Reovirus
mRNA's containing a 5'-terminal methylated cap structure (
m(7)GpppG(m)) were shown to be effective primers for
influenza viral RNA transcription in vitro catalyzed by the
influenza virion
transcriptase. Priming activity required the presence of methyl groups in the cap since reovirus
mRNA's with 5'-terminal GpppG were inactive as primers. Both the cap and internal
nucleotides were physically transferred from radiolabeled reovirus
mRNA to
influenza viral
complementary RNA (
cRNA) during transcription in vitro. By using reovirus
mRNA's with methyl-(3)H-labeled caps as primers, we showed that the
influenza viral
cRNA synthesized in the presence of unlabeled
nucleoside triphosphates contained [methyl-(3)H]
m(7)GpppG(m), identical to that found in the reovirus
mRNA primer. To demonstrate transfer of internal residues, reovirus
mRNA's synthesized in the presence of all four alpha-(32)P-labeled ribonucleoside triphosphates were used as primers. The resulting
influenza viral
cRNA was (32)P-labeled. Diethyl-aminoethyl-
Sephadex chromatography of the
RNase T2 digest of this
cRNA demonstrated (32)P radiolabel in both internal residues (charge -2) and the cap (charge -4.6). Approximately 25 internal
nucleotides along with the cap of reovirus
mRNA were transferred to each chain of
influenza viral
cRNA. Gel electrophoretic analysis indicated that the segments of
influenza viral
cRNA primed by reovirus
mRNA were approximately the same size as those primed by a different
mRNA,
globin mRNA, strongly suggesting that the
influenza virion
transcriptase complex transfers approximately the same number of
nucleotides plus the cap from different
mRNA primers to the 5' end of
influenza viral RNA transcripts.