Poly(A): Possible Role in Translation of Poly(A)-Rich RNA 1

A covalently linked region, 100–200 nucleotides long, rich in polyriboadenylic acid [poly(A)], has been identified in rapidly labeled polyribosome-associated RNA and heterogeneous nuclear RNA in a number of eukaryotic cells (1–3), in several specific eukaryotic mRNAs (4–6), and in viral mRNAs (7). It has been suggested that most if not all mRNAs in eukaryotic cells, with the exception of histone mRNA, contain a poly (A) tract (8) at the 3′-OH terminal end (9).

The function of the poly (A) region is not known although a role in mRNA transport from nucleus to cytoplasm has been suggested on the basis of drug inhibitor studies (10). The presence of the poly (A) region in the mRNA of viruses whose life cycle occurs entirely in the cytoplasm (11, 12) implies additional roles. It is possible that the poly (A) tract has a regulatory function related to translation or stability of mRNA. Histone mRNA does not appear to contain a poly (A) tract (13) but can be translated in a cell-free, protein-synthesizing system (14); however, histone mRNA differs from other mRNAs by the number of genomic copies and by the rapidity of its translation and degradation (15–18) perhaps implying a unique mode of regulation.

Rapidly labeled polyribosome-associated RNA containing poly (A) tracts (poly (A)-rich RNA) from cultured human peripheral lymphocytes has been shown to be digested by a putative ribonuclease which is associated with this RNA and which retains activity despite exposure to SDS and phenol (19). This study defines the conditions of incubation for selective cleavage of the poly (A) tract of lymphocyte poly (A)-rich RNA by a putative-associated ribonuclease. The effect of this cleavage on translation has been evaluated in a cell-free, protein-synthesizing system.