Strong function-related homology between the pore-forming colicins K and 5.

Sequence determination of the Escherichia coli colicin K determinant revealed identity with the E. coli colicin 5 determinant in the immunity and lysis proteins, strong homologies in the pore-forming region (93.7%) and the Tsx receptor-binding region (77%) of the colicins, and low levels of homology (20.3%) in the N-terminal region of the colicins. This latter region is responsible for the Tol-dependent uptake of colicin K and the Ton-dependent uptake of colicin 5 in the respective colicins. During evolution, the DNA encoding colicin activity and binding to the Tsx receptor was apparently recombined with two different DNA fragments that determined different uptake routes, leading to the differences observed in colicin K and colicin 5 import.