Are the 5′ ends of influenza viral mrnas synthesized in vivo donated by host mRNAs?

Eucaryotic mRNAs serve as primers for influenza viral RNA transcription in vitro and donate their 5′ terminal methylated cap and a short stretch of internal nucleotides to the resulting viral RNA transcripts (Plotch, Bouloy and Krug, 1979). If a similar mechanism operates in vivo, then the viral mRNA synthesized in the infected cell would contain a short stretch of nucleotides at its 5′ end, including the cap, which is not viral-coded. This study was undertaken to establish whether such a nonviral sequence exists in in vivo viral mRNA. Gel electrophoresis analysis indicated that the segments of in vivo viral mRNA were 10–15 nucleotides longer at their 5′ end than the segments of ApG-primed complementary RNA synthesized in vitro. As the latter segments initiate exactly at the 3′ end of the virion RNA templates, this result indicates that the segments of in vivo viral mRNA contain 10–15 nucleotides at their 5′ end which are not viral-coded. In addition, when 3H-methyl-labeled in vivo viral mRNA was hybridized to virion RNA, the 5′ terminal cap structure of the mRNA was not protected against pancreatic or T1 RNAase digestion. One of the three 6-methyladenosine (m 6A) residues found per chain of viral mRNA was also not protected. These results strongly suggest that host cell mRNAs serve as primers for viral RNA transcription in the infected cell, and that they donate their cap and 10–15 internal nucleotides, one of which is m 6A, to the resulting viral mRNA molecules.