The Formation and Metabolic Interchanges of Pseudouridine in Tetrahymena pyriformis

The formation and metabolism of pseudouridine (5-ribosyluracil) in growing Tetrahymena pyriformis have been studied with the aid of labeled uracil, uridine, pseudouridine, and glucose as precursors. The pyrimidine rings of uracil, uridine, and pseudouridine serve equally well as precursors of the pyrimidine component of uridine and pseudouridine of ribonucleic acid. The ribosyl moiety of uridine labels the ribosyl portions of RNA uridine and RNA pseudouridine equally. Although the latter was not degraded to establish unequivocally that uridine ribosyl is present in pseudouridine ribosyl, the equivalence of specific activities and the evidence of the presence of precursor uridine ribosyl in the uridine of the RNA make it appear that the uridine-pseudouridine interconversion takes place by an intramolecular rearrangement and not principally through a free uracil or diribosyluracil intermediate. Although some transribosylation involving purine and pyrimidine ribosyl moieties and some formation of RNA pyrimidine ribosyl from glucose were observed, these appear to be of secondary importance. The data are consistent with, but do not prove, a rearrangement of pyrimidine nucleosides at the polynucleotide level.