The eight genome RNA segment of influenza B virus codes for two nonstructural polypeptides (NS1 and NS2). These polypeptides are translated from separate mRNA species which have been identified by in vitro translation after gradient separation. The location of the gene coding for the NS2 polypeptide was determined by hybridization of its mRNA to individual virus genome RNA segments and the resulting specific inhibition of its synthesis in vitro after hybridization to genome segment 8. The NS1 and NS2 polypeptides do not share [35S]methionine-containing tryptic peptides, and the size of the NS1 polypeptide is such that the nucleotide sequences coding for it must occupy almost the entirety of genome segment 8. Thus the sequences coding for most, if not all, of the NS2 polypeptide must overlap with those coding for NS1 and be translated in a different reading frame. The results suggest that the proportion of the NS1 mRNA that is translated and the extent of the overlap between the NS1 and NS2 genes is greater in influenza B virus than was found previously with influenza A virus.
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