title: An endogenous protein inhibitor, YjhX (TopAI), for topoisomerase I from Escherichia coli.
abstract: Almost all free-living bacteria contain toxin-antitoxin (TA) systems on their genomes and the targets of toxins are highly diverse. Here, we found a novel, previously unidentified TA system in Escherichia coli named yjhX-yjhQ. Induction of YjhX (85 amino acid residues) causes cell-growth arrest resulting in cell death, while YjhQ (181 residues) co-induction resumes cell growth. The primary cellular target of YjhX was found to be topoisomerase I (TopA), inhibiting both DNA replication and RNA synthesis. Notably, YjhX has no homology to any other toxins of the TA systems. YjhX was expressed well with an N-terminal protein S (PrS) tag in soluble forms. PrS-YjhX specifically interacts with the N-terminal region of TopA (TopA67) but not full-TopA in the absence of plasmid DNA, while PrS-YjhX binds to full-TopA in the presence of DNA. Notably, YjhX does not directly interact with DNA and RNA. YjhX inhibits only topoisomerase I but not topoisomerase III and IV in vitro. Hence, yjhX is renamed as the gene for the TopA inhibitor (the topAI gene). TopAI is the first endogenous protein inhibitor specific for topoisomerase I.
authors.affiliation: The Osaka City University Advanced Research Institute for Natural Science and Technology (OCARINA), 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Faculty of Biology, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan yamaguchi@ocarina.osaka-cu.ac.jp.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Yamaguchi
authors.affiliation: Department of Biochemistry and Molecular Biology, Center for Advanced Biotechnology and Medicine, Rutgers-Robert Wood Johnson Medical School, Piscataway, NJ 08854, USA.
authors.forename: Masayori
authors.initials: M
authors.lastname: Inouye
mesh_terms: D001427:Bacterial Toxins / Q000235:genetics / Q000378:metabolism*; D004264:DNA Topoisomerases, Type I / Q000378:metabolism; D004269:DNA, Bacterial / Q000096:biosynthesis / Q000378:metabolism; D004278:DNA, Superhelical / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D012329:RNA, Bacterial / Q000096:biosynthesis; D059004:Topoisomerase I Inhibitors / Q000378:metabolism*
chemical_list: D001427:Bacterial Toxins; D004269:DNA, Bacterial; D004278:DNA, Superhelical; D029968:Escherichia coli Proteins; D012329:RNA, Bacterial; D059004:Topoisomerase I Inhibitors; C000606974:YjhQ protein, E coli; C000606973:YjhX protein, E coli; D004264:DNA Topoisomerases, Type I