A multidrug efflux system is involved in colony growth in Streptomyces lividans

1 Department of Life Sciences and Institute of Genome Sciences, National Yang-Ming University, University System of Taiwan, Shih-Pai, Taipei, Taiwan 2 Department of Biological Science and Technology, Chung Hwa College of Medical Technology, Jen-Te Hsiang, Tainan Hsien, Taiwan Correspondence Carton W. Chen cwchen{at}ym.edu.tw Multidrug resistance (MDR) genes are abundant in Streptomyces genomes, and yet these bacteria are generally drug sensitive under routine laboratory conditions, indicating low or no expression of these genes. Drug-resistant mutations have been isolated that lie in regulatory genes adjacent to the MDR genes, suggesting that resistance arises by derepression. This study identified a divergently oriented pair consisting of a TetR-family regulator ( ebrS ) and a major facilitator-family MDR pump ( ebrC ) gene in Streptomyces lividans , which is widely conserved in Streptomyces species. EbrS represses transcription of ebrC as well as its own transcription. Deletion of ebrS causes overexpression of ebrC , resulting in elevated resistance to many drugs. The ebrS and ebrC promoters were used in a reporter system to test inducibility by various chemicals. Among the 15 compounds (including five EbrC target drugs) tested, none induced ebrC transcription. On the other hand, the ebrS promoter was induced by rifampicin and high concentrations of calcium and magnesium. Deletion of ebrS-ebrC did not change rifampicin sensitivity, indicating that the EbrC pump is not involved in rifampicin efflux. Moreover, deletion of ebrC caused retardation of colony growth on selected media, and the defect could be suppressed by supplementation with high concentrations of Ca 2+ , Mg 2+ , Na + or K + . Based on these results, it is proposed that the primary biological role of most MDR systems in Streptomyces species is not removal of extrinsic drugs, but rather export of specific toxic compounds endogenously synthesized during growth. Abbreviations: EB, ethidium bromide; MDR, multidrug resistance; MFS, major facilitator superfamily Details of the microarray results are available as supplementary data with the online version of this paper. These authors contributed equally to this work. Present address: Biofibre and Biotechnology Application Department, Union Chemical Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan.