The SFV Gene Expression System.

The SFV Expression System is a DNA expression system used to produce recombinant protein in eukaryotic cells (1,2). The SFV system is based on the Semliki Forest Vnus (SFV), which has several features that provide distinct advantages for a good cDNA expression system. These are: 1. The SFV RNA genome has a positive polarity and thus functions directly as an mRNA. Infectious RNA molecules can be therefore obtained by transcription from a full-length cDNA copy of the genome, 2. The SFV RNA molecule codes for its own RNA rephcase. Consequently, within a few hours after infection, up to 200,000 copies of the plus-RNAs are made in a single cell (3), 3 SFV replication occurs in the cell cytoplasm, where the virus replicase transcribes and caps the subgenomes for production of the structural proteins (4) This eliminates problems, such as mRNA splicing, limitations in transcription factors, problems with capping effctency, and mRNA transport, 4. The cytopathic effects in infected cells appear late during infection. Thus, there is an extensive time window of approx 4-24 h after infection during which a very high expression level of the SFV structural proteins is obtained without significant morphologrcal change of host cells, and 5. Under laboratory conditions, SFV infects a broad range of different cultured cells mammalian, avian, reptilian, amphibian, and insect cells.