Identification of the Escherichia coli tonB gene product in minicells containing tonB hybrid plasmids.

Sodium dodecyl sulfate/polyacrylamide gel analysis of proteins encoded by a series of tonB⁺ plasmids in minicells has identified the ton B gene product as a protein with an apparent molecular weight of 36,000. A parallel analysis of seven ton B mutations which have been genetically crossed onto a tonB⁺ plasmid supports this identification; the 36,000 M_r protein is absent from the set of proteins encoded by each tonB^? plasmid. Four of the tonB mutations are apparently IS1 insertions. The locations of these insertions within tonB have been determined by restriction endonuclease mapping. Correlation of these IS1 insertion sites with the molecular weights of prematurely terminated tonB polypeptides, suggests that tonB is transcribed in the direction opposite to that of the nearby tryptophan operon. In addition, a protein encoded by one of the inverted repeat sequences of the transposable element Tn5 has been tentatively identified.