Comparative sequence analysis of the 5'terminal noncoding regions of poliovirus vaccine strain Sabin 1, Sabin 2, and Sabin 3 genomes

Complementary DNAs (cDNAs) were synthesized with virion RNAS from immunologically distinct vaccine strains of poliovirus, that is, Sabin 1 (LSc, 2ab), Sabin 2 (p712, Ch, 2ab), and Sabin 3 (Leon 12a₁b). Restriction endonuclease HaeIII cleavage patterns of these cDNAs suggested striking difference in RNA sequences among these poliovirus serotypes. The virion RNA of Sabin 3 was shown to have a genome-linked protein (VPg) and physical properties of the VPg were identical to those of VPgs of the other poliovirus strains. Virion RNA from each serotype was labeled with ¹²⁵I-Bolton-Hunter reagent, after proteinase K treatment to remove all but a small peptide of the VPg which covalently linked to the 5′ terminus. The specifically 5′-end-labeled RNAs were analyzed by rapid RNA sequencing techniques. Sequences of the first 20 nucleotides were identical in Sabin 1, Sabin 2, and Sabin 3 strains, that is, VPg-pU-U-A-A-A-A-C-A-G-C-U-C-U-G-GG-G-U-U-G. Complete sequence homology was observed between virion RNAs of Sabin 1 and Sabin 2 strains up to the 43rd nucleotide from the 5′ end except for the 25th nucleotide by a “wandering spot” analysis of ¹²⁵I-labeled RNA mixture of these two strains. These results suggested that serotypes of poliovirus were derived by numerous mutations from a putative prototype poliovirus, and that the homologous sequence at 5′ termini, conserved through a long evolutional process, may have essential roles for replication and viral function of poliovirus.