SENDAI VIRUS Y-PROTEINS ARE INITIATED BY A RIBOSOMAL SHUNT

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 A UG(104) (the second initiation site) and encodes the 568-amino-acid P protein, an essential subunit of the viral polymerase. The first (ACG( 81)), third (ATG(114)), fourth (ATG(183)), and fifth (ATG(201)) initia tion sites are used to express a C-terminal nested set of polypeptides (collectively named the C proteins) in the fl ORF relative to P, name ly, C', C, Y1, and Y2, respectively. Leaky scanning accounts for trans lational initiation at the first three start sites (a non-ATG followed by ATGs in progressively stronger contexts). Consistent with this, ch anging ACG(81/C') to ATG (GCCATG(81)G) abrogates expression from the d ownstream ATG(104/P) and ATG(114/C) initiation codons. However, expres sion of the Y1 and Y2 proteins remains normal in this background. We n ow have evidence that initiation from ATG(183/Y1) and ATG(201/Y2) take s place via a ribosomal shunt or discontinuous scanning. Scanning comp lexes appear to assemble at the 5' cap and then scan ca. 50 nucleotide s (nt) of the 5' untranslated region before being translocated to an a cceptor site at or close to the Y initiation codons. No specific donor site sequences are required, and translation of the Y proteins contin ues even when their start codons are changed to ACG. Curiously, ATG co dons tin 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 e xpressed even in the ACG(Y1)/ACG(Y2) background. This indicates that A TG(183/Y1) and ATG(201/Y2) are privileged start sites within the accep tor site. Our observations suggest that the shunt delivers the scannin g complex directly to the Y start codons.