Inactivation of ribosomes in vitro by colicin E 3 and its mechanism of action.

The incubation of purified ribosomes with colicin E(3) results in the cleavage of a terminal fragment from the 16S ribosomal RNA. The cleavage reaction requires three components: colicin E(3), the 30S ribosomal subunit, and the 50S ribosomal subunit. An immunity factor found in extracts derived from colicinogenic cells prevents the in vitro inactivation of ribosomes by colicin E(3). Evidence is presented suggesting that it does so by binding to the colicin molecule. The mode of action of colicin E(3)in vivo can be explained by the assumption that a small fraction of the adsorbed colicin penetrates into the cell and catalytically inactivates the ribosomes.