A mutation in the amino terminus of a hybrid TrpC-TonB protein relieves overproduction lethality and results in cytoplasmic accumulation

We have developed a selection for mutations in a trpC-tonB gene fusion that takes advantage of the properties of the plasmid-encoded TrpC-TonB hybrid protein. The TrpC-TonB hybrid protein consists of amino acids 1 through 25 of the normally cytoplasmic protein, TrpC, fused to amino acids 12 through 239 of TonB. It is expressed from the trp promoter and is regulated by the trpR gene and the presence or absence of tryptophan. Under repressing conditions in the presence of tryptophan, the trpC-tonB gene can restore Φ80 sensitivity to a tonB deletion mutant, which indicates that TrpC-TonB can be exported and is functional. High-level expression of TrpC-TonB protein in the absence of tryptophan results in virtually immediate cessation of growth for strains carrying the trpC-tonB plasmid. By selecting for survivors of the induced growth inhibition (overproduction lethality), we have isolated a variety of mutations. Many of the mutations decrease expression of the TrpC-TonB protein, as expected. In addition, three independently isolated mutants expressing normal levels of TrpC-TonB protein result in a Gly→Asp substitution within the hydrophobic amino terminus of TonB. The mutant proteins are designated TrpC-TonBG26D. The mutations are suppressed by prlA alleles, known to suppress export (signal sequence) mutations. TrpC-TonB proteins carrying the Gly→Asp substitution accumulate in the cytoplasm. We conclude that the Gly→Asp substitution is an export mutation. TrpC-TonBG26D protein has been purified and used to raise polyclonal antibodies that specifically recognize both TrpC-TonB protein and wild-type TonB protein.