Characterization of MarR, the repressor of the multiple antibiotic resistance (mar) operon in Escherichia coli.

The marRAB operon is one of two operons in the mar locus of Escherichia coli that are divergently transcribed from a central regulatory region, marO. The marRAB operon, transcribed from marOII, controls intrinsic resistance or susceptibility to multiple antibiotics and is inducible by structurally unrelated compounds such as tetracycline and chloramphenicol (S. P. Cohen, H. Hachler, and S. B. Levy, J. Bacteriol. 175:1484-1492, 1993). To clarify the role of the operon in response to environmental signals, its transcription was studied under different conditions, using a marOII-lacZ transcriptional fusion introduced into the chromosome of wild-type or mar-deleted cells. In wild-type cells, uncoupling agents (such as carbonyl cyanide m-chlorophenylhydrazone) and different redox-cycling compounds (e.g., menadione and plumbagin) induced expression from the marOII-lacZ fusion two- to sevenfold. In the mar-deleted strain, LacZ expression from the fusion was 10-fold higher than in wild-type cells. This activity was temperature sensitive (3-fold lower at 42 than at 30 degrees C) and decreased 20-fold with the introduction of the gene for MarR. Structurally different compounds which induce the mar operon in wild-type cells reversed the MarR repression of marOII-lacZ expression. To determine the size of MarR, it was fused to MalE as a MarR fusion protein of 144 amino acids [MarR(144)] or of 125 amino acids (deleted of 19 amino acids at the N terminus) [MarR(125)]. Only the MarR(144) fusion showed repressor ability. The purified MarR(144) fusion, but not the MarR(125) fusion, bound specifically to marO in vitro, as revealed by gel retardation, with an apparent dissociation constant of 5 x 10(-9) M. MarR, therefore, controls expression of the marRAB operon presumably by binding to marO. MarR repression in cells can be reversed by different compounds, facilitating the response of bacteria to multiple environmental stress conditions.