REPRESSOR MUTATIONS IN THE MARRAB OPERON THAT ACTIVATE OXIDATIVE STRESS GENES AND MULTIPLE ANTIBIOTIC-RESISTANCE IN ESCHERICHIA-COLI

Resistance to multiple antibiotics and certain oxidative stress compou nds was conferred by three independently selected mutations (marR1, so Q1, and cfxB1) that mapped to 34 min on the Escherichia coli chromosom e. Mutations at this locus can activate the marRAB operon, in which ma rR encodes a putative repressor of mar transcription and marA encodes a putative transcriptional activator of defense genes against antibiot ics and oxidants. Overexpression of the wild-type MarR protein reverse d the phenotypes (antibiotic resistance and increased antioxidant enzy me synthesis) of all three mutants. DNA sequence analysis showed that, like marR1, the other two mutations were alterations of marR: a 285-b p deletion in cfxB1 and a GC-->AT transition at codon 70 (Ala-->Thr) i n soxQ1. All three mutations cause increased amounts of mar-specific R NA, which supports the hypothesis that MarR has a repressor function i n the expression of the marRAB operon. The level of mar RNA was furthe r induced by tetracycline in both the marR1 and soxQ1 strains but not in the cfxB1 deletion mutant. In the cfxB1 strain, the level of expres sion of a truncated RNA, with or without tetracycline exposure, was th e same as the fully induced level in the other two mutants. Overproduc tion of MarR in the cfxB1 strain repressed the transcription of the tr uncated RNA and restored transcriptional inducibility by tetracycline. Thus, induction of the marRAB operon results from the relief of the r epression exerted by MarR. The marRAB operon evidently activates both antibiotic resistance and oxidative stress genes.