NOTES

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. The properties of colicin K (6, 10) and the mode of action of colicin K on cells and in artificial membranes (8, 24) have been studied in detail. Colicin K forms defined channels in mem-branes that destroy the electrochemical potential of the cyto-plasmic membrane, and thus it kills cells. Uptake into sensitive cells requires the outer membrane proteins Tsx, OmpF, and OmpA (20) and the Tol system, composed of TolA, TolQ, TolR, and TolB (25). Colicin K has been purified from spen