The replicase polyprotein of coronaviruses 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. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Produced by -1 ribosomal frameshifting at the 1a-1b genes boundary. Produced by conventional translation. Belongs to the coronaviruses polyprotein 1ab family.