Regulation of pyrimidine nucleotide biosynthesis in Pseudomonas synxantha

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.