Changes in cytoplasmic terminal transferase activities during chick embryo skeletal muscle development

Abstract

Ribonucleotidyl transferases are a group of primer-dependent terminal transferases, of nuclear and cytoplasmic origin, that have been known to exist for many years [l-6]. The best known perhaps are the cytoplasmic tRNA nucleotidyl transferases, shown some time ago to be responsible for the addition of the 3 ’ -terminal trinucleotide CCA to tRNA [7,8]. Poly(A)polymerase activities have also been described and shown to play a role in the polyadenylation of newly synthesized premRNA [g-11]. We have been concerned with a terminal transferase activity in the ribosome fraction of maturing avian erythrocytes responsible for the synthesis of polymers of oligo(U) [12]. We have shown the product of this enzyme activity to be present in the cytoplasm of intact cells [ 131, but the enzyme is not related to cytoplasmic tRNA nucleotidyl transferase [14]. This lends considerable support to the idea that this terminal transferase is not an artefact of isolation or assay. What then is the function of this enzyme? We have previously provided correlative evidence [ 121 suggesting that such a ribosome-associated terminal transferase may play a role in the synthesis or control of synthesis of globin, during the development of the immature avian erythrocyte. In order to investigate a possible translational control function more directly we have continued these studies using developing embryonic chick skeletal muscle. The development of mononucleated myoblasts to form nonproliferating myotubes has been extensively studied in terms of the synthesis of musclespecific proteins and the translational regulation of this synthesis [ 15,161. It represents therefore a suitable system for the analysis of primerdependent terminal transferases and their possible role in regulating differential mRNA availability and translation. This paper reports the presence of cytoplasmic terminal transferases in developing chick embryo skeletal muscle and the changes in the activities of these polysome- and 80 S-associated terminal transferases during myoblast development in vivo.