大腸菌異物排出系遺伝子の発現制御とMdtBCトランスポーターの形成機構

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Escherichia coli has five RND-type xenobiotic efflux systems. Among them, only the AcrAB-TolC complex is constitutively expressed. The MdtABC-TolC complex is unique in that it has two inner membrane transporters MdtBC. The expression of the mdt operon is regulated by the two-component system BaeS-BaeR that senses indole. Here I measured mRNA levels of mdtA and baeS in the presence or absence of the outer membrane channel TolC. The mRNA levels of both genes in the tolC-deleted strain were higher than those in the tolC+ parent, suggesting that the accumulation of indole in the cytosol, which presumably results from the lack of TolC, induces the expression of these genes. Next I visualized how the MdtB and MdtC transporters are assembled in vivo using green fluorescent protein (GFP) and red fluorescent protein (TagRFP). When expressed alone, MdtB-GFP formed a homotrimer, whereas MdtC-TagRFP did not. When co-expressed, a heterotrimer consisting of two MdtB-GFP molecules and one MdtC-TagRFP molecule was formed predominantly. These results suggest that MdtB and MdtC favor to form the 2:1 heterotrimer, which has been proposed, by the study on tandem-linked trimers, to have the highest drug efflux activity among all possible trimers.

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