Sendai Virus Y Proteins Are Initiated by a Ribosomal Shunt

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Abstract
The Sendai virus P/C mRNA expresses eight primary translation products by using a combination of ribosomal choice and cotranscriptional mRNA editing. The longest open reading frame (ORF) of the mRNA starts at AUG104 (the second initiation site) and encodes the 568-amino-acid P protein, an essential subunit of the viral polymerase. The first (ACG81), third (ATG114), fourth (ATG183), and fifth (ATG201) initiation sites are used to express a C-terminal nested set of polypeptides (collectively named the C proteins) in the +1 ORF relative to P, namely, C′, C, Y1, and Y2, respectively. Leaky scanning accounts for translational initiation at the first three start sites (a non-ATG followed by ATGs in progressively stronger contexts). Consistent with this, changing ACG81/C′ to ATG (GCCATG81G) abrogates expression from the downstream ATG104/P and ATG114/C initiation codons. However, expression of the Y1 and Y2 proteins remains normal in this background. We now have evidence that initiation from ATG183/Y1 and ATG201/Y2 takes place via a ribosomal shunt or discontinuous scanning. Scanning complexes appear to assemble at the 5′ cap and then scan ca. 50 nucleotides (nt) of the 5′ untranslated region before being translocated to an acceptor site at or close to the Y initiation codons. No specific donor site sequences are required, and translation of the Y proteins continues even when their start codons are changed to ACG. Curiously, ATG codons (in good contexts) in the P ORF, placed either 16 nt upstream of Y1, 29 nt downstream of Y2, or between the Y1 and Y2 codons, are not expressed even in the ACGY1/ACGY2 background. This indicates that ATG183/Y1 and ATG201/Y2 are privileged start sites within the acceptor site. Our observations suggest that the shunt delivers the scanning complex directly to the Y start codons.