STRONG FUNCTION-RELATED HOMOLOGY BETWEEN THE PORE-FORMING COLICIN-K AND COLICIN-5

Sequence determination of the Escherichia coli colicin K determinant r evealed identity with the E. coli colicin 5 determinant in the immunit y and lysis proteins, strong homologies in the pore-forming region (93 .7%) and the Tsx receptor-binding region (77%) of the colicins, and lo w levels of homology (20.3%) in the N-terminal region of the colicins. This latter region is responsible for the Tol-dependent uptake of col icin K and the Ton-dependent uptake of colicin 5 in the respective col icins. 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 diff erences observed in colicin K and colicin 5 import.