CONTRASTS IN THE METABOLIC STABILITY OF DIFFERENT NUCLEOTIDES IN THE RIBONUCLEIC ACIDS OF ISOLATED NUCLEI

Abstract

Nuclei isolated from calf thymus tissue incorporate adenosine-8-C14, orotic acid-6-C14, uridine-C14, and P32-ortho-phosphate into their ribonucleic acids (RNA). Adenosine-8-C14, once incorporated into RNA, is relatively stable and is well retained by nuclei during a prolonged incubation. However, some loss of adenosine-C14 from nuclear RNA occurs when unlabeled adenosine is added to the medium. Orotic acid and uridine are utilized for the synthesis of both cytidylic and uridylic acids in nuclear RNA. Most of the label appears in the uridylic acid. However, much of the C14 taken up by nuclear RNA when orotic acid-6-C14 or uridine-Cl4 is used as a precursor is rapidly lost on subsequent incubation of the nuclei in C14-free medium. The C14 loss is restricted to the uridine-containing nucleotides of nuclear RNA. No corresponding "turnover" is observed in the cytidylic acid. The C14-turnover reaction involves both the base and the sugar of uridylic acid in RNA. p32-iabeling experiments indicate that phosphate is also lost in the process. The rate of C14 loss is accelerated by adding uridine to the medium, but the corresponding free base (uracil) or the nucleotide (UMP) do not promote the "turnover" reaction. Some mechanisms are considered. The "nucleolar RNA" fraction, which is the site of most active RNA synthesis, is also the site of greatest "turnover". Some evidence is presented to show that uridylic acid "turnover" is a normal aspect of nuclear RNA metabolism.