VPg linkage to the 5' ends of picornavirus RNAs requires production of VPg-pUpU. VPg-pUpU is templated by an RNA stem-loop (the cre or oriI) found at different locations in picornavirus genomes. At least one adaptive mutation is required for human rhinovirus type 14 (HRV-14) to use poliovirus type 3 (PV-3) or PV-1 oriI efficiently. One mutation changes Leu-94 of 3C to Pro; the other changes Asp-406 of 3Dpol to Asn. By using an in vitro VPg uridylylation system for HRV-14 that recapitulates biological phenotypes, we show that the 3C adaptive mutation functions at the level of 3C(D) and the 3D adaptive mutation functions at the level of 3Dpol. Pro-94 3C(D) has an expanded specificity and enhanced stability relative to wild-type 3C(D) that leads to production of more processive uridylylation complexes. PV-1/HRV-14 oriI chimeras reveal sequence specificity in 3C(D) recognition of oriI that resides in the upper stem. Asn-406 3Dpol is as active as wild-type 3Dpol in RNA-primed reactions but exhibits greater VPg uridylylation activity due to more efficient recruitment to and retention in the VPg uridylylation complex. Asn-406 3Dpol from PV-1 exhibits identical behavior. These studies suggest a two-step binding mechanism in the assembly of the 3C(D)-oriI complex that leads to unwinding of at least the upper stem of oriI and provide additional support for a direct interaction between the back of the thumb of 3Dpol and 3C that is required for 3Dpol recruitment to and retention in the uridylylation complex.