Riboadenylate Transferase

Abstract: Summary Cordycepin [3′-deoxyadenosine (3′-dAdo)] is an adenosine analog that interferes with nucleic acid synthesis both in vivo and in vitro. Our in vivo studies were performed with L5178Y cells. 3′-dAdo strongly inhibited cell proliferation (concentration that induces 50% inhibition of cell proliferation, 0.27 µm); 3′-dAdo-treated cells did not show unbalanced growth. The inhibitory potency of 3′-dAdo could be abolished to some extent by coincubation with adenosine, but not with 2′-deoxyadenosine. In precursor studies, 3′-dAdo strongly reduced protein synthesis and to a lesser extent total RNA synthesis. The reduction of protein synthesis was most probably the result of an inhibition of mRNA synthesis, since in the presence of 3′-dAdo the number of polysomes decreased. In an intact cell system, [3H]-3′-dAdo was incorporated into RNA but not into DNA. Incorporated [3H]-3′-dAdo was found in the polyadenylate [poly(A)] stretch of poly(A)-containing RNA and mainly in the 10 S and 55 S species of poly(A)-free RNA. Cordycepin triphosphate (3′-dATP) had no influence on the activity of DNA-dependent DNA polymerase α and β from L5178y cells. The incorporation rate of adenosine triphosphate into RNA by DNA-dependent RNA polymerases I, II, and III from mouse liver was moderately inhibited by 3′-dATP. The strongest inhibitory effect of 3′-dATP was observed in the enzyme systems containing nuclear poly(A) polymerase (from oviduct) or cytoplasmic terminal riboadenylate transferase (from calf thymus). The inhibition type was competitive with respect to adenosine triphosphate; in the case of poly(A) polymerase and terminal riboadenylate transferase, the enzyme activity was also inhibited competitively with respect to the oligo(pA)10 initiator. 3′-dATP was used as substrate by poly(A) polymerase; incorporated 3′-deoxyadenosine 5′-monophosphate acted as chain terminator.