Studies on the Biosynthesis of 5-ribosyluracil 5'-monophosphate in Tetrahymena Pyriformis.

Since the discovery (1, 2) and elucidation of the structure of 5-ribosyluracil ($ uridine) (3-5)) increasing attention has been turned toward the metabolism of this cotnpound. $ Uridine, along with the other new]> discovered constituents of ribonucleic acid, has been shown to be present in highest proportions, if not exclusively, in soluble, or amino acid transfer RX.1 (2, 6). Because of the central role of sRNA’ in protein biosynthesis, investigations into the metabolic formation of 1c/ uridine and its incoq)oration into I)olynucleotide have become of iml)ortance in gaining not only a clearer insight into the chemical events of ljrotein formation, but also a better understanding of the mode of synthesis of transfer RSh. In preliminary experiments, we observed that extracts from the pyrimidine-requiring protozoan, Tetrah!Jmena pyriformis, catalyzed the conversion of uracil and 5-phosphoribosyl pyrephosphate (PI’-ribosr-I’) to both uridine monophosphate and 5-ribosyluracil 5’-monol,hosphate (4 UMl’) (7). The Ixesent paper describes the separation and I,artial purification of the enzymes responsible for ITMP and fi XXI1 biosynthesis in this organism. During the course of this investigation, it was shown t.hat whereas UMP synthesis proceeds via the conventional pyrophosphorylase }jathtvay (8)) # 1-1\IP is formed directly from uracil and ribose 5-phosphate by way of an entirely distinct mechanism involving a new enzyme, which has been tentatively called + UMP synthetase. This latter finding constitutes the first clearly defined description of the enzymatic formation of the carbon-carbon ribosyl linkage of $ uridine and its derivatives. A preliminary account of some of thl. ‘s \vork has been published (9).