DNA and amino acid sequence analysis of structural and immunity genes of colicins Ia and Ib

Abstract

The nucleotide sequences for colicin Ia and colicin Ib structural and immunity genes were determined. The two colicins each consist of 626 amino acid residues. Comparison of the two sequences along their lengths revealed that the two colicins are nearly identical in the N-terminal 426 amino acid residues. The C-terminal 220 amino acid residues of the colicins are only 60% identical, suggesting that this is the region most likely recognized by their cognate immunity proteins. The predicted proteins for the colicin immunity proteins would contain 111 amino acids for the colicin Ia immunity protein and 115 amino acids for the colicin Ib immunity protein. The colicin immunity proteins have no detectable DNA or amino acid homology but do exhibit a conservation of overall hydrophobicity. The colicin immunity genes lie distal to and in opposite orientation to the colicin structural genes. The colicin Ia immunity protein was purified to apparent homogeneity by a combination of isoelectric focusing and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The N-terminal amino acid sequence of the purified Ia immunity protein was determined and was found to be in perfect agreement with that predicted from the DNA sequence of its structural gene. The Ia immunity protein is not a processed membrane protein.