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Journal of Bacteriology, August 2005, p. 5146-5155, Vol. 187, No. 15
0021-9193/05/$08.00+0     doi:10.1128/JB.187.15.5146-5155.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

CitI, a Transcription Factor Involved in Regulation of Citrate Metabolism in Lactic Acid Bacteria

Mauricio G. Martin,^1,2 Christian Magni,² Diego de Mendoza,² and Paloma López^1*

Departamento de Estructura y Función de Proteínas, Centro de Investigaciones Biológicas (C.S.I.C.), Madrid, Spain,¹ Instituto de Biología Molecular y Celular de Rosario, Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Rosario, Argentina²

Received 23 February 2005/ Accepted 13 May 2005

A large variety of lactic acid bacteria (LAB) can utilize citrate under fermentative conditions. Although much information concerning the metabolic pathways leading to citrate utilization by LAB has been gathered, the mechanisms regulating these pathways are obscure. In Weissella paramesenteroides (formerly called Leuconostoc paramesenteroides), transcription of the citMDEFCGRP citrate operon and the upstream divergent gene citI is induced by the presence of citrate in the medium. Although genetic experiments have suggested that CitI is a transcriptional activator whose activity can be modulated in response to citrate availability, specific details of the interaction between CitI and DNA remained unknown. In this study, we show that CitI recognizes two A+T-rich operator sites located between citI and citM and that the DNA-binding affinity of CitI is increased by citrate. Subsequently, this citrate signal propagation leads to the activation of the cit operon through an enhanced recruitment of RNA polymerase to its promoters. Our results indicate that the control of CitI by the cellular pools of citrate provides a mechanism for sensing the availability of citrate and adjusting the expression of the cit operon accordingly. In addition, this is the first reported example of a transcription factor directly functioning as a citrate-activated switch allowing the cell to optimize the generation of metabolic energy.

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* Corresponding author. Mailing address: Centro de Investigaciones Biológicas, Ramiro de Maeztu 9, 28040 Madrid, Spain. Phone: (34) 918373112, ext. 4202. Fax: (34) 915360432. E-mail: plg{at}cib.csic.es.

Supplemental material for this article may be found at http://jb.asm.org/.

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Journal of Bacteriology, August 2005, p. 5146-5155, Vol. 187, No. 15
0021-9193/05/$08.00+0     doi:10.1128/JB.187.15.5146-5155.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

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