The wild-type allele of tonB in Escherichia coli is dominant over the tonB1 allele, encoding TonBQ160K, which suppresses the btuB451 mutation.

The entire coding sequence of the tonB gene, except for nine codons at the 3' end, was deleted from the chromosome of Escherichia coli. Introduction of the btuB451 suppressor mutant tonB1 into the chromosome of such a tonB deletion strain showed that the tonB1 allele was active as a suppressor in a single copy at 37 degrees C and 42 degrees C but not at 28 degrees C. No temperature dependence was seen when FepA- or FhuA-dependent activities of the tonB1 gene product (TonBQ160K) were tested. The btuB451 suppressor activity of tonB1 was inhibited by the simultaneous presence within the cells of the tonB+ allele on a multicopy plasmid. This represents the first case of dominance among different tonB alleles. Inhibition of suppression was abolished by overexpression of the btuB451-encoded receptor protein. Competition for binding of TonB+ and TonBQ150K to ExbB was excluded as the cause of dominance. Based on our data we conclude that competition for binding of TonB+ and TonBQ160K to the btuB451 gene product is the reason for the observed dominance. The implications of these findings for the mechanism of btuB451 suppression by tonB1 are discussed.