The plant UMPK is insensitive to GTP and UTP. Eukaryotic UMPKs all share a conserved glycine-rich sequence in their N-terminal regions which is referred to as the phosphate-binding loop and may play a role in ATP binding and/or enzyme catalysis. Site-specific muta- tions within this glycine-rich conserved region of the Arabidopsis enzyme resulted in significant changes in its catalytic activity. Mutations that showed reduced ATP binding showed increased UMP binding. The cDNAs encoding UMPK of rice and Arabidopsis share roughly equivalent identity to the enzymes from yeast (45.5% and 49.7%), Dictyostelium (48.9% and 50.5%), and mammals (55.6% and 53.0%, respectively)