Mutants in tolA, B, Q, and R genes have been isolated on the basis of their tolerance to bacterial toxins (colicins) and their resistance to the infection of filamentous phages (Ml3, fd, and fl) (Davies and Reeves, 1975a, 1975b ; Nagel de Zwaig and Luria, 1967). These genes form a cluster at 16,8 min on the chromosomal map of E. coli. tol mutants are hypersensitive to detergents and to certain drugs, and they release periplasmic proteins into the growth medium (Nagel de Zwaig and Luria, 1967). Mutations in a contiguous gene, pal, which encodes the outer membrane Peptidoglycan Associated Lipoprotein (PAL), generate a similar phenotype (Fognini-Lefebvre et al., 1987). This suggests that the Tol/PAL proteins are involved in maintaining the integrity of the outer membrane of E. coli. However, the exact physiological role of the Tol/PAL system has not yet been elucidated.