Point mutations in the "TonB box" of fhuA were suppressed by point mutations in the tonB gene, suggesting both a functional and physical interaction between the FhuA receptor protein in the outer membrane and the TonB protein in the cytoplasmic membrane of Escherichia coli K12. Mutations in fhuA were classified into four types according to the extent by which they impaired mutant cells in their growth on ferrichrome as sole iron source and in their sensitivity to the antibiotic albomycin and to colicin M. The tonB mutation with a glutamine to leucine replacement at position 165 was less efficient in restoring the FhuA functions than the glutamine to lysine exchange at the same position. The better the coupling between FhuA and TonB the poorer was the inhibition of phage T1 binding to FhuA by ferrichrome. A working model is proposed in which the TonB protein assumes different conformations in response to the energized state of the cytoplasmic membrane and thereby allosterically regulates the activity of the FhuA receptor. This model implies an intermembrane coupling between two proteins in adjacent membranes.