Displays RNA-directed RNA polymerase activity. The viral mRNA guanylyl transferase displays a different biochemical reaction than the cellular enzyme. The template is composed of the viral RNA tightly encapsidated by the nucleoprotein (N). Functions either as transcriptase or as replicase. The transcriptase synthesizes subsequently three subgenomic RNAs, assuring their capping and polyadenylation by a stuttering mechanism. In replicase mode, the polymerase replicates the whole viral genome without recognizing the transcriptional signals. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Interacts with the P protein. Truncated N-terminus.