Both the 7-methyl and the 2'-O-methyl groups in the cap of mRNA strongly influence its ability to act as primer for influenza virus RNA transcription.

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The ability of eukaryotic mRNAs to serve as primers for influenza virus RNA transcription depends on the presence of a 5'-terminal methylated can structure, the absence of which eliminates essentially all priming activity [Plotch, S. J., Bouloy, M. & Krug, R. M. (1979) Proc. Natl. Acad. Sci. USA 76, 1618-1622]. The present study was undertaken to determine the extent to which each of the methyl groups in the cap influences the priming activity of a mRNA. To assess the importance of the 2'-O-methyl group on the penultimate base of the cap, we used several plant viral RNAs containing the monomethylated cap 0 structure, m7GpppG. Brome mosaic virus (BMV) RNA 4 stimulated influenza virus RNA transcription only about 10-15% as effectively as did globin mRNA, which has a cap with a 2'-O-methyl group. When the cap of BMV RNA 4 was enzymatically 2'-O-methylated, its priming activity was increased 14-fold. Qualitatively similar results were obtained with other plant virus RNAs. To assess the importance of the terminal 7-methyl group, BMV RNA 4 containing the cap structure GpppGm was prepared by a series of chemical and enzymatic steps. These molecules were found to be only about 15% as active in priming as BMV RNA 4 molecules containing the fully methylated cap, m7GpppGm, indicating that the terminal 7-methyl group also strongly enhances priming activity. These results indicate that the cap 1 structure (m7GpppXm) found in all mammalian cellular mRNAs is more stringently required for priming influenza virus RNA transcription than for translation in cell-free systems.

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