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 subgenomic RNAs, assuring their capping and polyadenylation by a stuttering mechanism (By similarity). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) a 5'-end (5'-triphosphoguanosine)-adenylyl-adenylyl-cytidylyl-adenosine in mRNA + 2 S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyladenylyl)-adenylyl-cytidylyl-adenosine in mRNA + H(+) + 2 S-adenosyl-L-homocysteine a 5'-end (5'-triphosphoguanosine)-adenylyl-adenylyl-cytidylyl-adenosine in mRNA + S-adenosyl-L-methionine = a 5'-end (5'-triphosphoguanosine)-(2'-O-methyladenylyl)-adenylyl-cytidylyl-adenosine in mRNA + H(+) + S-adenosyl-L-homocysteine GTP + H2O = GDP + H(+) + phosphate a 5'-end (5'-triphosphoguanosine)-(2'-O-methyladenylyl)-adenylyl-cytidylyl-adenosine in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyladenylyl)-adenylyl-cytidylyl-adenosine in mRNA + S-adenosyl-L-homocysteine a 5'-end triphospho-adenylyl-adenylyl-cytidylyl-adenosine in mRNA + GDP + H(+) = a 5'-end (5'-triphosphoguanosine)-adenylyl-adenylyl-cytidylyl-adenosine in mRNA + diphosphate Belongs to the varicosavirus protein L family.