This is the catalytic subunit of the viral RNA-dependent RNA polymerase complex. This complex is involved in viral RNA replication that produces (+)-stranded genomes via a complementary, (-)-stranded intermediate. Binds RNA cooperatively with the host ribosomal protein S1. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Binds 2 Mg(2+) per subunit, Ca(2+) is used in crystallization to prevent RNA polymerase activity. Homodimer; the replicase complex can dimerize. Part of the viral RNA-dependent RNA polymerase complex, the other subunits are the host ribosomal protein S1, EF-Tu and EF-Ts. S1 is needed for the initiation of genomic RNA (+)-strand replication. In order to produce high amounts of RNA polymerase catalytic core, a fusion protein consisting of tsf-tufB-replicase with a cleavable linker between tufB and the viral replicase subunit is frequently used.