Tryptic digestion of colicin E2 and its active fragment

Tryptic digestion of colicin E2, a bacteriocin of Escherichia coli, was carried out to obtain an active fragment. Native colicin E2, a complex of protein A or E2* (molecular weight 50,000) possessing DNase activity and protein B or immunity protein (MW 10,000), was digested with a low concentration of trypsin and two fragments, T1 and T2, were obtained. The T1 fragment was a polypeptide having a probable molecular weight of 35,000, while T2 fragment was further dissociated by urea treatment into two components, T2A (MW 16,000) and protein B. These tryptic fragmentation patterns suggest a close structural similarity of colicin E2 to colicin E3. Tryptic fragment T2A retained almost all the DNase activity of protein A, which was neutralized by protein B. At high concentrations, protein A or T2A caused rapid single strand scission of closed-circular ColE1 DNA followed by endonucleolytic cleavage. Protein A showed full activities of native colicin E2 as regards killing sensitive cells and leakage of intracellular methyl-β-D-thiogalactoside (TMG) in λ-lysogenic cells, but none of T1, T2, and T2A showed these activities in vivo.