Transient and inducible expression of vaccinia/T7 recombinant viruses

Recombinant DNA technology has made it possible to develop molecular cloning vectors that allow the expression of heterologous genes in a variety of animal viruses. This chapter discusses the use of vaccinia virus encoding bacteriophage T7 RNA polymerase as an expression vector system. A chosen gene is inserted into a plasmid vector designed to express genes under the control of the T7 promoter. Transient expression can then be achieved either by transfecting this plasmid into cells infected with the recombinant vaccinia virus expressing T7 RNA polymerase, vTF7-3 or by crossing this plasmid into the vaccinia virus genome and coinfecting cells with both viruses. Moreover, placement of lacO downstream of the vaccinia virus P11 late promoter regulating T7 RNA polymerase expression, and integration of lacI under vaccinia promoter control into the viral genome, vT7lacOI, yielded a recombinant virus capable of IPTG-inducible T7 promoter-controlled expression of foreign genes.