title: Characterization of the citrate/acetate antiporter CitW of Klebsiella pneumoniae.
abstract: The genome of Klebsiella pneumoniae contains at least three different genes encoding citrate transporters. Recently, a third and hitherto unknown gene encoding a citrate transport system ( citW) was identified. Escherichia coli transformed with a plasmid expressing citW was able to grow on citrate as sole carbon and energy source, identifying CitW as a citrate carrier. In this report, we provide evidence that further specifies CitW as a Na(+)-independent citrate/citrate and citrate/acetate exchanger. Kinetic analysis of citrate uptake at different pH values identified Hcitrate(2-) as the transported citrate species, with a K(m) of 25 microM. Since citW is expressed under anoxic conditions and acetate is the main end-product of citrate fermentation in K. pneumoniae, citrate/acetate exchange might be its in vivo function. Sequence similarity searches identified CitW (454 amino acids, 48.15 kDa) as a member of the 2-hydroxycarboxylate transporter family (TC 2.A.24). The substrate specificity seems to partially contradict this phylogenetic classification, but appears logical with respect to the putative functional role of CitW in the citrate fermentation pathway of K. pneumoniae.
authors.affiliation: Institut für Mikrobiologie, Eidgenössische Technische Hochschule Zürich, Schmelzbergstrasse 7, 8092 Zürich, Switzerland.
authors.forename: Christopher N
authors.initials: CN
authors.lastname: Kästner
authors.forename: Karin
authors.initials: K
authors.lastname: Schneider
authors.forename: Peter
authors.initials: P
authors.lastname: Dimroth
authors.forename: Klaas M
authors.initials: KM
authors.lastname: Pos
mesh_terms: D000085:Acetates / Q000378:metabolism*; D000332:Aerobiosis; D000595:Amino Acid Sequence; D017920:Antiporters / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D019343:Citric Acid / Q000378:metabolism; D005285:Fermentation; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D007711:Klebsiella pneumoniae / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D008956:Models, Chemical; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D013379:Substrate Specificity
chemical_list: D000085:Acetates; D017920:Antiporters; D002352:Carrier Proteins; C020723:citrate-binding transport protein; D019343:Citric Acid