Pyrimidine biosynthesis in Serratia marcescens: a possible role for nonsequential enzyme interactions in mimicking coordinate gene expression

De novo synthesis of pyrimidine nucleotides by Helicobacter pylori

The incorporation of pyrimidine nucleotide precursors into Helicobacter pylori and the activities of enzymes involved in their synthetic pathways were investigated by radioactive tracer analysis and 31P nuclear magnetic resonance spectroscopy. The bacterium was found to take up aspartate and bicarbonate and to incorporate carbon atoms from these precursors into its genomic DNA. Orotate, an intermediate of de novo pyrimidine biosynthesis, and uracil and uridine, precursors for pyrimidine pathways, were also incorporated by the micro-organism. Radiolabelled substrates were used to assess the activities of aspartate transcarbamoylase, orotate phosphoribosyltransferase, orotidylate decarboxylase, CTP synthetase, uracil phosphoribosyltransferase, thymidine kinase and deoxycytidine kinase in bacterial lysates. The study provided evidence for the presence in H. pylori of an operational de novo pathway, and a less active salvage pathway for the biosynthesis of pyrimidine nucleotides.

Pyrimidine biosynthesis in Neisseria meningitidis. 2. Regulation of enzyme synthesis

In Neisseria meningitidis aspartate carbamoyltransferase (ACTase), ornithine carbamoyltransferase (OCTase), and carbamoylphosphate synthetase (CPSase) showed incomplete repression by uracil and arginine. De-repression studies with pyrimidine and arginine mutants showed no de-repressed levels of these enzymes.

Regulation of carbamylphosphate synthesis in Serratia marcescens

Serratia marcescens HY possessed a single carbamylphosphate synthase (CPSase) which was subject to cumulative repression by arginine and a pyrimidine. CPSase did not appear to be a part of a multifunctional enzyme complex as is the case for other enzymes of pyrimidine biosynthesis in this organism. CPSase was purified to homogeneity. The molecular weight of the enzyme was estimated to be 167,000 by sucrose density gradient ultracentrifugation. The double-reciprocal plot for magnesium adenosine triphosphate was linear, yielding a Km value of 2.5 mM. The enzyme utilized either glutamine (Km, 0.1 mM) or NH3 (Km, 10.5 mM) as a nitrogen donor in the reaction. CPSase activity was subject to activation by ornithine and feedback inhibition by uridine monophosphate, as is the case for other enteric bacteria. Carbamate kinase activity, detected in crude extracts of S. marcescens, was shown to be due to a constitutive acetate kinase. The absence of carbamate kinase from S. marcescens HY is consistent with the inability of this organism to utilize arginine as a source of energy under anaerobic conditions.

Pyrimidine biosynthesis in Serratia marcescens: polypeptide interactions of three nonsequential enzymes

Orotidine-5'-monophosphate pyrophosphorylase (OMPppase, E.C. 2.4.2.10) and orotidylate decarboxylase (OMPdecase, E.C. 4.1.1.23) were purified from Serratia marcescens HY. These enzymes required physical association for maximal catalytic activities and formed a fragile complex with dihydroorotase (DHOase, E.C. 3.5.2.3). OMPppase reversibly lost 50% of its activity upon separation from DHOase. The kinetic characteristics of OMPppase were modified by this separation. In the presence of DHOase, the Kms for PRPP and orotate were stoichiometric: 2.3 X 10(-6) M and 2.6 X 10(-6) M, respectively. Following separation, the Kms were significantly different: 1.3 X 10(-6) M for PRPP and 4.1 X 10(-6) M for orotate. OMPppase and OMPdecase could be reversibly separated by acrylamide gel electrophoresis, but the separation was accompanied by a loss of catalytic efficiency for both enzymes. DHOase readily associated into multiple molecular forms and could not be purified. The DHOase-OMPppase-OMPdecase interactions demonstrate that a weakly aggregated, multifunctional enzyme complex participates in the biosynthesis of pyrimidine nucleotides in S. marcescens. This unique association of non-sequential biosynthetic enzymes may represent a larger complex which provides a channeling or regulatory unit.