Differential biochemical regulation of the URA7- and URA8-encoded CTP synthetases from Saccharomyces cerevisiae

The URA7- and URA8-encoded CTP synthetases (EC 6.3.4.2, UTP:ammonia ligase (ADP-forming)) are functionally overlapping enzymes responsible for the biosynthesis of CTP in the yeast Saccharomyces cerevisiae. URA8-encoded CTP synthetase was purified to apparent homogeneity by ammonium sulfate fractionation of the cytosolic fraction followed by chromatography with Q- Sepharose, Affi-Gel Blue, Mono Q, and Superose 6. The subunit molecular mass (67 kDa) of purified URA8-encoded CTP synthetase was in good agreement with the predicted size of the URA8 gene product. Antibodies raised against a fusion protein constructed from the coding sequences of the URA8 gene and expressed in Escherichia coli reacted with purified URA8-encoded CTP synthetase. Native URA8-encoded CTP synthetase existed as a dimer which oligomerized to a tetramer in the presence of its substrates UTP and ATP. Maximum URA8-encoded CTP synthetase activity was dependent on Mg²⁺ ions (K(a) = 2.4 mM) and 2-mercaptoethanol at the pH optimum of 7.5. The enzyme followed saturation kinetics toward UTP (K(m) = 74 μM), ATP (K(m) = 22 μM), and glutamine (K(m) = 0.14 mM). GTP stimulated (K(a) = 26 μM) URA8-encoded CTP synthetase activity 12-fold. CTP potently inhibited (IC₅₀ = 85 μM) URA8-encoded CTP synthetase activity and, in addition, caused the dependence of activity toward UTP to become cooperative. The URA8-encoded CTP synthetase and the previously purified URA7-encoded CTP synthetase differed significantly with respect to several biochemical properties including turnover number, pH optimum, substrate dependences, and sensitivity to inhibition by CTP. The URA7-encoded CTP synthetase mRNA was 2-fold more abundant when compared with URA8-encoded CTP syntherase mRNA. Both CTP synthetase isoforms were maximally expressed in the exponential phase of growth.