Site of Synthesis of the Enzymes of the Pyrimidine Biosynthetic Pathway in Oat (Avena sativa L.) Leaves

Heat-bleached oat (Avena sativa L. cv Porter) leaves lacking 70S chloroplast ribosomes have been used to demonstrate that four chloroplast-localized enzymes of pyrimidine nucleotide biosynthesis: aspartate carbamoyl-transferase, dihydroorotase, orotidine phosphoribosyl-transferase, and orotidine-5′-phosphate decarboxylase, are synthesized on cytoplasmic ribosomes. Two other chloroplast enzymes, carbamoyl phosphate synthetase, involved in both pyrimidine and arginine biosynthesis, and ornithine carbamoyltransferase, an enzyme of arginine biosynthesis, were also shown to be made on 80S ribosomes.