A SINGLE AMINO-ACID CHANGE IN THE E2 SPIKE PROTEIN OF A VIRULENT-STRAIN OF SEMLIKI-FOREST VIRUS ATTENUATES PATHOGENICITY

The virulent strain SFV4 of Semliki Forest virus (SFV), produced from the infectious clone pSP6-SFV4, is lethal after intranasal (i.n.) infe ction of adult mice and for pregnant mice after intraperitoneal (i.p.) infection. In contrast, the A7 strain of SFV is avirulent when given i.n. to adult mice, but induces fetal death in pregnant mice after i.p . infection. The nucleotide and deduced amino acid sequences of part o f the core and all of the envelope region of A7-SFV were determined an d compared to those of SFV4. A7 differed from SFV4 at 80 nucleotides ( nt) in the coding sequence, 15 of which were associated with amino aci d differences and seven of which (two in the E2 protein and five in E1 ) were nonconservative. The 3' non-coding sequence of A7 was longer (4 15 nt) than that of SFV4 (263 nt) and a divergent sequence of 181 nt w as present adjacent to the end of the F1 coding region. The effects on virulence of two mutations in the E2 gene of SFV4, resulting in the n on-conservative amino acid substitutions present in A7, were analysed. One mutation (mut 8729 a/c) resulted in only slight attenuation, wher eas the other (mut 8902 a/g) resulted in avirulence for pregnant mice. However, mut 8902 a/g was lethal for the majority of developing fetus es after i.p. infection of the mother.