Uridine Nucleotide Incorporation into Pigeon Liver Microsome Ribonucleic Acid

The role of nucleotides as precursors of ribonucleic acid in animal tissues has been well documented (l-4). ilside from the formation of polymers by the enzyme polynucleotide phosphorylase (5), only one preliminary report of net synthesis of polyribonucleotides in cell-free systems has been made (6). Hilmoe and Heppel (7) have demonstrated the phosphorolysis of added polyadenylic acid by a partially purified enzyme system from guinea pig liver nuclei but were unable to obtain incorporation of adenosine diphosphate into RNA. The incorporation of mononucleotides into the RNA of nuclear (7-11)) microsomal (3, II), and supernatant (11-13) fractions of cell-free systems has been described. All of these systems apparently utilize nucleoside triphosphates as the direct precursors of RNA. Some of these systems incorporate nucleotides nonterminally (3,8,9, 12) whereas the others (10, 11,13) involve terminal addition of nucleoside triphosphates to RNA. The nonterminal incorporation of adenosine monophosphate, under conditions of oxidative phosphorylation, into cytoplasmic RNA of pigeon liver has previously been reported by one of us (1). Herbert et al. (3) have shown that erotic acid is first converted to uridine monophosphate and the latter is then incorporated into rat liver microsomal RNA presumably after conversion to uridine diphosphate or UTP. It seemed pertinent to determine the phosphorylation level of the nucleotide which was the direct precursor of microsomal RNA since most cytoplasmic RNA is in this fraction and because this RNA may serve as templates for the biosynthesis of proteins. In this study the conditions required for nucleotide incorporation into pigeon liver microsomal RNA have been determined and a comparison made between UMP, UDP, and UTP as RNA precursors. The quantities of terminal and nonterminal incorporation have been determined, as well as some of the other properties of the system.