Poliovirus CRE-Dependent VPg Uridylylation Is Required for Positive-Strand RNA Synthesis but Not for Negative-Strand RNA Synthesis

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Abstract
The cis -acting replication element (CRE) is a 61-nucleotide stem-loop RNA structure found within the coding sequence of poliovirus protein 2C. Although the CRE is required for viral RNA replication, its precise role(s) in negative- and positive-strand RNA synthesis has not been defined. Adenosine in the loop of the CRE RNA structure functions as the template for the uridylylation of the viral protein VPg. VPgpUpU OH , the predominant product of CRE-dependent VPg uridylylation, is a putative primer for the poliovirus RNA-dependent RNA polymerase. By examining the sequential synthesis of negative- and positive-strand RNAs within preinitiation RNA replication complexes, we found that mutations that disrupt the structure of the CRE prevent VPg uridylylation and positive-strand RNA synthesis. The CRE mutations that inhibited the synthesis of VPgpUpU OH , however, did not inhibit negative-strand RNA synthesis. A Y3F mutation in VPg inhibited both VPgpUpU OH synthesis and negative-strand RNA synthesis, confirming the critical role of the tyrosine hydroxyl of VPg in VPg uridylylation and negative-strand RNA synthesis. trans -replication experiments demonstrated that the CRE and VPgpUpU OH were not required in cis or in trans for poliovirus negative-strand RNA synthesis. Because these results are inconsistent with existing models of poliovirus RNA replication, we propose a new four-step model that explains the roles of VPg, the CRE, and VPgpUpU OH in the asymmetric replication of poliovirus RNA.

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