isolated an A. thaliana cDNA encoding a UPRT of 198 amino acids which is structurally and func- tionally similar to other phosphoribosyltransferases and is predicted to be localized in the cytoplasm. This subcellu- lar localization is consistent with that of the enzyme cat- alyzing the next step in pyrimidine metabolism, UMP kinase. Comparison of the Arabidopsis UPRT sequence with those identified from sources including four species of bacteria and three species of eukaryotes revealed that sequences clustered into two distinct gene subfamilies. A. thaliana UPRT1 is most closely related to the Toxoplasma gondii and S. cerevisiae enzymes