Catalyzes the ATP-dependent amination of UTP to CTP with either L-glutamine or ammonia as the source of nitrogen. Constitutes the rate-limiting enzyme in the synthesis of cytosine nucleotides. Required for assembly of cytoophidium in female germline cells (PubMed:23459760, PubMed:25223282). In nurse cells, CTPsyn filament assembly in the cytoophidium is regulated by Ack kinase which may thereby contribute to the control of CTP production at specific stages of oogenesis and development of the nurse cell membrane (PubMed:25223282). ATP + H2O + L-glutamine + UTP = ADP + CTP + 2 H(+) + L-glutamate + phosphate Pyrimidine metabolism; CTP biosynthesis via de novo pathway; CTP from UDP: step 2/2. Localizes to cytoophidium, a subcellular filamentary structure where CTP synthase is compartmentalized. Does not localize to cytoophidium. In ovary, expressed in oocytes, follicle cells and nurse cells. Also expressed in larval and adult testis (at protein level). In larvae, expressed in lymph gland, salivary gland, regions of the midgut, testis, optical lobe and trachea. Isoform 1 is expressed in adult testis, ovary, accessory gland and head. Isoform 2 is weakly expressed in ovary. Isoform 1 expression is high during embryogenesis, modest during larval and pupal stages and abundant in adult. Isoform 2 is expressed at low levels in 0-4h embryos and at very low levels during the rest of developmental stages and in adult. Larval lethal. Belongs to the CTP synthase family.