Influenza virus RNA segment 7 generates three poly(A) RNAs, M1 mRNA, M2 mRNA and mRNA3, the last of which has almost no coding capacity; M2 mRNA and mRNA3 derive from M1 mRNA by removal of a single intron. The kinetics of M1 and M2 mRNA accumulation in the cytoplasm of productively infected cells were studied by means of a quantitative RNA protection assay; the ratio of M2 mRNA to M1 mRNA increased 2·7-fold during the course of infection. To analyse the basis for this change, the kinetics of M1 and M2 mRNA synthesis and nuclear accumulation, their stability and nucleocytoplasmic transport were studied. Under the experimental conditions used, the synthesis of segment 7-specific RNA showed a peak at 4 h post-infection and continued later at a slower rate. The half-lives of M1 and M2 mRNAs were indistinguishable (2·73 h for M1 mRNA and 2·70 h for M2 mRNA) and the kinetics of nucleocytoplasmic transport or showed no preference for either mRNA early or late in infection. Consequently, regulation at the level of mRNA splicing is proposed. Using the mRNA synthesis and stability data, a simulation was performed to predict the change in splicing efficiency required to account for the mRNA accumulation results. The best fit was obtained when splicing efficiency changed about 20 times during a period in which viral gene expression was maximal.


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