6-Azauracil-Resistant Variants of Cultured Plant Cells Lack Uracil Phosphoribosyltransferase Activity

6-Azauracil-resistant variants of Haplopappus gracilis (Nutt.) Gray and Datura innoxia Mill. lack activity of uracil phosphoribosyltransferase, a pyrimidine salvage enzyme that catalyzes the conversion of uracil and 6-azauracil to uridine-5'-monophosphate and 6-azauridine-5'-monophosphate, respectively. Resistant cells are competent to take up uracil from their growth medium but do not convert it into a form that can be used for macromolecular synthesis. In extracts from resistant cells, orotate monophosphate decarboxylase, a target enzyme of 6-azauridine monophosphate, is fully sensitive to the phosphorylated analog. These results strongly suggest that uracil phosphoribosyltransferase is the major pathway of pyrimidine salvage in cells of these species and that loss of this enzyme activity confers on the variants resistance to 6-azauracil.