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 two differents sites in 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. Needs as cofactors the phosphoprotein for processivity and the M2-1 anti-termination protein. Polyribonucleotidyl transferase (PRNTase) adds the cap structure when the nascent RNA chain length has reached few nucleotides (By similarity). Ribose 2'-O methylation of viral mRNA cap precedes and facilitates subsequent guanine-N-7 methylation (By similarity). 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 (By similarity). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) GTP + H2O = GDP + H(+) + phosphate a 5'-end triphospho-adenylyl-adenylyl-cytidylyl-adenosine in mRNA + GDP + H(+) = a 5'-end (5'-triphosphoguanosine)-adenylyl-adenylyl-cytidylyl-adenosine in mRNA + diphosphate 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 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 For RNA-directed RNA polymerase activity. Mn(2+) can stimulate de novo initiation but it is inefficient at supporting elongation of de novo initiated RNA. Interacts with the phosphoprotein (via C-terminus); the association of P and L forms the polymerase complex. Localizes in cytoplasmic inclusion bodies. Contains an RNA-dependent RNA polymerase (RdRp) domain, a polyribonucleotidyl transferase (PRNTase or capping) domain and a methyltransferase (MTase) domain. Belongs to the paramyxovirus L protein family.