Identification of specific residues in colicin E1 involved in immunity protein recognition

The basis of specificity between pore-forming colicins and immunity protein s was explored by interchanging residues between colicins El (ColE1) and 10 (Col10) and testing for altered recognition by their respective immunity p roteins, Imm and Cti, A total of 34 divergent residues in the pore-forming domain of ColE1 between residues 419 and 501, a region previously shown to contain the specificity determinants for Imm, were mutagenized to the corre sponding Col10 sequences. The residue changes most effective in converting ColE1 to the Col10 phenotype are residue 448 at the N terminus of helix VI and residues 470, 472, and 474 at the C terminus of helix VII. Mutagenesis of helix VI residues 416 to 419 in Col10 to the corresponding ColE1 sequenc e resulted in increased recognition by Imm and loss of recognition by Cti.