Influenza A virus RNA-dependent RNA polymerase cleaves influenza mRNA in vitro

Influenza virion-derived viral ribonucleoproteins synthesize both mRNA and cRNA in vitro

The mechanisms regulating the synthesis of mRNA, cRNA, and viral genomic RNA (vRNA) by the influenza A virus RNA-dependent RNA polymerase are not fully understood. Early results suggested that the RNA polymerase "switched" from a transcriptase to a replicase during the viral life cycle in response to the expression of viral proteins. However, recently an alternative model suggesting that replication of influenza virus is regulated by stabilization of the replicative intermediates was proposed. According to this model, the virion-associated polymerase is capable of synthesizing both mRNA and cRNA. We now demonstrate that virion-derived viral ribonucleoproteins (vvRNPs) synthesize both mRNA and cRNA in vitro in the absence of non-virion-associated RNA polymerase or nucleoproteins. The authenticity of the in vitro-transcribed mRNA and cRNA was confirmed by terminal sequence analysis. The addition of non-virion-associated polymerase or NP had no effect on vvRNP activity. De novo synthesis of cRNA was found to be more sensitive than the capped primer-dependent synthesis of mRNA to the concentration of ATP, CTP, and GTP. We conclude that vvRNPs intrinsically possess both transcriptase and replicase activities and that there is no switch in the synthesis of mRNA to cRNA during the influenza virus life cycle.

Tomato spotted wilt virus transcriptase in vitro displays a preference for cap donors with multiple base complementarity to the viral template

Transcription of segmented negative-strand RNA viruses is initiated by cap snatching: a host mRNA is cleaved generally at 10-20 nt from its 5' capped end and the resulting capped leader used to prime viral transcription. For Tomato spotted wilt virus (TSWV), type species of the plant-infecting Tospovirus genus within the Bunyaviridae, cap donors were previously shown to require a single base complementarity to the ultimate or penultimate viral template sequence. More recently, the occurrence in vitro of "re-snatching" of viral mRNAs, i.e., the use of viral mRNAs as cap donors, has been demonstrated for TSWV. To estimate the relative occurrence of re-snatching compared to snatching of host mRNAs, the use of cap donors with either single, double, or multiple complementarity to the viral template was analyzed in pair-wise competition in TSWV in vitro transcription assays. A strong preference was observed for multiple-basepairing donors.

Definition of the minimal viral components required for the initiation of unprimed RNA synthesis by influenza virus RNA polymerase

The first 11 nt at the 5' end of influenza virus genomic RNA were shown to be both necessary and sufficient for specific binding by the influenza virus polymerase. A novel in vitro transcription assay, in which the polymerase was bound to paramagnetic beads via a biotinylated 5'-vRNA oligonucleotide, was used to study the activities of different forms of the polymerase. Complexes composed of co-expressed PB1/PB2/PA proteins and a sub-complex composed of PB1/PA bound to the 5'-vRNA oligonucleotide, whereas PB1 expressed alone did not. The enriched 5'-vRNA/PB1/PB2/PA complex was highly active for ApG and globin mRNA primed transcription on a model 3'-vRNA template. RNA synthesis in the absence of added primers produced products with 5'-terminal tri- or diphosphate groups, indicating that genuine unprimed initiation of transcription also occurred. No transcriptase activity was detected for the PB1/PA complex. These results demonstrate a role for PA in the enhancement of 5' end binding activity of PB1, a role for PB2 in the assembly of a polymerase complex able to perform both cap-dependent and -independent synthesis and that NP is not required for the initiation of replicative transcription.