16 Poly(A) Metabolism and Translation: The Closed-loop Model

Abstract

More than two decades ago, experiments on the ribonuclease susceptibility of different RNAs led to the realization that uninterrupted tracts of polyadenylic acid (poly[A]) are present within eukaryotic messenger RNAs (Lim and Canellakis 1970; Darnell et al. 1971a,b; Edmonds et al. 1971; Kates 1971; Lee et al. 1971). It is now well recognized that almost all mRNAs whose biosynthesis originates within nuclei contain a 3′ poly(A) tail. Poly(A) sequences are not encoded within genes (Philipson et al. 1971; Birnboim et al. 1973; Jacobson et al. 1974) but are added to nascent pre-mRNAs in a processing reaction that involves site-specific cleavage and subsequent polyadenylation. The site of cleavage is determined by a highly conserved AAUAAA sequence usually 5–30 nucleotides 5′ to the site and by other less well-conserved sequences 3′, and sometimes 5′, to the site. Cleavage and polyadenylation are dependent on poly(A) polymerase and several other factors that impart specificity and processivity to the reaction. For reviews on the sequence and factor requirements of polyadenylation, see Wickens (1990), Proudfoot (1991), Wahle and Keller (1992), and Manley and Proudfoot (1994).

Pulse-chase experiments demonstrated that a large fraction of newly synthesized nuclear poly(A) was conserved in cytoplasmic mRNA (Puckett et al. 1975). These experiments provided early support for the notion that heterogeneous nuclear RNA (hnRNA) was the precursor to mRNA and also forced the conclusion that at least one function of poly(A) must be cytoplasmic. Evidence that the latter conclusion had merit is presented in this chapter, where I consider the...