The tonB gene is required for energy-dependent transport processes across the outer membrane of gram-negative bacteria. Using the antibiotics albomycin and ferrimycin, a tonB mutant of Yersinia enterocolitica was isolated. Comparison of the tonB mutant with the parent strain revealed that in Y. enterocolitica the uptake of ferrioxamine, ferrichrome, pesticin and heme is TonB-dependent. The tonB gene from Y. enterocolitica was sequenced and found to be similar to those of other Enterobacteria. The Y. enterocolitica tonB gene complemented a Y. enterocolitica tonB mutant. In contrast, some TonB functions of an Escherichia coli tonB mutant were not restored by the tonB gene of Y. enterocolitica. The observed differences in the ability to complement E. coli TonB functions correlated with the degree to which the TonB boxes of the receptors and colicins differed from the TonB box consensus sequence. Furthermore, the N-terminal membrane anchor of the TonB proteins and the TolA protein are likely to form an alpha-helix with an identical sequence motif (SHLS) located at one face of the alpha-helix, suggesting this region to be involved in the functional cross-talk between the TonB-ExbBD- and TolABQR-dependent transport systems across the outer membrane.