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.
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