Characterization of Colicin Ia and Colicin Ib

The chemical properties of purified colicin Ia-CA53 and colicin Ib-P9 have been compared. Colicins Ia and Ib are basic proteins having isoelectric points of greater than 10 and 9.5, respectively. The colicins exhibit similar amino acid compositions having a high content of basic amino acids and low amounts of hydrophobic residues. Neither colicin contains cysteine. Both colicins have serine as an NH2-terminal residue. Comparative fingerprint analyses of the products obtained from trypsin digestion of the iodinated colicins showed that approximately half of the resulting iodinated peptides were identical in the 2 molecules. Similar analyses showed that approximately 80% of the arginine containing tryptic peptides were identical. These results indicate extensive primary sequence homology between the colicins. Both colicins consist of a single polypeptide chain of molecular weight 80,000.