The 5'-terminal 88 nt of poliovirus RNA fold into a cloverleaf RNA structure and form ribonucleoprotein complexes with poly(rC) binding proteins (PCBPs; AV Gamarnik, R Andino, RNA, 1997, 3:882-892; TB Parsley, JS Towner, LB Blyn, E Ehrenfeld, BL Semler, RNA, 1997, 3:1124-1134). To determine the functional role of these ribonucleoprotein complexes in poliovirus replication, HeLa S10 translation-replication reactions were used to quantitatively assay poliovirus mRNA stability, poliovirus mRNA translation, and poliovirus negative-strand RNA synthesis. Ribohomopoly(C) RNA competitor rendered wild-type poliovirus mRNA unstable in these reactions. A 5'-terminal 7-methylguanosine cap prevented the degradation of wild-type poliovirus mRNA in the presence of ribohomopoly(C) competitor. Ribohomopoly(A), -(G), and -(U) did not adversely affect poliovirus mRNA stability. Ribohomopoly(C) competitor RNA inhibited the translation of poliovirus mRNA but did not inhibit poliovirus negative-strand RNA synthesis when poliovirus replication proteins were provided in trans using a chimeric helper mRNA possessing the hepatitis C virus IRES. A C24A mutation prevented UV crosslinking of PCBPs to 5' cloverleaf RNA and rendered poliovirus mRNA unstable. A 5'-terminal 7-methylguanosine cap blocked the degradation of C24A mutant poliovirus mRNA. The C24A mutation did not inhibit the translation of poliovirus mRNA nor diminish viral negative-strand RNA synthesis relative to wild-type RNA. These data support the conclusion that poly(rC) binding protein(s) mediate the stability of poliovirus mRNA by binding to the 5'-terminal cloverleaf structure of poliovirus mRNA. Because of the general conservation of 5' cloverleaf RNA sequences among picornaviruses, including C24 in loop b of the cloverleaf, we suggest that viral mRNA stability of polioviruses, coxsackieviruses, echoviruses, and rhinoviruses is mediated by interactions between PCBPs and 5' cloverleaf RNA.