A NEW INTERNAL RIBOSOMAL ENTRY SITE 5'-BOUNDARY IS REQUIRED FOR POLIOVIRUS TRANSLATION INITIATION IN A MOUSE SYSTEM

Four mutants of the virulent Mahoney strain of poliovirus were generat ed by introducing mutations in nucleotides (nt) 128 to 134 of the geno me, a region that contains a part of the stem loop II (SLII) structure located within the internal ribosomal entry site (IRES; nt 120 to 590 ) (K. Shiroki, T. Ishii, T. Aoki, Y. Ota, W.-X. Yang, T. Komatsu, Y. A mi, M. Arita, S. Abe, S. Hashizume, and A. Nomoto, J. Virol, 71:1-8, 1 997). These mutants (SLII mutants) replicated well in human HeLa cells but not in mouse TgSVA cells that had been established from the kidne y of a poliovirus-sensitive transgenic mouse, Their neurovirulence in mice was also greatly attenuated compared to that of the parental viru s, The poor replication activity of the SLII mutants in TgSVA cells ap peared to be attributable to reduced activity of the IRES, Two and thr ee naturally occurring revertants that replicated well in TgSVA cells were isolated from mutants SLII-1 and SLII-5, respectively. The revert ants recovered IRES activity in a cell-free translation system from Tg SVA cells and returned to a neurovirulent phenotype like that of the M ahoney strain in mice, Two of the revertant sites that affected the ph enotype were identified as being at nt 107 and within a region from nt 120 to 161, A mutation at nt 107, specifically a change from uridine to adenine, was observed in all the revertant genomes and exerted a si gnificant effect on the revertant phenotype, Exhibition of the full re vertant phenotype required mutations in both regions, These results su ggested that nt 107 of poliovirus RNA is involved in structures requir ed for the IRES activity in mouse cells.