specific IgG antibodies in rabbits. For enzyme localization experiments, paraformaldehyde-glutaraldehyde-fixed cells were embedded and polymerized by a low-temperature procedure using Lowicryl K4M. The immuno electron microscopic protein A-gold technique and an immuno-gold method revealed that 70% of the enzyme-specific labeling were located in the cell envelope whereas 30% were found in the cytoplasm. In metal-shadowed preparations, no indications for the presence of the enzyme could be found on the cell surface; however, on the surface of cell protoplasts enzyme specific labeling could be detected. The results indicate the presence of the major amount of EcoRI in the periplasmic space of the cell where it might be loosely bound or even freely diffusible.