Poliovirus (PV) RNA is translated by a cap-independent mechanism involving the internal entry of ribosomes onto the 5′ noncoding region (NCR). Using the vaccinia virus-T7 RNA polymerase transient expression system, we showed previously that deletion of certain individual predicted secondary structures within the PV 5′ NCR rendered the element defective in directing internal initiation when assayed alone. However, these defective 5′ NCRs were functional when coexpressed within cells with full-length PV cDNA (N. Percy, G. J. Belsham, J. K. Brangwyn, M. Sullivan, D. M. Stone, and J. W. Almond, J. Virol. 66:1695-1701, 1992). We have extended the study to demonstrate that when these predicted secondary structures are deleted in combination, the enhanced activity in the presence of the full-length PV cDNA is still observed. Indeed, a poliovirus 5′ NCR devoid of all predicted secondary structures is capable of initiating protein synthesis under these conditions. Surprisingly, we also found that this enhancement of activity requires neither any PV protein nor the inhibition of cap-dependent translation. The results indicate that the defective PV 5′ NCR elements can be complemented in trans by functional 5′ NCRs in a highly sequence specific manner.