CONTROL OF THE PYRIMIDINE BIOSYNTHETIC-PATHWAY IN PSEUDOMONAS PSEUDOALCALIGENES

The five de novo enzyme activities unique to the pyrimidine biosynthet ic pathway were found to be present in Pseudomonas pseudoalcaligenes A TCC 17440. A mutant strain with 31-fold reduced orotate phosphoribosyl transferase (encoded by pyrE) activity was isolated that exhibited a p yrimidine requirement for uracil or cytosine. Uptake of the nucleoside s uridine or cytidine by wild-type or mutant cells was not detectable; explaining the inability of the mutant strain to utilize either nucle oside to satisfy its pyrimidine requirement. When the wildtype strain was grown in the presence of uracil, the activities of the five de nov o enzymes were depressed. Pyrimidine limitation of the mutant strain l ed to the increase in aspartate transcarbamoylase and dihydroorotate d ehydrogenase activities by more than 3-fold, and dihydroorotase and or otidine 5'-monophosphate decarboxylase activities about 1.5-fold, as c ompared to growth with excess uracil. It appeared that the syntheses o f the de novo enzymes were regulated by pyrimidines. In vitro regulati on of aspartate transcarbamoylase activity in P. pseudoalcaligenes ATC C 17440 was investigated using saturating substrate concentrations; tr anscarbamoylase activity was inhibited by P-i, PPi, uridine ribonucleo tides, ADP, ATP, GDP, GTP, CDP, and CTP.