The tolC locus affects the expression of sbmA through σE activity increase

The SbmA protein is involved in the transport of MccB17-, MccJ25-, bleomycin- and proline-rich peptides into the Escherichia coli cytoplasm. sbmA gene homologues were found in a variety of bacteria. However, the physiological role of this protein still remains unknown. Previously, we found that a combination of sbmA and tolC mutations in Tn10-carrying E. coli K-12 strains results in hypersusceptibility to tetracycline. In this work, we studied sbmA expression in a tolC mutant background and observed an increased expression throughout growth. We ruled out the global transcriptional regulator RpoS and the small RNA micF as intermediates in this regulation. The tolC mutation induced the expression of other well-characterized strong σ(E) -dependent promoters in E. coli. We observed that the increase in σ(E) activity led to a greater sbmA expression, conversely eliminating σ(E) prevented expression of sbmA. We also observed that the sbmA upregulation in a tolC mutant context was abolished in an rpoE-null strain. These results suggest a σ(E) -dependent positive regulation on sbmA by the tolC mutation. We hypothesize that this mechanism might be part of a compensatory cell envelope stress response.