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Appl Environ Microbiol, March 1998, p. 850-857, Vol. 64, No. 3
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

The Citrate Transport System of Lactococcus lactis subsp. lactis biovar diacetylactis Is Induced by Acid Stress

Nieves García-Quintáns,1 Christian Magni,2 Diego de Mendoza,2 and Paloma López1,*

Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain,1 and Programa Multidisciplinario de Biología Experimental, Consejo Nacional de Investigaciones Científicas and Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmaceúticas, Universidad Nacional de Rosario, 2000 Rosario, Argentina2

Received 15 October 1997/Accepted 26 December 1997

Citrate transport in Lactococcus lactis subsp. lactis biovar diacetylactis is catalyzed by citrate permease P (CitP), which is encoded by the plasmidic citP gene. We have shown previously that citP is included in the citQRP operon, which is mainly transcribed from the P1 promoter in L. lactis subsp. lactis biovar diacetylactis. Furthermore, transcription of citQRP and citrate transport are not induced by the presence of citrate in the growth medium. In this work, we analyzed the influence of the extracellular pH on the expression of citP. The citrate transport system is induced by natural acidification of the medium during cell growth and by a shift to media buffered at acidic pHs. This inducible response to acid stress takes place at the transcriptional level and seems to be due to increased utilization of the P1 promoter. Increased transcription correlates with increased synthesis of CitP and results in higher citrate transport activity catalyzed by the cells. Finally, this acid stress response seems to provide L. lactis subsp. lactis biovar diacetylactis with a selective advantage resulting from cometabolism of glucose and citrate at low pHs.

* Corresponding author. Mailing address: Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Velázquez 144, 28006 Madrid, Spain. Phone: 34-1-5611800, ext. 4203. Fax: 34-1-5627518. E-mail: CIBPL40{at}FRESNO.CSIC.ES.




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