Heterologous expression of bacterial and human multidrug resistance proteins protect Escherichia coli against mercury and zinc contamination

In order to determine the role of multidrug resistance proteins in mercury and zinc resistance, human MDR1, Lactococcus lactis lmrA, and Oenococcus oeni omrA genes were expressed in an Escherichia coli tolC mutant which is hypersensitive to metals. The three transporters conferred an increased mercury and zinc resistance to E. coli as compared to the control bacteria. This improved resistance correlated with a decreased zinc and mercury bioaccumulation. Indeed, quantification of intracellular metal concentrations by atomic absorption spectrometry (AAS) showed a 2.1-, 3-, and 5.1-fold decrease in zinc in cells expressing hMDR1, omrA, and lmrA, respectively, and a 2.7-, 7.5-, and 7.7-fold decrease in mercury in cells expressing omrA, lmrA, and hMDR1, respectively, as compared to the control bacteria. This means that hMDR1, LmrA, and OmrA proteins which are specialised in xenobiotic scavenging, their main known function, are nevertheless able to confer some resistance against metals. Our results show that the tolC mutated strain is well adapted to the study of MDR transporter activity and could be used to screen substrates and competitive hMDR1 inhibitors.