Abstract
The Sendai virus P/C mRNA contains five ribosomal initiation sites between positions 81 and 201 from the 5' end. One of these sites initiates in the P open reading frame (ORF) (ATG/104), whereas four initiate in the C ORF (ACG/81 and ATGs/114, 183, 201), to give a nested set of C proteins (C', C, Y1, Y2). Introduction of new ATGs or physically breaking the mRNA upstream of these natural sites was used in vitro to prevent ribosomal scanning downstream. The results suggest that a minority of the ribosomes which initiate C (ATG/114) and all of those which initiate Y1 and Y2 (ATGs/183 and 201) do so by a scanning independent mechanism. When the leaky ACG/81 site is changed to a non-leaky ATG site in in vivo experiments, ribosomal initiation at Y is again not diminished, whereas that at C as well as at P becomes undetectable. Ribosomal initiation at Y appears to be scanning independent in vitro and in vivo. That at C is partly independent in vitro, but completely dependent in vivo. These results are discussed in terms of a model of internal initiation at Y.