A MUTATION IN THE RNA-POLYMERASE OF POLIOVIRUS TYPE-1 CONTRIBUTES TO ATTENUATION IN MICE

The attenuated Sabin strain of poliovirus type 1 (PV-1) differs from t he neurovirulent PV-1 Mahoney strain by 55 nucleotide mutations. Only one of these mutations (A-480-->G), in the 5' noncoding (5' NC) region of the genome, is well characterized, and it confers a strong attenua ting effect. We attempted to identify genetic attenuation determinants in the 3'-terminal part of the Sabin 1 genome including the 3D polyme rase (3D(pol)) gene and the 3' NC region. Previous studies suggested t hat some of the 11 mutations in this region of the Sabin 1 genome, and in particular a mutation in the polymerase gene (U-6203-->C, Tyr-73-- >His), are involved to some extent in the attenuation of PV-1. We anal yzed the attenuating effect in the mouse model by using the mouse-adap ted PV-1/PV-2 chimeric strain v510 (a Mahoney strain carrying nine ami no acids of the VP1 capsid protein from the Lansing strain of PV-2). M utagenesis of locus 6203 was performed on the original v510 (U-6203--> C) and also on a hybrid v510/Sabin 1 (C-6203-->U) carrying the downstr eam 1,840 nucleotides of the Sabin 1 genome including the 3D(pol) and 3' NC regions. Statistical analysis of disease incidence and time to d isease onset in numerous mice inoculated with these strains strongly s uggested that nucleotide C-6203 is involved in the attenuation of the Sabin 1 strain. Results also suggested that, among the mutations locat ed in the 3D(pol) and 3' NC regions, nucleotide C-6203 may be the prin cipal or the only one to be involved in attenuation in this mouse mode l. We also found that the effect of C-6203 was weaker than that of nuc leotide G-480; the two nucleotides acted independently and may have a cumulative effect on attenuation. The U-6203-->C substitution also app eared to contribute to the thermosensitivity of the Sabin 1 strain.