We report the construction of a poliovirus genome [pPVM-VPg(3F4A)] harboring a double mutation in VPg. This mutant, in which the tyrosine and the threonine at residues 3 and 4 of the VPg region were replaced by phenylalanine and alanine, respectively, is lethal, that is, all RNA synthesis was abolished and no revertants could be isolated. Using the properties of dicistronic polioviruses (with the general genotype PV 5'NTR-3AB-EMCV IRES-PV ORF-3'NTR), we have observed that the defect in RNA synthesis of the VPg(3F4A) mutant could be rescued by providing wild-type protein 3AB from the first open reading frame in trans. We conclude that the 3AB provided by the first cistron of the dicistronic construct was capable of "intragenomic complementation." Intragenomic complementation, however, was inefficient. Thus, the dicistronic RNAs were only quasi-infectious, and even first-passage viruses were found to have reverted to a functioning VPg in the polyprotein. This phenomenon underlines the role of polypeptide 3AB in multiple functions of viral proliferation. First-passage viruses, all of which expressed a small-plaque phenotype, had retained the original dicistronic genotype. At the fourth passage, however, all isolates were monocistronic, and they displayed complex genetic rearrangements revealing interesting information regarding IRES function.