The peptide antibiotic microcin 25 is imported through the TonB pathway and the SbmA protein.

Selection of spontaneous mutants for insensitivity to the peptide antibiotic microcin 25 led to the isolation of five categories of mutants. Phenotypic and mapping studies showed the mutations to be located in the fhuA, exb, tonB, and sbmA genes. The latter encodes a cytoplasmic membrane protein which is also required for the penetration of microcin B17.