Regulation of the Pyrimidine Biosynthetic Pathway in Pseudomonas mucidolens

The pyrimidine biosynthetic pathway and its regulation in Pseudomonas jessenii

The control of the pyrimidine biosynthetic pathway by pyrimidine bases was examined in Pseudomonas jessenii ATCC 700870. The pyrimidine biosynthetic enzymes aspartate transcarbamoylase, dihydroorotase, dihydroorotate dehydrogenase, orotate phosphoribosyltransferase, and orotidine 5'-monophosphate (OMP) decarboxylase activities were found to be higher in the succinate-grown ATCC 700870 cells than the glucose-grown cells. All the enzyme activities were depressed in uracil-supplemented ATCC 700870 glucose-grown cells relative to the unsupplemented cells which was indicative of possible repression of enzyme synthesis by uracil. In the succinate-grown, ATCC 700870 cells, transcarbamoylase, dihydroorotase and dehydrogenase activities were decreased by uracil and orotate supplementation while decarboxylase activity was decreased following uracil addition. A pyrimidine auxotroph was isolated by conventional chemical mutagenesis and resistance to 5-fluoroorotic acid whose pyrimidine requirement was met by uracil or cytosine. The mutant strain was deficient for orotate phosphoribosyltransferase activity. Pyrimidine limitation of the mutant strain cells for 1 or 2 h caused about a two-fold increase in aspartate transcarbamoylase or dihydroorotase activity independent of carbon source relative to excess uracil growth conditions. At the level of enzyme activity, aspartate transcarbamoylase activity in P. jessenii ATCC 700870 was inhibited strongly by pyrophosphate, ATP, UTP, GTP and UMP under saturating substrate concentrations.

Pyrimidine biosynthesis in Pseudomonas veronii and its regulation by pyrimidines

Pyrimidine biosynthesis in the nutritionally versatile bacterium Pseudomonas veronii ATCC 700474 appeared to be controlled by pyrimidines. When wild type cells were grown on glucose in the presence of uracil, four enzyme activities were depressed while all five enzyme activities increased in succinate-grown cells supplemented with uracil. Independent of carbon source, orotic acid-grown cells elevated aspartate transcarbamoylase, dihydroorotase, orotate phosphoribosyltransferase or OMP decarboxylase activity. Pyrimidine limitation of glucose-grown pyrimidine auxotrophic cells lacking OMP decarboxylase activity resulted in at least a doubling of the enzyme activities relative to their activities in uracil-grown cells. Less derepression of the enzyme activities was observed after pyrimidine limitation of succinate-grown mutant cells possibly due to catabolite repression. Aspartate transcarbamoylase activity in Ps. veronii was regulated at the level of enzyme activity since the enzyme was strongly inhibited by pyrophosphate, UDP, UTP, ADP, ATP and GTP. Overall, the regulation of pyrimidine biosynthesis in Ps. veronii could be used to differentiate it from other taxonomically related species of Pseudomonas.

Regulation of pyrimidine formation in Pseudomonas lundensis

The regulation of pyrimidine formation in the food spoilage agent Pseudomonas lundensis ATCC 49968 by pyrimidines was examined. In P. lundensis cells grown on glucose as a carbon source, the enzymes aspartate transcarbamoylase, dihydroorotase, and orotidine 5'-monophosphate decarboxylase were induced by orotic acid. Pyrimidine auxotrophs containing reduced transcarbamoylase or orotate phosphoribosyltransferase activity were isolated using chemical mutagenesis and selection procedures. Independent of carbon source, the maximum derepression of enzyme activity was observed for orotidine 5'-monophosphate decarboxylase after pyrimidine limitation of either auxotroph. In the glucose-grown cells of the transcarbamoylase mutant strain, orotic acid induced dihydroorotase and decarboxylase activities. Aspartate transcarbamoylase activity in succinate-grown P. lundensis cells was highly regulated by pyrophosphate as well as by pyrimidine and purine ribonucleotides. It was concluded that pyrimidine formation in P. lundensis was controlled both at the level of de novo pyrimidine biosynthetic enzyme synthesis and at the level of transcarbamoylase activity.

Regulation of pyrimidine nucleotide biosynthesis in Pseudomonas synxantha

Regulation of pyrimidine nucleotide biosynthesis in Pseudomonas synxantha ATCC 9890 was investigated and the pyrimidine biosynthetic pathway enzyme activities were affected by pyrimidine supplementation in cells grown on glucose or succinate as a carbon source. In pyrimidine-grown ATCC 9890 cells, the activities of four de novo enzymes could be depressed which indicated possible repression of enzyme synthesis. To learn whether the pathway was repressible, pyrimidine limitation experiments were conducted using an orotate phosphoribosyltransferase (pyrE) mutant strain identified in this study. Compared to excess uracil growth conditions for the succinate-grown mutant strain cells, pyrimidine limitation of this strain caused dihydroorotase activity to increase about 3-fold while dihydroorotate dehydrogenase and orotidine 5'-monophosphate decarboxylase activities rose about 2-fold. Regulation of de novo pathway enzyme synthesis by pyrimidines appeared to be occurring. At the level of enzyme activity, aspartate transcarbamoylase activity in P. synxantha ATCC 9890 was strongly inhibited in vitro by pyrophosphate, UTP, ADP, ATP, CTP and GTP under saturating substrate concentrations.

Regulation of the pyrimidine biosynthetic pathway in apyrD knockout mutant ofPseudomonas aeruginosa

In research to date, regulation of the pyrimidine biosynthetic pathway at the level of gene expression has not been shown for wild type Pseudomonas aeruginosa. No repression was observed when uracil was added to the growth medium nor was any derepression seen when Pyr(-) auxotrophs were limited for pyrimidines. Here we show that the addition of uracil to Pseudomonas minimal medium influenced the synthesis of pyrimidine enzymes, while starvation of a pyrimidine knockout mutant (pyrD) elicited derepression of the pyrimidine enzymes. Moreover, the inclusion of orotate in the growth medium induced the synthesis of dihydroorotase in both wild type and mutant. These results suggest that the pyrimidine pathway in P. aeruginosa is regulated at the level of enzyme synthesis in a manner similar to a number of other Pseudomonas species.

Characterization of Treponema denticola pyrF encoding orotidine-5'-monophosphate decarboxylase

The Treponema denticola ATCC 35405 genome annotation contains most of the genes for de novo pyrimidine biosynthesis. To initiate characterization of pyrimidine synthesis in Treponema, we focused on TDE2110 (the putative pyrF, encoding orotidine-5'-monophosphate decarboxlyase). Unlike the parent strain, an isogenic pyrF mutant was resistant to 5-fluoroorotic acid. In complex medium, growth of the pyrF mutant was independent of added uracil, indicating activity of a uracil uptake/salvage pathway. Transcription of pyrF was greatly reduced in T. denticola grown in excess uracil, demonstrating that de novo pyrimidine synthesis is regulated and suggesting a feedback mechanism. Treponema denticola PyrF complemented uracil auxotrophy in an Escherichia coli pyrF mutant. This study provides biochemical confirmation of T. denticola genome predictions of de novo and salvage pyrimidine pathways and provides proof of concept that pyrF has potential as a selectable marker in T. denticola.