The replicase polyprotein 1ab is a multifunctional protein: it contains the activities necessary for the transcription of negative stranded RNA, leader RNA, subgenomic mRNAs and progeny virion RNA as well as proteinases responsible for the cleavage of the polyprotein into functional products. The Nsp1 chain is essential for viral subgenomic mRNA synthesis. The 3C-like serine proteinase chain is responsible for the majority of cleavages as it cleaves the C-terminus of the polyprotein. The helicase chain, which contains a zinc finger structure, displays RNA and DNA duplex-unwinding activities with 5' to 3' polarity. Plays a role in viral transcription/replication and prevents the simultaneous activation of host cell dsRNA sensors, such as MDA5/IFIH1, OAS, and PKR (By similarity). Acts by degrading the 5'-polyuridines generated during replication of the poly(A) region of viral genomic and subgenomic RNAs. Catalyzes a two-step reaction in which a 2'3'-cyclic phosphate (2'3'-cP) is first generated by 2'-O transesterification, which is then hydrolyzed to a 3'-phosphate (3'-P) (By similarity). If not degraded, poly(U) RNA would hybridize with poly(A) RNA tails and activate host dsRNA sensors (By similarity). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) ATP + H2O = ADP + H(+) + phosphate ATP + H2O = ADP + H(+) + phosphate uridylyl-uridylyl-ribonucleotide-RNA = a 3'-end uridylyl-2',3'-cyclophospho-uridine-RNA + a 5'-end dephospho-ribonucleoside-RNA The hydrophobic domains (HD) could mediate the membrane association of the replication complex and thereby alter the architecture of the host cell membrane. Specific enzymatic cleavages in vivo by its own proteases yield mature proteins. There are two alternative pathways for processing. Either nsp4-5 is cleaved, which represents the major pathway or the nsp5-6 and nsp6-7 are processed, which represents the minor pathway. The major pathway occurs when nsp2 acts as cofactor for nsp4 (By similarity). Produced by -1 ribosomal frameshifting at the 1a-1b genes boundary. Produced by conventional translation. Belongs to the arteriviridae polyprotein family.