Translational Gymnastics on the Sendai Virus P/C mRNA

The Sendai virus (SeV) P/C mRNA expresses eight different polypeptide chains using a combination of ribosomal choice and cotranscriptional editing (an internal open reading frame (ORF) is accessed by the addition of a single G residue after a short run of Gs at position 1053 on the mRNA). The longest ORF within the mRNA starts at ATG104 (the second initiation site) and encodes the 568-aa P protein, an essential viral structural protein which serves both as a cofactor for the RNA-dependant RNA polymerase (L protein) and as a part of the assembly complex. The first (ACG81), third (ATG114), fourth (ATG183) and fifth (ATG201) initiation sites are used to express a C-terminal nested set of polypeptides which are in the +1 ORF relative to P, namely C′, C, Y1, and Y2, respectively (collectively named the C proteins). 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 the C′ ACG to an ATG (GCCATG81G; ATG81/C′) ablates all expression from the downstream ATG104/P and ATG114/C initiation codons, whereas initiation from ATG183/Y1 and ATG201/Y2 remains normal in this background. Initiation from ATG183/Y1/ ATG201/Y2 probably takes place by discontinuous scanning via a ribosomal shunt. Scanning complexes appear to assemble at the 5′ cap and then scan the first ≈30 nt of the 5′ UTR before being translocated to an acceptor site close to the Y initiation codons. No specific 5′ UTR or donor site sequence elements are required, and translation of the Y proteins continues even when their start codons are changed to ACG.