CTP formation from UTP is catalyzed by CTPS in the fol- lowing reaction: UTP + ATP + glutamine à CTP + ADP + glutamate + Pi. This reaction requires ATP and glutamine in addition to GTP which is a strong activator of this enzyme (Weinfeld et al., 1978). A gene encoding a CTP synthetase-like protein has been annotated on chromo- some 4 of A. thaliana (Atg02120) but has not yet been characterized.