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Journal of Bacteriology, September 2004, p. 5649-5660, Vol. 186, No. 17
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.17.5649-5660.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Acid-Inducible Transcription of the Operon Encoding the Citrate Lyase Complex of Lactococcus lactis Biovar diacetylactis CRL264

Mauricio G. Martín, Pablo D. Sender, Salvador Peirú, Diego de Mendoza, and Christian Magni*

Instituto de Biología Molecular y Celular de Rosario (IBR-CONICET) and Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Rosario, Argentina

Received 28 January 2004/ Accepted 24 May 2004

Although Lactococcus is one of the most extensively studied lactic acid bacteria and is the paradigm for biochemical studies of citrate metabolism, little information is available on the regulation of the citrate lyase complex. In order to fill this gap, we characterized the genes encoding the subunits of the citrate lyase of Lactococcus lactis CRL264, which are located on an 11.4-kb chromosomal DNA region. Nucleotide sequence analysis revealed a cluster of eight genes in a new type of genetic organization. The citM-citCDEFXG operon (cit operon) is transcribed as a single polycistronic mRNA of 8.6 kb. This operon carries a gene encoding a malic enzyme (CitM, a putative oxaloacetate decarboxylase), the structural genes coding for the citrate lyase subunits (citD, citE, and citF), and the accessory genes required for the synthesis of an active citrate lyase complex (citC, citX, and citG). We have found that the cit operon is induced by natural acidification of the medium during cell growth or by a shift to media buffered at acidic pHs. Between the citM and citC genes is a divergent open reading frame whose expression was also increased at acidic pH, which was designated citI. This inducible response to acid stress takes place at the transcriptional level and correlates with increased activity of citrate lyase. It is suggested that coordinated induction of the citrate transporter, CitP, and citrate lyase by acid stress provides a mechanism to make the cells (more) resistant to the inhibitory effects of the fermentation product (lactate) that accumulates under these conditions.

* Corresponding author. Mailing address: Departamento de Microbiología, Suipacha 531, S2002LRK Rosario, Argentina. Phone: 54-341-435 0661. Fax: 54-341-439 0465. E-mail: chmagni{at}infovia.com.ar.

Journal of Bacteriology, September 2004, p. 5649-5660, Vol. 186, No. 17
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.17.5649-5660.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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