Coronavirus nsp8 bears a second, noncanonical RdRp activity that synthesizes short oligonucleotides (<6nt), acting as an RNA primase that produces the primers required for nsp12-mediated RNA synthesis. Structural studies have shown that nsp8 interacts with nsp7, forming a hexadecameric protein complex (eight molecules of each nsp) that contains a channel capable of encircling RNA due to its internal dimensions and electrostatic properties. This complex, which is active in both de novo initiation and primer extension, confers processivity to the RdRp in an in vitro assay using purified proteins.