An ∼11,000 molecular weight (mol. wt.) polypeptide (designated 4) has been found in cells of various types infected with the WSN strain of influenza virus and, in small amount, in purified virions. Peptides mapping and immunoprecipitation studies have shown that this polypeptide is distinct from the eight defined influenza virus gene products. A polypeptide of similar peptide composition has been synthesized in vitro in systems programmed with viral mRNA isolated from infected cells and could be precipitated with antiserum against infected cells. Polypeptide 4 was not synthesized in infected cells treated with the amino acid analog p-fluorophenylalanine to restrict viral protein synthesis to that directed by primary mRNA transcripts, nor in an in vitro system programmed with mRNA from infected cells which were treated with cycloheximide to limit viral RNA synthesis to primary transcripts. Thus the available evidence suggests that the synthesis of polypeptide 4 requires "early" protein synthesis. In infected cells, small polypeptides related to the nonstructural polypeptide NS have also been found. A hypothetical scheme for the synthesis of the mRNA for polypeptide 4 is discussed.
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