RNA-directed RNA polymerase that catalyzes the transcription of viral mRNAs, their capping and polyadenylation. 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, and transcribes subsequently all viral mRNAs with a decreasing efficiency. The first gene is the most transcribed, and the last the least transcribed. The viral phosphoprotein acts as a processivity factor. Capping is concommitant with initiation of mRNA transcription. Indeed, a GDP 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. Polyadenylation of mRNAs occur by a stuttering mechanism at a slipery stop site present at the end viral genes. After finishing transcription of a mRNA, the polymerase can resume transcription of the downstream gene. RNA-directed RNA polymerase that catalyzes the replication of viral genomic RNA. The template is composed of the viral RNA tightly encapsidated by the nucleoprotein (N). The replicase mode is dependent on intracellular N protein concentration. In this mode, the polymerase replicates the whole viral genome without recognizing transcriptional signals, and the replicated genome is not caped or polyadenylated. 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 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)-(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 GTP + H2O = GDP + H(+) + phosphate Homooligomer. Interacts with the P and C proteins. The L protein complexes with P protein to form the functional polymerase. C protein binding to L has an inhibitory effect (By similarity). The N-terminal part (about 1-400) seems to be involved in binding to the P protein. Least abundant structural protein (approximately 50 copies per virion). Unstable in the absence of P protein (By similarity). Belongs to the paramyxovirus L protein family.