To investigate the role of the 73-kDa poly(A)-binding protein in protein synthesis, the effect of the addition of homo-polyribonucleotides on the translation of polyadenylated and non-adenylated mRNA was studied in the rabbit reticulocyte lysate. Poly(A) was found to be the most effective polynucleotide in inhibiting duck-globin mRNA translation, whereas it had no effect on the translation of polyribosomal duck-globin mRNP, or on the endogenous synthesis of the rabbit reticulocyte lysate. The translation of poly(A)-free mRNA was not affected by the addition of poly(A).Furthermore, we found that the inhibiting effect of poly(A) can be reversed by addition of purified poly(A)-binding protein. It is thus likely that the 73-kDa poly(A)-binding protein is an essential factor necessary for poly(A)-rich mRNA translation.The presence of a stretch of adenylate residues at the 3' end of eukaryotic mRNA has been known for over a decade, but its precise role still remains undefined [l -41. Only a few classes of somatic-cell mRNA do not possess a poly(A) tail long enough to be retained on an oligo(dT) column, as those coding for the histones and some viral mRNAs such as reovirus, TMV and TYMV [5 -71. Investigation of the cytoplasmic role of poly(A) has concentrated in the past on mRNA stability and efficiency of translation.A role of the poly(A) sequence in maintaining message stability was suggested by microinjection experiments showing, in the case of rabbit-globin mRNA and human-histone mRNA, a stringent relationship between the presence of a poly(A) tail and mRNA stability in the frog oocyte and HeLa cells [6,[8][9][10]. This relationship is however apparently not true for all types of mRNA. For instance interferon [ l l ] and mengovirus mRNA [12], whether in their natural polyadenylated or enzymatically deadenylated form, have the same half-life in oocytes. Furthermore, all ten reovirus mRNAs have been shown to be stable for several days following microinjection [7]. A study of the endogenous Xenopus leavis oocyte mRNA coding for the histones has shown that as much as 25-50% is non-adenylated and yet is clearly stable during the long period of oogenesis [13, 141. As another possibility, a function of the poIy(A) tail (and the protein associating with it) in mRNA translation was suggested [15, 161. The efficiency of translation of enzymatically deadenylated mRNA has been compared to that of natural poly(A)-rich mRNA in a variety of in vitro proteinsynthesizing systems. A significant difference between the two forms of mRNA, suggesting a facilitating effect of the poly(A) tail on translation, was only observed when systems with a high rate of re-initiation, such as the reticulocyte lysate, were employed. On the other hand, in the wheat germ or Krebs In eukaryotic cells, the mRNA recovered from polyribosomes dissociated with EDTA or puromycin in the form of mRNP is found tightly associated with a major protein of about 73 kDa, as well as with several other minor proteins [18, 191. This 73-kDa protein appears to be ubiqui...

Abstract. The small RNP complexes of defined morphology

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