The nucleotide sequence of a cloned section of the Escherichia coli chromosome containing the tonB gene has been determined. Transcription initiation and termination sites for tonB RNA have been determined by SI nuclease mapping. The tonB promoter and terminator resemble other E. coli promoters and terminators; the sequence of the tonB terminator region suggests that it may function bidirectionally. The DNA sequence specifies an open translation reading frame between the 5' and3' RNA termini whose location is consistent with the position of previously isolated tonB::IS1 mutations. The DNA sequence predicts a proline-rich protein with a calculated size of 26.1-26.6 kilodaltons (239-244 amino acids), depending-on.which of three potential initiation codons is utilized. The predicted NH2 terminus of tonB-protein resembles the proteolytically cleaved signal sequences of E coli periplasmic and outer membrane proteins; the overall hydrophilic character of the protein sequence suggests that the bulk of the tonB protein is not embedded within the inner or outer membrane. A significant discrepancy exists between the calculated size of tonB protein and the apparent size of, 36 kilodaltons determined by sodium dodecyl sulfate/polyacrylamide gel electrophoresis.The function of the Escherichia coli gene tonB has been the subject of much speculation due to the pleiotropic nature oftonB mutations; tonB mutants were originally isolated as a class of bacteria surviving exposure to bacteriophage Ti (1). They were subsequently shown to be insensitive to bacteriophage 480 (2) and to the group B colicins (3, 4). In addition, tonB mutants are defective in vitamin B-12 transport (5) and, perhaps most significantly, they are defective in all known high-affinity iron transport systems, including iron transport mediated by enterochelin, ferrichrome, citrate, rhodotorulic acid, and aerobactin (6-9). The tonB gene product appears to be essential for the energy-dependent phase of vitamin B-12 transport (10) and in the energy-dependent, irreversible step of 080 and T1 infection (11).The cellular location of the tonB gene product remains uncertain. The observation that spheroplasts of a tonB mutant can transport ferrichrome suggests either an outer cell membrane or periplasmic location for the tonB gene-product (12). The energy-dependent phase of vitamin B-12 transport is sensitive to osmotic shock (13, 14), a finding that is consistent with a periplasmic location. However, it has been reported that the tbnB gene product resides in the inner cell membrane on~the basis of Sarkosyl fractionation of the E. coli envelope (15). It has also been reported that spheroplasts of tonB mutants are unable to transport enterochelin (16), a finding that suggests an inner cell membrane location for the tonB gene product; it should be noted, however, that other investigators have been unable to confirm this result (12). A number of possible functions for the tonB gene product have been suggested including control of outer membrane permeability (6), to...

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