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J. Bacteriol. doi:10.1128/JB.01505-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Identification of a Spermidine Excretion Protein Complex (MdtJI) in Escherichia coli

Graduate School of Pharmaceutical Sciences, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8675, Department of Cell Membrane Biology, Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 560-0047, and Faculty of Pharmacy, Chiba Institute of Science, 15-8 Shiomi-cho, Choshi, Chiba 288-0025, Japan

^* To whom correspondence should be addressed. Email: iga16077{at}p.chiba-u.ac.jp.

	Abstract

A spermidine excretion protein in Escherichia coli was looked for amongst 33 putative drug exporters thus far identified. Cell toxicity and inhibition of growth due to overaccumulation of spermidine was examined in E. coli deficient in spermidine acetyltransferase, an enzyme that metabolizes spermidine. Toxicity and inhibition of cell growth by spermidine were recovered in cells transformed with pUCmdtJI or pMWmdtJI encoding MdtJ and MdtI which belong to the small multidrug resistance family of drug exporters. Both mdtJ and mdtI are necessary for recovery from toxicity of overaccumulated spermidine. It was also found that the level of mdtJI mRNA was increased by spermidine. Spermidine content in cells cultured in the presence of 2 mM spermidine was decreased, and excretion of spermidine from cells was enhanced by MdtJI, indicating that the MdtJI complex can catalyze excretion of spermidine from cells. It was found that Tyr⁴, Trp⁵, Glu¹⁵, Tyr⁴⁵, Tyr⁶¹ and Glu⁸² in MdtJ and Glu⁵, Glu¹⁹, Asp⁶⁰, Trp⁶⁸ and Trp⁸¹ in MdtI were involved in the excretion activity of MdtJI.

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