Comparison of incorporation and metabolism of RNA pyrimidine nucleotide precursors in leaf tissues.

Abstract

The pathway of uridylic and cytidylic acid synthesis was investigated in leaf discs of bean (Phaaeolus vulgaris) and of cocklebur (Xanthium pensylvanicum). Carbon-14 labeled pyrimidine nucleotide precursors were fed to the tissues for 4-hour periods, and the fate of the radioactivity in various tissue fractions was then determined. The approximate order of efficiency as RNA precursors in the bean was orotic acid, uridine, carbamyl aspartic acid, uracil, and aspartic acid. Cytosine was metabolized very little by either plant. Uridine nucleotides, including uridine diphosphate glucose, were formed from the first 4 listed precursors. The formation of orotidine-5[image]-phosphate (OMP) and orotidine by the plants was demonstrated, following carboxyl-labeled orotic acid addition, especially in the presence of 6-azauridine. This result, and the observed large stimulation of CO2 loss from orotic acid carboxyl-C14 by 5-phosphoribo-sylpyrophosphate, indicate a normal function for OMP in plant RNA synthesis. The evidence obtained suggests that much of the pathway proposed for other organisms also occurs in these plants. Results also indicate that orotic acid is principally catabolized after it conversion to uridine nucleotides, then by the normal pathway for uracil breakdown through ureidopropionic acid.