Catalyzes the ATP-dependent amination of UTP to CTP with either L-glutamine or ammonia as the source of nitrogen. Is essential for the synthesis of CTP de novo. Contrary to other bacterial CTP synthases, the lactococcal enzyme is also able to convert dUTP to dCTP, but this reaction may not play a significant physiological role (PubMed:11500486). Regulates intracellular CTP levels through interactions with the four ribonucleotide triphosphates (By similarity). ATP + H2O + L-glutamine + UTP = ADP + CTP + 2 H(+) + L-glutamate + phosphate H2O + L-glutamine = L-glutamate + NH4(+) ATP + NH4(+) + UTP = ADP + CTP + 2 H(+) + phosphate Allosterically activated by GTP, when glutamine is the substrate (PubMed:11500486). GTP has no effect on the reaction when ammonia is the substrate. The allosteric effector GTP functions by stabilizing the protein conformation that binds the tetrahedral intermediate(s) formed during glutamine hydrolysis (By similarity). Also activated by magnesium (PubMed:11500486). Allosterically inhibited by CTP (PubMed:11500486). Pyrimidine metabolism; CTP biosynthesis via de novo pathway; CTP from UDP: step 2/2. Homotetramer. In contrast to E.coli CTP synthase, remains a tetramer at dilute enzyme concentrations even in the absence of Mg(2+), ATP and UTP. Cells lacking this gene require cytidine for growth, proving that in L.lactis, the pyrG product is the only enzyme responsible for the amination of UTP to CTP. CTPSs have evolved a hybrid strategy for distinguishing between UTP and CTP. The overlapping regions of the product feedback inhibitory and substrate sites recognize a common feature in both compounds, the triphosphate moiety. To differentiate isosteric substrate and product pyrimidine rings, an additional pocket far from the expected kinase/ligase catalytic site, specifically recognizes the cytosine and ribose portions of the product inhibitor. Belongs to the CTP synthase family.