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)