Spliced and unspliced messenger RNAs synthesized from cloned influenza virus M DNA in an SV40 vector: Expression of the influenza virus membrane protein (M₁)

To investigate the splicing potential of influenza virus mRNAs derived from virion RNA segment 7, which codes for the viral M proteins, RNA segment 7 was cloned (M DNA) into a simian virus 40 vector (SV40-M) using the SV40 late region promoter and control sequences. Sizing and nuclease S1 analysis of mRNAs showed that both interrupted and uninterrupted mRNAs containing influenza virus M sequences are synthesized in cells infected with the vector. The uninterrupted SV40-M mRNA, which contains the coding region for influenza virus M₁ protein, can be translated to yield authentic M₁ protein. The nucleotide sequences found at the junctions of the interrupted mRNA are identical to those found for a mRNA (M mRNA₃) derived from influenza virus RNA segment 7 that is found in influenza virus infected cells. These data therefore indicate that influenza virus mRNA synthesized from RNA segment 7 can be spliced by the host cell and eliminate other possibilities involving "transcriptional jumping." Analysis of the mRNAs containing M sequences derived from the SV40-M vector has shown that there is a similarity between the 5′ ends of spliced and unspliced mRNAs.