title: Semliki Forest virus expression system: production of conditionally infectious recombinant particles.
abstract: In the recently developed Semliki Forest virus (SFV) DNA expression system, recombinant RNA encoding the viral replicase, and helper RNA molecules encoding the structural proteins needed for virus assembly are cotransfected into cells. Since the helper RNA lacks the sequence needed for its packaging into nucleocapsids, only recombinant RNAs should be packaged. We have found, however, that small amounts of replication-proficient SFV particles can still be produced. Here we describe the construction of a helper variant with a mutation in the gene encoding the viral spike protein such that its product cannot undergo normal proteolytic processing to activate viral entry functions. Hence, the recombinant stock is noninfectious, but may be activated by cleavage with chymotrypsin. When recombinant virus produced with the new helper was examined in a variety of assays, including sensitive animal tests, we were unable to detect any replication-competent SFV particles. We therefore conclude that this conditional expression system meets extremely stringent biosafety requirements.
authors.affiliation: Department of Molecular Biology, Karolinska Institute, Novum, Huddinge, Sweden.
authors.forename: P
authors.initials: P
authors.lastname: Berglund
authors.forename: M
authors.initials: M
authors.lastname: Sjöberg
authors.forename: H
authors.initials: H
authors.lastname: Garoff
authors.forename: G J
authors.initials: GJ
authors.lastname: Atkins
authors.forename: B J
authors.initials: BJ
authors.lastname: Sheahan
authors.forename: P
authors.initials: P
authors.lastname: Liljeström
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D002918:Chymotrypsin / Q000378:metabolism; D006224:Cricetinae; D005455:Fluorescent Antibody Technique; D015870:Gene Expression; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D016296:Mutagenesis; D012313:RNA; D012367:RNA, Viral / Q000235:genetics*; D012672:Semliki forest virus / Q000235:genetics*; D014162:Transfection; D014759:Viral Envelope Proteins / Q000235:genetics*; D014779:Virus Replication
chemical_list: D012367:RNA, Viral; C039919:RNA, recombinant; D014759:Viral Envelope Proteins; D012313:RNA; D002918:Chymotrypsin