Colicin E3 is a toxic protein secreted by certain strains of E. coli in order to eliminate related strains of bacterial living in the same ecological niche. Colicin E3 inactivates the protein biosynthetic machinery of the infected cell by nicking 16S ribosomal RNA at a specific site. Colicin E3 thus acts enzymatically as an endoribonuclease. The producing organism is immune to the toxicity of its own colicin E3 by virtue of a "immunity protein" which binds to colicin E3 and thus renders it inactive. Dr. Shoham proposed a crystallographic structure determination of colicin E3 and its inhibitor, the immunity protein gain understanding at the molecular level of the ribonuclease action of colicin E3 and its inhibition of immunity protein. Detailed knowledge of the enzyme-inhibitor interactions will also shed light on the structure of the ribosome at large, in the vicinity of the cutting site. the immunity protein is one the very few known ribonuclease inhibitors. Its three-dimensional structure may provide a basis for the design of other RNAse inhibitors with desired specificity.
