Transfer of 5′-terminal cap of globin mRNA to influenza viral complementary RNA during transcription in vitro

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Abstract
Globin mRNAs [from rabbits] are effective primers for influenza viral RNA transcription in vitro catalyzed by the virion transcriptase. The 5''-terminal methylated cap of the globin mRNAs is transferred to viral complementary cRNA during transcription. Chemical (.beta.-elimination) or enzymatic removal of the cap of globin mRNA eliminated essentially all their priming activity. Much of this activity could be restored by recapping the .beta.-eliminated globin mRNA with the vaccinia virus guanylyl and methyl transferases. Globin mRNA containing 32P label only in the cap (m7G32pppm6Am) were prepared by recapping .beta.-eliminated globin mRNA with the vaccinia virus enzymes, [a-32P]GTP and unlabeled S-adenosylmethionine. By using this labeled globin mRNA as primer and unlabeled nucleoside triphosphates as precursors, the viral cRNA segments that were synthesized were shown to contain a 32P-labeled 5''-terminal cap structure. Gel electrophoretic analysis indicated that the globin mRNA-primed cRNA segments were 10-15 nucleotides longer at their 5'' end than ApG-primed cRNA segments, which initiate exactly at the 3'' end of the virion RNA templates. This suggests that, in addition to the cap, about 10-15 other nucleotides are also transferred from the globin mRNA to viral cRNA. A mechanism for the priming of influenza viral cRNA synthesis by globin mRNA is proposed.