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The uracil salvage pathway in Lactobacillus plantarum was demonstrated to be dependent on the upp-pyrP gene cluster. PyrP was the only high-affinity uracil transporter since a pyrP mutant no longer incorporated low concentrations of radioactively labeled uracil and had increased resistance to the toxic uracil analogue 5-fluorouracil. The upp gene encoded a uracil phosphoribosyltransferase (UPRT) enzyme catalyzing the conversion of uracil and 5-phosphoribosyl-[alpha]-l-pyrophosphate to UMP and pyrophosphate. Analysis of mutants revealed that UPRT is a major cell supplier of UMP synthesized from uracil provided by preformed nucleic acid degradation. In a mutant selection study, seven independent upp mutants were isolated and all were found to excrete low amounts of pyrimidines to the growth medium. Pyrimidine-dependent transcription regulation of the biosynthetic pyrimidine pyrR1-B-C-Aa1-Ab1-D-F-E operon was impaired in the upp mutants. Despite the fact that upp and pyrP are positioned next to each other on the chromosome, they are not cotranscribed. Whereas pyrP is expressed as a monocistronic message, the upp gene is part of the lp_2376glyA-upp operon. The lp_2376 gene encodes a putative protein that belongs to the conserved protein family of translation modulators such as Sua5, YciO, and YrdC. The glyA gene encodes a putative hydroxymethyitransferase involved in [C.sub.1] unit charging of tetrahydrofolate, which is required in the biosynthesis of thymidylate, pantothenate, and purines. Unlike upp transcription, pyrP transcription is regulated by exogenous pyrimidine availability, most likely by the same mechanism of transcription attenuation as that of the pyr operon.

Carbamoyl phosphate is a precursor for both arginine and pyrimidine biosynthesis. In Lactobacillus plantarum, carbamoyl phosphate is synthesized from glutamine, ATP, and carbon dioxide by two sets of identified genes encoding carbamoyl phosphate synthase (CPS). The expression of the carAB operon (encoding CPS-A) responds to arginine availability, whereas pyrAaAb (encoding CPS-P) is part of the pyrR1BCAaAbDFE operon coding for the de novo pyrimidine pathway repressed by exogenous uracil. The pyr operon is regulated by transcription attenuation mediated by a trans-acting repressor that binds to the pyr mRNA attenuation site in response to intracellular UMP/phosphoribosyl pyrophosphate pools. Intracellular pyrimidine triphosphate nucleoside pools were lower in mutant FB335 (carAB deletion) harboring only CPS-P than in the wild-type strain harboring both CPS-A and CPS-P. Thus, CPS-P activity is the limiting step in pyrimidine synthesis. FB335 is unable to grow in the presence of uracil due to a lack of sufficient carbamoyl phosphate required for arginine biosynthesis. Forty independent spontaneous FB335-derived mutants that have lost regulation of the pyr operon were readily obtained by their ability to grow in the presence of uracil and absence of arginine; 26 harbored mutations in the pyrR1-pyrB loci. One was a prototroph with a deletion of both pyrR1 and the transcription attenuation site that resulted in large amounts of excreted pyrimidine nucleotides and increased intracellular UTP and CTP pools compared to wild-type levels. Low pyrimidine-independent expression of the pyr operon was obtained by antiterminator site-directed mutagenesis. The resulting AE1023 strain had reduced UTP and CTP pools and had the phenotype of a high-[CO.sub.2]-requiring auxotroph, since it was able to synthesize sufficient arginine and pyrimidines only in [CO.sub.2]-enriched air. Therefore, growth inhibition without [CO.sub.2] enrichment may be due to low carbamoyl phosphate pools from lack of CPS activity.