FIG. 3.

Identification of specific mutations involved in SVNI neurovirulence toward 3- and 5-week-old rats. Recombinants were used to estimate the contribution of the E2 (recombinant 40) and the 5′ end determinants (recombinants 43 and 45) or the combined effect (recombinant 47) of SVNI determinants. For SVN and SVNI, the differing nucleotides (positions 8 and 45 in the 5′NCR) and amino acids (positions 121, 190, and 266 in E2) in the neurovirulent loci are indicated. For the recombinants, the indicated nucleotides or amino acids are those normally present in the other strain. PFU/ICLD₅₀ titer ± the standard error (SE) values were determined as described in the legend to Fig. 2.