O4-(5'-uridylyl)tyrosine is the bond between the genome-linked protein and the RNA of poliovirus.

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Abstract
Virion RNA of poliovirus type 1 was analyzed for the linkage between gemome-protein VPg and the polyribonucleotide chain. Hydrolysis of the linkage with acid or alkali and enzymatic degradation showed that the bond is neither a phosphodiester such as nucleotidyl-(P-O)-serine (or threonine) nor a phosphoramidate such as nucleotidyl-(P-N)-amino acid. VPg-RNA can be iodinated by the Bolton and Hunter reagent [iodinated 3-(4-hydroxyphenyl)propionic acid N-hydroxysuccinimide ester] but not by the chloramine-T or lactoperoxidase procedures, an observation suggesting that VPg does not contain accessible tyrosine. VPg can be labeled with [3H]tyrosine in vivo. Hydrolysis of VPg-[32P]pUp with 5.6 M HCl at 110.degree. C yielded 32P-labeled O4-(3''-phospho-5''-uridylyl)tyrosine that could be cleaved with micrococcal nuclease to O4-[32P]phosphotyrosine and uridine 3''-[32P]phosphate. These data establish that VPg is linked to the poliovirus genome by a bond between the O4 of tyrosine and the 5''-P atom of the terminal uridylic acid residue. The 5'' end of polio genome RNA can now be described as VPg(Tyr-O)-pU-U-A-A-A-A-C-A-G..
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