THE WILD-TYPE ALLELE OF TONB IN ESCHERICHIA-COLI IS DOMINANT OVER THETONB1 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. Intr oduction of the btuB451 suppressor mutant tonB1 into the chromosome of such a tonB deletion strain showed that the tonB1 allele was active a s a suppressor in a single copy at 37-degrees-C and 42-degrees-C but n ot at 28-degrees-C. No temperature dependence was seen when FepA- or F huA-dependent activities of the tonB1 gene product (TonB(Q160K)) were tested. The btuB451 suppressor activity of tonB1 was inhibited by the simultaneous presence within the cells of the tonB+ allele on a multic opy plasmid. This represents the first case of dominance among differe nt tonB alleles. Inhibition of suppression was abolished by overexpres sion of the btuB451-encoded receptor protein. Competition for binding of TonB+ and TonB(Q150K) to ExbB was excluded as the cause of dominanc e. Based on our data we conclude that competition for binding of TonB and TonB(Q160K) to the btuB451 gene product is the reason for the obs erved dominance. The implications of these findings for the mechanism of btuB451 suppression by tonB1 are discussed.