Expression Of The Pyr Operon Of Lactobacillus Plantarum Is Regulated By Inorganic Carbon Availability Through A Second Regulator, Pyrr(2), Homologous To The Pyrimidine-Dependent Regulator Pyrr(1)

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 pyrR(1)BCAa(1)Ab(1)DFE 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. PyrR, 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 PyrR(2) was investigated by allelic gene replacement. Unlike the pyrR, mutant, the Delta pyrR(2) 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 pyrR(1) and pyrR(2) mutants suggested that only PyrR(2) 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 PyrR, and PyrR(2) in the pyr regulon expression.