Influenza virus RNA segment 7 encodes two proteins, M1 and M2, depending on the optional removal of an intron from its primary transcript. To investigate the mechanism of this regulated splicing, an influenza virus segment 7 cDNA was cloned under the control of simian 40 virus (SV40) early promoter and poly(A) signals in an SV40 recombinant virus (SVM), and expressed in COS-1 cells. Expression of both M1 and M2 proteins was detected in SVM-infected cells, suggesting (i) the appropriate splicing events to generate M2 mRNA occur in these cells and (ii) significant amounts of unspliced M1 mRNA are transported to the cytoplasm. Analysis of the relative proportion of M2 mRNA to mRNA3 indicated that the use of the alternative 5′ splice sites is reversed in SVM-infected cells compared with those infected with influenza virus. In addition, a different intranuclear distribution of segment 7 transcripts was found in each type of infected cell. We speculate that these differences in splicing efficiency and splice site choice might be related to different subnuclear localizations of segment 7 transcripts synthesized by the different transcriptional machineries.


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