Initiation of translation directed by 42S and 26S RNAs from Semliki Forest virus in vitro.

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Abstract
The proteins synthesized in vitro in response to 42S and 26S RNA from Semliki Forest virus were labeled with formyl-[35S]methionine from initiator tRNA. One protein which comigrated with viral capsid protein was labeled under the direction of 26S RNA and only 1 labeled peptide was detected after digestion with trypsin. Further digestion with pronase gave rise to the dipeptide fMet-AsN. Several labeled polypeptides were found in the 42S RNA directed product and these had MW of up to 150,000. Tryptic digestion of the product yielded only 1 formylmethionyl-labeled peptide, which had a different mobility from that directed by the 26S RNA. Further digestion with pronase gave a single dipeptide, fMet-Ala. This indicates that nonstructural proteins as large as 150,000 daltons are probably synthesized from 1 initiation site on the 42S RNA. Translation starting from the internal initiation site on the 42S RNA, which is equivalent to that on the 26S RNA, could not be detected under the conditions used. Internal initiation sites which are similarly inactive were also detected in other viral RNA (e.g., brome mosaic virus, tobacco mosaic virus and polyoma 19S RNA). Although eukaryotic mRNA can contain more than 1 initiation site for protein synthesis, only the site nearer the 5'' terminus may be active in vitro.