Poly(A) shortening and degradation of the 3' A+U-rich sequences of human c-myc mRNA in a cell-free system.

Abstract

The early steps in the degradation of human c-myc mRNA were investigated, using a previously described cell-free mRNA decay system. The first detectable step was poly(A) shortening, which generated a pool of oligoadenylated mRNA molecules. In contrast, the poly(A) of a stable mRNA, gamma globin, was not excised, even after prolonged incubation. The second step, degradation of oligoadenylated c-myc mRNA, generated decay products whose 3' termini were located within the A+U-rich portion of the 3' untranslated region. These products disappeared soon after they were formed, consistent with rapid degradation of the 3' region. In contrast, the 5' region, corresponding approximately to c-myc exon 1, was stable in vitro. The data indicate a sequential degradation pathway in which 3' region cleavages occur only after most or all of the poly(A) is removed. To account for rapid deadenylation, we suggest that the c-myc poly(A)-poly(A)-binding protein complex is readily dissociated, generating a protein-depleted poly(A) tract that is no longer resistant to nucleases.

Developmental regulation of CRD-BP, an RNA-binding protein that stabilizes c-myc mRNA in vitro

“…Without competitor CRD RNA, the mRNA is still relatively unstable in vitro but is protected from endonucleolytic attack because the CRD-BP shields the CRD. mRNA to which the CRD-BP is bound is degraded by a previously characterized 3' to 5' decay pathway (Brewer and Ross, 1988). In the presence of competitor CRD RNA, the CRD-BP is titrated from the mRNA, which deprotects the CRD, allows the CRD to be attacked by the endonuclease, and accounts for the observed destabilization of the mRNA.…”

Oat phytochrome A mRNA degradation appears to occur via two distinct pathways.

“…Some mRNAs are degraded while associated with polysomes (Brewer and Ross, 1988;Pei and Calame, 1988;Brown and Harland, 1990;Byrne et al, 1993;Pastori and Schoenberg, 1993;Tanzer and Meagher, 1994). POly(A) shortening was detected with human c-myc mRNA in a polysome-based cell-free system (Brewer and Ross, 1988), suggesting that deadenylation of mRNAs can occur on polysomes.…”

“…A common theme in mRNA turnover is first the removal of the poly(A) tail, either by a 3'-5' exoribonuclease or an endoribonucleolytic cleavage in the Tuntranslated region, followed by the degradation of the body of the message. Some eukaryotic mRNAs, for example c-myc (Brewer and Ross, 1988), apolipoprotein II (Binderet al, 1989), and p-globin (Albrecht et al, 1984), are degraded first by poly(A) tail removal followed by hydrolysis of the body of the message by a 3'+5'exoribonuclease. The nonpolyadenylated human H4 histone is also degraded by a 3'+5'exoribonuclease (Peltz et al, 1991).…”

“…Efforts have been made to identify sequences within mRNAs, cis elements, that are involved in regulating stability. Some mammalian mRNAs have been found to contain one or more cis elements that have a large impact on mRNA stability (Brewer and Ross, 1988;Klausner and Harford, 1990;Shyu et al, 1991;Theodorakis and Cleveland, 1992;Sachs, 1993). The control of mRNA stability in plants has recently been reviewed by Gallie (1993) and Green (1993).…”

The Poly(A) Tail Inhibits the Assembly of a 3′-to-5′ Exonuclease in an In Vitro RNA Stability System

“…The tail is a dynamic structure which serves as a mediator through which several important cellular processes including the initiation of translation (31), nucleocytoplasmic transport (18), and mRNA stability (7), are regulated. Most mRNAs are rapidly degraded following deadenylation of their 3Ј ends to approximately 30 adenylate residues (8,21,27,34). Many destabilizing elements appear to act by increasing the rate of deadenylation (11).…”