The periplasmic protein MppA requires an additional mutated locus to repress marA expression in Escherichia coli.

Escherichia coli strain TP985, which has an insertional mutation in the gene for the periplasmic murein tripeptide binding protein MppA, was previously reported to overproduce MarA and exhibit a multiple-antibiotic resistance (Mar) phenotype (H. Li and J. T. Park, J. Bacteriol. 181:4842-4847, 1999). We found that TP985 contained a previously unrecognized marR mutation which was responsible for the Mar phenotype. Transduction of the mppA mutation from TP985 to another wild-type strain did not affect antibiotic susceptibility. Overproduction of MppA repressed marA transcription in TP985 but not in other mppA or marR mutants. Therefore, TP985 contains an additional unknown mutation(s) that facilitates the repression of marA expression by MppA.