PROTEIN-PRIMED RNA-SYNTHESIS BY PURIFIED POLIOVIRUS RNA-POLYMERASE

A small protein, VPg, is covalently linked to the 5' end of the plus-s tranded poliovirus genomic RNA(1-3). Poliovirus messenger RNA, identic al in nucleotide sequence to genomic RNA,is not capped at its 5' end b y the methylated structure that is common to most eukaryotic mRNAs, Th ese discoveries presented two problems. First, as cap structures are u sually required for transition of mRNA into protein, how does this unc apped viral RNA act as a template for translation? Second, what is the function of VPg? The identification of the internal ribosomal-entry s ite, which allows the entry of ribosomes into viral mRNA independently of the 5' mRNA end, has solved the first conundrum(4-6). Here we desc ribe the resolution of the second problem. VPg is linked to the genomi c RNA through the 5'-terminal uridylic acid of the RNA. We show that V Pg can be uridylylated by the poliovirus RNA polymerase 3D(pol). Uridy lylated VPg can then prime the transcription of polyadenylate RNA by 3 D(pol) to produce VPg-linked poly(U). Initiation of transcription of t he poliovirus genome from the polyadenylated 3' end therefore depends on VPg.