Influenza A virus genome RNA segment 7 encodes three known mRNAs, two of which, M2 mRNA and M mRNA3, are derived by alternative splicing of the primary collinear mRNA transcript using alternative 5′ splice sites. The function of M mRNA3 is currently unknown, therefore we attempted to determine whether it is essential for virus replication. Recombinant viruses unable to produce M mRNA3 and/or M2 mRNA were created by mutating the shared 3′ splice site. Growth of the mutant viruses in M2-expressing MDCK cells was not significantly affected by the lack of M mRNA3. During the course of a wild-type virus infection, levels of M mRNA3 began to decrease while those of M2 mRNA increased, which may indicate a potential mechanism of alternative splicing control. These data suggest that neither M mRNA3 nor any potential protein product are essential for influenza virus replication in tissue culture.
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