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Journal of Bacteriology, December 2006, p. 8607-8616, Vol. 188, No. 24
0021-9193/06/$08.00+0     doi:10.1128/JB.00985-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Expression of the pyr Operon of Lactobacillus plantarum Is Regulated by Inorganic Carbon Availability through a Second Regulator, PyrR₂, Homologous to the Pyrimidine-Dependent Regulator PyrR₁

UMR7156 Université Louis Pasteur/CNRS, Génétique Moléculaire, Génomique, Microbiologie, Département Microorganismes, Génomes, Environnement, 28 Rue Goethe, 67083 Strasbourg, France,¹ Biocentrum-DTU, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark²

Received 6 July 2006/ Accepted 3 October 2006

Inorganic carbon (IC), such as bicarbonate or carbon dioxide, stimulates the growth of Lactobacillus plantarum. At low IC levels, one-third of natural isolated L. plantarum strains are nutritionally dependent on exogenous arginine and pyrimidine, a phenotype previously defined as high-CO₂-requiring (HCR) prototrophy. IC enrichment significantly decreased the amounts of the enzymes in the pyrimidine biosynthetic pathway encoded by the pyrR₁BCAa₁Ab₁DFE operon, as demonstrated by proteomic analysis. Northern blot and reverse transcription-PCR experiments demonstrated that IC levels regulated pyr genes mainly at the level of transcription or RNA stability. Two putative PyrR regulators with 62% amino acid identity are present in the L. plantarum genome. PyrR₁ is an RNA-binding protein that regulates the pyr genes in response to pyrimidine availability by a mechanism of transcriptional attenuation. In this work, the role of PyrR₂ was investigated by allelic gene replacement. Unlike the pyrR₁ mutant, the pyrR₂ strain acquired a demand for both pyrimidines and arginine unless bicarbonate or CO₂ was present at high concentrations, which is known as an HCR phenotype. Analysis of the IC- and pyrimidine-mediated regulation in pyrR₁ and pyrR₂ mutants suggested that only PyrR₂ positively regulates the expression levels of the pyr genes in response to IC levels but had no effect on pyrimidine-mediated repression. A model is proposed for the respective roles of PyrR₁ and PyrR₂ in the pyr regulon expression.

Published ahead of print on 13 October 2006.

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