3′ poly(A) is dispensable for translation

Abstract

In wild-type cells, the 3' poly(A) structure is necessary for translation of mRNA and for mRNA stability. The superkiller 2 (ski2), ski3, ski6, ski7, and ski8 mutations enhance the expression of the poly(A)(-) mRNAs of yeast RNA viruses. Ski2p is a DEVH-box RNA helicase and Slh1p resembles Ski2p. Both repress L-A double-stranded RNA (dsRNA) virus copy number, further suggesting that their functions may overlap. We find that slh1Delta ski2Delta double mutants are healthy (in the absence of viruses) and show normal rates of turnover of several cellular mRNAs. The slh1Delta ski2Delta strains translate electroporated nonpoly(A) mRNA with the same kinetics as polyA(+) mRNA. Thus, the translation apparatus is inherently capable of efficiently using nonpoly(A) mRNA even in the presence of normal amounts of competing poly(A)(+) mRNA, but is normally prevented from doing so by the combined action of the nonessential proteins Ski2p and Slh1p.

Linking the 3′ Poly(A) Tail to the Subunit Joining Step of Translation Initiation: Relations of Pab1p, Eukaryotic Translation Initiation Factor 5B (Fun12p), and Ski2p-Slh1p

“…7), nor was the stability of ACT1 or PGK1 mRNAs affected (35). This confirmed earlier results indicating that unless the major Xrn1p 5Ј33Ј mRNA degradation system is blocked, the ski2 mutation has no effect on mRNA turnover (18).…”

“…Differences in expression at the earliest times are most likely due to translation rates and, in cases where the expression is proceeding linearly, rates can be clearly distinguished from yields. Functional half-lives of electroporated CϩAϩ and CϩAϪ LUC mRNAs varied little between wildtype cells and ski2⌬ slh1⌬ double mutants (35). Moreover, the half-life in wild-type cells of electroporated CϩAϩ mRNA (62 min) was only about twice that of CϩAϪ mRNA (36 min), a difference insufficient to account for the Ͼ40-fold difference in the expression of these two species (35).…”

“…Like Ski2p, Slh1p is a nonessential RNA helicase that represses dsRNA virus copy number (20). In the ski2⌬ slh1⌬ double mutant poly(A) ϩ and non-poly(A) mRNAs were translated with equal efficiency for the same duration (35) (Fig. 6).…”

“…Further, ski2⌬ slh1⌬ double mutants treat poly(A) ϩ and poly(A) Ϫ mRNAs the same, translating them at the same rate and for the same duration (35). Ski2p, Ski3p, and Ski8p are found in a cytoplasmic complex (4), but their precise role is unclear.…”

“…Although there is a strict requirement for the 3Ј poly(A) structure for translation, both in vivo and in vitro, recent work indicates that this requirement is one imposed by the inhibition of translation of non-poly(A) mRNAs by two homologous nonessential RNA helicases, Ski2p and Slh1p (20,21,35,41). Mutations in SKI2 and SLH1, or other nonessential genes involved in this activity (SKI3, SKI7, and SKI8; see also reference 3), derepress translation of non-poly(A) mRNAs introduced by electroporation, the naturally poly(A)…”