Scanning independent ribosomal initiation of the Sendai virus X protein.

Both peptide antisera and monoclonal antibodies detect a 10-kd protein (X) in Sendai virus infected cells, which represents approximately the last 95 residues of the 568-amino-acid-long P protein. The X protein does not appear to be derived from a precursor P in vivo, and in in vitro X can be made under conditions in which P synthesis has been arrested by hybridized DNA fragments or specific cleavage of the mRNA. X initiation is nevertheless cap dependent. Ribosomes which initiate X appear to pass directly from the cap to the initiation codon.