Anaerobic citrate metabolism and its regulation in enterobacteria
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© Springer-Verlag 1997
MINI-REVIEW
Michael Bott
Anaerobic citrate metabolism and its regulation in enterobacteria
Received: 25 September 1996 / Accepted: 18 November 1996
Abstract Several species of enterobacteria are able to utilize citrate as carbon and energy source. Under oxic conditions in the presence of a functional tricarboxylic acid cycle, growth on this compound solely depends on an appropriate transport system. During anaerobiosis, when 2oxoglutarate dehydrogenase is repressed, some species such as Klebsiella pneumoniae and Salmonella typhimurium, but not Escherichia coli, are capable of growth on citrate by a Na+-dependent pathway forming acetate, formate, and CO2 as products. During the last decade, several novel features associated with this type of fermentation have been discovered in K. pneumoniae. The biotin protein oxaloacetate decarboxylase, one of the key enzymes of the pathway besides citrate lyase, is a Na+ pump. Recently it has been shown that the proton required for the decarboxylation of carboxybiotin is taken up from the side to which Na+ ions are pumped, and a membrane-embedded aspartate residue that is probably involved both in Na+ and in H+ transport was identified. The Na+ gradient established by oxaloacetate decarboxylase drives citrate uptake via CitS, a homodimeric carrier protein with a simultaneous-type reaction mechanism, and NADH formation by reversed electron transfer involving formate dehydrogenase, quinone, and a Na+-dependent NADH:quinone oxidoreductase. All enzymes specifically required for citrate fermentation are induced under anoxic conditions in the presence of citrate and Na+ ions. The corresponding genes form a cluster on the chromosome and are organized as two divergently transcribed operons. Their co-ordinate expression is dependent on a two-component system consisting of the sensor kinase CitA and the response
Dedicated to Prof. Achim Kröger on the occasion of his 60th birthday M. Bott (Y) Mikrobiologisches Institut, Eidgenössische Technische Hochschule, ETH-Zentrum, Schmelzbergstrasse 7, CH-8092 Zürich, Switzerland Tel. +41-1-632-3830; Fax +41-1-632-1148 e-mail [email protected]
regulator CitB. The citAB genes are part of the cluster and are positively autoregulated. In addition to CitA/CitB, the cAMP receptor protein (Crp) is involved in the regulation of the citrate fermentation enzymes, subjecting them to catabolite repression. Key words Catabolite repression · Citrate fermentation · Citrate transport · Enterobacteria · Gene regulation · Two-component system · Escherichia coli · Klebsiella pneumoniae · Providencia rettgeri · Salmonella typhimurium Abbreviations ACP Acyl carrier protein · Crp cAMP receptor protein · HQNO Heptylhydroxyquinoline-Noxide · TCA cycle Tricarboxylic acid cycle · ∆Ψ Transmembrane electrical gradient · ∆pH Transmembrane chemical gradient of H+ · ∆pNa Transmembrane chemical gradient of Na+ · ∆µH+ Transmembrane electrochemical gradient of H+ · ∆µNa+ Transmembrane electrochemical gradient of Na+
1 Introduction Citrate is a ubiqu
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