Lactobacillus plantarum response to inorganic carbon concentrations:: PyrR2-dependent and -independent transcription regulation of genes involved in arginine and nucleotide metabolism

Lactobacillus plantarum susbp. plantarum is a capnophilic Gram-positive heterotroph with optimal growth in 4% CO2-enriched air. At low inorganic carbon (C-i) concentrations, the pyr genes encoding the enzymes of the pyrimidine biosynthetic pathway were overexpressed, in agreement with a previous study showing that these genes are regulated at the transcription level in response to Q via a PyrR(2)-mediated mechanism. A previous study of high-CO2-requiring (HCR) mutants revealed an unknown genetic link between arginine regulation and C-i-dependent nutritional needs. To better understand L. plantarum's adaptation to C-i availability, additional C-i-responsive genes were sought in the arginine biosynthetic pathway (arg and car genes) using slot-blot hybridization and a proteomic differential 2D gel electrophoresis (DIGE) global approach. Besides the nine pyr-encoded proteins, 16 new Icr (inorganic-carbon-regulated) proteins accumulated differentially in response to C-i availability, suggesting that the C-i response involves several metabolic pathways and adaptation processes. Among these Icr proteins only argininosuccinate Iyase, encoded by argH, was involved in arginine biosynthesis. Three proteins involved in the purine biosynthetic pathway and nucleotide conversion, adenylate kinase (Adk), GMP synthase (GuaA), and IMP dehydrogenase (GuaB), accumulated differentially in response to changes in C-i levels. Expression of the Icr protein-encoding genes argH and guaB was regulated at the transcription level or by RNA stability in response to C-i availability, as previously demonstrated for the pyr genes. However, PyrR(2) was not essential for the C-i-regulated transcription of argH and guaB, demonstrating that PyrR2 modulates only a subset of C-i-regulated genes. These results suggest that the C-i response may involve at least two regulatory mechanisms in L. plantarum.