Molecular characterisation of the Stc mutation of Escherichia coli K-12

The previously described Stc ^- (suppressor of TolC) mutation modifies the phenotype of tolC mutants from OmpF^? to OmpF⁺. Restriction mapping of chromosomal DNA from Stc ⁺ and Stc^? strains was performed to investigate the nature of the mutation which was shown to be a deletion, upstream of the ompC gene. DNA from the region of the deletion was cloned into pUC18 and a 650-bp PstI-EcoRI fragment was sequenced. The deletion started 49 bp upstream of the AUG start codon of the ompC gene, thus removing part of the ompC promoter and the whole of the micF gene. We suggest that the deletion of micF gives rise to the Stc^? phenotype since the effect of micF expression is assumed to reduce ompF expression, and the Stc^? phenotype involves increase in ompF expression.