DNA sequence of the Escherichia coli tonB gene.

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Abstract
The nucleotide sequence of a cloned section of the E. coli chromosome containing the tonB gene was determined. Transcription initiation and termination sites for tonB RNA were determined by S1 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'' and 3'' RNA termini whose location is consistent with the position of previously isolated tonB::IS[insertion sequence]l 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 3 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.