Microcin B17 (MccB17) is a ribosomally synthesized peptide
antibiotic of 43 amino acids that induces double-strand breaking
of DNA in a DNA gyrase-dependent reaction. As a consequence, the
SOS regulon is induced and massive DNA degradation occurs. In
this work we have characterized an Escherichia coli gene, sbmC,
that in high copy number determines high cell resistance to
MccB17. sbmC encodes a cytoplasmic polypeptide of 157 amino
acids (Mr, 18 095) that has been visualized in
SDS—polyacrylamide gels. The gene is located at min 44 of the E.
coli genetic map, close to the sbcB gene. sbmC expression is
induced by DNA-damaging agents and, also, by the entry of cells
into the stationary growth phase. A G → T transversion at the
fifth nucleotide of the quasicanonical LexA-box preceding the
gene makes recA cells 16-fold more resistant to exogenous
MccB17. The gene product, SbmC, also blocks MccB17 export from
producing cells. Altogether, our results suggest that SbmC
recognizes and sequesters MccB17 in a reversible