Studies on cytoplasmic ribonucleic acid from rat liver: V. Cofactor requirements for the metabolic interaction between soluble and microsomal ribonucleic acid

Abstract 1. 1. The metabolic transfer of nucleotide label from [ 32 P]s-RNA to microsomal RNA has been studied in a system containing rat-liver microsomes and varying concentrations of the four nucleoside triphosphates. 2. 2. ATP and GTP stimulate the reaction, whereas the other nucleoside triphosphates are much less effective. Optimal conditions, however, vary from batch to batch of s-RNA. By contrast, the transfer of [ 14 C]leucine from [ 14 C]leucyl-s-RNA to microsomal protein requires GTP only. 3. 3. Incubation of s-RNA with microsomes in a reaction mixture devoid of cofactors reduces its metabolic transferring abilities as compared with s-RNA incubated in the presence of these factors. Evidence is presented in this and the subsequent paper that the structural integrity of the s-RNA terminus is a requisite for the metabolic transfer of nucleotide as well as for amino acid transfer to the microsomal template. It is suggested that GTP is specifically required for the interaction between s-RNA (or part of the soluble polynucleotides) and the microsomal template whereas the stimulatory activity of ATP may be ascribed to its capacity to restore the functional end of s-RNA, although a more specific function of the latter nucleotide cannot be excluded. 4. 4. The experiments further indicate that s-RNA following incubation with microsomes, has fully retained its ability to interact with microsomes, as judged by its amino acid carrier function as well as by its capacity to exchange nucleotide material with the microsomal template, provided that ATP and GTP are present during the transfer reaction.