A small protein, VPg, is covalently linked to the 5' end of the plus-stranded poliovirus genomic RNA. Poliovirus messenger RNA, identical in nucleotide sequence to genomic RNA, is not capped at its 5' end by the methylated structure that is common to most eukaryotic mRNAs. These discoveries presented two problems. First, as cap structures are usually required for translation of mRNA into protein, how does this uncapped viral RNA act as a template for translation? Second, what is the function of VPg? The identification of the internal ribosomal-entry site, which allows the entry of ribosomes into viral mRNA independently of the 5' mRNA end, has solved the first conundrum. Here we describe the resolution of the second problem. VPg is linked to the genomic RNA through the 5'-terminal uridylic acid of the RNA. We show that VPg can be uridylylated by the poliovirus RNA polymerase 3Dpol. Uridylylated VPg can then prime the transcription of polyadenylate RNA by 3Dpol to produce VPg-linked poly(U). Initiation of transcription of the poliovirus genome from the polyadenylated 3' end therefore depends on VPg.