Simultaneous Translation of Structural and Nonstructural Proteins from Semliki‐Forest‐Virus RNA in Two Eukaryotic Systems in vitro

The Semliki Forest virus genome, 42‐S RNA, and the virus‐specific intracellular 26‐S RNA were translated in two cell‐free protein‐synthesising systems, the wheat germ extract, and a partially purified system from mammalian tissues. The 26‐S RNA directed the synthesis of structural proteins only, as revealed by tryptic peptide mapping. About 75–80% of the radioactivity in the products comigrated with capsid and about 4–8% with envelope protein peptides. All the capsid peptides and the full‐sized capsid protein were found in the products in vitro, whereas no complete envelope protein was formed and fewer than half of the envelope peptides were detected. This result is consistent with reports that there is only one initiation site for the translation ofvirus structural proteins, and that the capsid protein is N‐terminal in the polyprotein followed by envelope proteins. The systems programmed with 42‐S RNA yielded virtually the same structural peptides. However, the bulk of the radioactivity was in peptides which did not comigrate with the structural ones. These peptides were mostly associated with relatively small‐sized products. This shows that Semliki Forest virus 42‐S RNA has at least two initiation sites, one for the structural proteins and the other(s) for the nonstructural proteins.