Improvement of organic solvent tolerance level of Escherichia coli by overexpression of stress-responsive genes.

Water-immiscible organic solvents can be toxic to microorganisms. The tolerance levels differ among strains of Escherichia coli, suggesting that the organic solvent tolerance level is strain specific and determined genetically. We constructed several mutants from E. coli, of which the organic solvent tolerance levels were improved. The mutants were defective in the marR gene encoding a repressor protein for the mar operon that is responsible for environmental stress factors. High expression of stress-responsive genes, soxS, marA, and robA, elevated organic solvent tolerance levels of several strains of E. coli. These genes code for DNA-binding proteins that are transcriptional activators belonging to the AraC subfamily with the helix-turn-helix motif. It was shown that expression of the AcrAB-TolC system, a major efflux pump in E. coli, was positively regulated by the proteins. This system was highly expressed in the organic solvent-tolerant mutants. Strains defective in one of the genes, acrA, acrB, or tolC, were remarkably sensitive to organic solvents.