Three deletion mutants of the structural protein region of the Semliki Forest virus (SFV) genome, including one which encompassed all the viral structural protein genes, induced apoptosis in BHK cells at 48 h after transfection, as shown by DNA laddering and TUNEL staining, as did the wild-type SFV4 RNA. A similar result was obtained for the SFV1 expression vector, which has a multicloning site inserted in place of the structural protein genes. However, in cells transfected with viral RNA containing a deletion of the nsP2 gene, neither viral RNA synthesis nor the induction of apoptosis occurred. Both SFV1 vector and wild-type SFV4 RNA induced apoptosis in human H358a lung carcinoma cells, which have a homozygous deletion of the p53 gene. It is concluded that the SFV vector encodes a function in the nonstructural coding region which induces p53-independent apoptosis and is dependent on viral RNA synthesis.

The nonstructural region of the genome of the avirulent A7 strain of Semliki Forest virus (SFV) has been sequenced, so that the complete nucleotide sequence is available. Compared to the virulent SFV4 strain (produced from the infectious clone pSP6-SFV4), A7 contains 226 nucleotide changes in the translated region, which result in 47 amino acid changes. The 5h nontranslated region has two nucleotide changes, and the 3h nontranslated region is longer in A7 than SFV4, and contains divergent