Responsible for RNA synthesis (replicase and transcriptase), cap addition, and cap methylation. Performs also the polyadenylation of subgenomic mRNAs by a stuttering mechanism at a slipery stop site present at the end of viral genes. The template is composed of the viral RNA tightly encapsidated by the nucleoprotein (N). The viral polymerase binds to the genomic RNA at the 3' leader promoter, thereby initiating either genome replication or mRNA transcription. In the transcription mode, the polymerase performs the sequential transcription of all mRNAs using a termination-reinitiation mechanism responding to gene start and gene end signals. Some polymerase disengage from the template at each gene junction, resulting in a decreasing abundance of transcripts from the 3' to the 5' end of the genome. The first gene is the most transcribed, and the last the least transcribed. The viral phosphoprotein helps the polymerase to engage the N-RNA template and acts as processivity factor. Polyribonucleotidyl transferase (PRNTase) adds the cap structure when the nascent RNA chain length has reached few nucleotides. Ribose 2'-O methylation of viral mRNA cap precedes and facilitates subsequent guanine-N-7 methylation, both activities being carried by the viral polymerase. In the replication mode, the polymerase replicates the whole viral genome without recognizing the gene end transcriptional signals. The ability of the polymerase to override the gene end signals as it is producing the antigenome is probably due to replicative RNA becoming encapsidated with nucleoprotein as it is synthesized.

> 85% as determined by SDS-PAGE.