The 5'-terminal structures of poliovirion RNA and poliovirus mRNA differ only in the genome-linked protein VPg.

The 5'-terminal, RNase T1-resistant oligonucleotide of poliovirus mRNA has been isolated. Its sequence is pU-U-A-A-A-A-C-A-Gp, which is identical to that of virion RNA except that the genome-linked protein VPg is absent [Nomoto, A., Detjen, B., Pozzatti, R. & Wimmer, E. (1977) Nature 268, 208-213]. Because all newly synthesized viral RNAs are VPg-linked, we propose that VPg is cleaved from progeny RNA at the linkage between protein and nucleic acid prior to polyribosome formation. This may represent a new mode of processing of viral macromolecules. Virion RNA from which VPg has been cleaved proteolytically retains its specific infectivity, an observation suggesting that VPg is not involved in early steps (penetration and translation) of the infectious cycle initiated by RNA.