Globin mRNAs are primers for the transcription of influenza viral RNA in vitro

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Abstract
Because influenza viral RNA transcription in vitro is greatly enhanced by the addition of a primer dinucleotide, ApG or GpG, it was proposed that viral RNA transcription in vivo requires initiation by primer RNA synthesized by the host cell, specifically by RNA polymerase II, thereby explaining the .alpha.-amanitin sensitivity of viral RNA transcription in vivo. Such primer RNA are identified initially in reticulocyte [rabbit] extracts, where they are globin mRNA. Purified globin mRNA very effectively stimulated viral RNA transcription in vitro, and the resulting transcripts directed the synthesis of all the nonglycosylated virus-specific proteins in micrococcal nuclease-treated [mouse fibroblast] L cell extracts. The viral RNA transcripts synthesized in vitro primed by ApG also directed the synthesis of the nonglycosylated virus-specific proteins, but the globin mRNA-primed transcripts were translated about 3 times more efficiently. The translation of the globin mRNA-primed, but not the ApG-primed, viral RNA transcripts was inhibited by 7-methylguanosine 5''-phosphate in the presence of S-adenosylhomocysteine, suggesting that the globin mRNA-primed transcripts contained a 5''-terminal methylated cap structure. This cap was apparently transferred from the globin mRNA primer to the newly synthesized viral RNA transcripts, because no detectable de novo synthesis of a methylated cap occurred during globin mRNA-primed viral RNA transcription. Preliminary experiments indicate that other purified eukaryotic mRNAs also stimulate influenza viral RNA transcription in vitro.