The tonB gene product is necessary for the energy-dependent transport of ferric chelates and vitamin B12 across the Escherichia coli outer membrane. When carried on multicopy plasmids, the cloned tonB gene complemented tonB hosts, restoring transport of ferri-siderophone complexes and vitamin B12 , and susceptibility to the group B colicins and phage ~80. The levels of these activities were all markedly lower than when the tonB + gene was present in single copy. This depression of TonB function occurred even when the chromosome carried the normal tonB + allele, but plasmids carrying only a portion of the tonB gene, including the 5'-regulatory region, were not inhibitory. entry into the cell, often in a tonB-dependent manner [3]. It has been proposed that the tonB product transmits metabolic energy to the outer membrane receptors to allow them to release their bound substrates into the periplasm [4,5].The tonB gene has been cloned [6] and its nucleotide sequence determined [7]. However, the biological activity of the cloned gene has not been extensively characterized. During studied of the effect of the cloned tonB gene on vitamin B~2 transport, we found a considerably lower level of uptake than in wild-type strains. We show here that the cloned gene corrects all of the TonB mutant phen0types assayed, but that the levels of these activities are depressed. DISCUSSION 3. MATERIALS AND METHODSThe other membrane of E. coli contains several receptor proteins necessary for the transport of various iron chelates and vitamin B~2. Transport into the cell of these substrates after their binding to the receptor requires the action of the tonB product [1,12]. In addition, several colicins and phages employ these receptor proteins for their Bacterial strains and plasmidsThe strains and plasmids used in this study are listed in Table 1. The colicinogenic strains and phages BF23, T5, P1 and ~80vir were from the laboratory collection. Transformations and transduction were carried out as described previously [8].