Displays RNA-directed RNA polymerase, mRNA guanylyl transferase, mRNA (guanine-N(7)-)-methyltransferase and poly(A) synthetase activities. 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 five subgenomic RNAs, assuring their capping and polyadenylation by a stuttering mechanism. The replicase mode is dependent on intracellular N protein concentration. In this mode, the polymerase replicates the whole viral genome without recognizing the transcriptional signals (By similarity).