Specific binding of influenza A virus NS1 protein to the virus minus-sense RNA in vitro

The non-structural protein NS1, encoded by genome segment 8 of influenza A virus, was expressed in Escherichia coli from cloned cDNA and purified. The NS1 protein had a specific RNA-binding activity, binding to influenza A virus minus-sense but not plus-sense RNA synthesized in vitro from cloned DNA using phage RNA polymerase. NS1 bound preferentially to the regions of RNA containing either 5′- or 3′-terminal common sequences of the genomic RNA. Binding was inhibited by virion RNA, but not by single-stranded minus-sense cDNA and oligo DNAs having the common sequences. In addition, binding was also inhibited by 28S rRNA but not 18S rRNA prepared from MDCK cells.