A novel vaccinia virus expression system allowing construction of recombinants without the need for selection markers, plasmids and bacterial hosts.

Vaccinia virus is one of the most widely applied expression systems for use in eukaryotes in molecular biology. Expression of heterologous genes in the vaccinia virus system, however, requires integration of the foreign DNA into the vaccinia virus genome by means of homologous recombination or by direct molecular cloning. In both cases, plasmid vector constructs are required that contain the gene of interest and, usually, a marker gene, both of which are controlled by suitable promoter sequences. In order to simplify the construction of recombinants and to eliminate the need for a marker gene we have developed a modified vaccinia virus genome that allows the direct targeted insertion of DNA fragments downstream of a strong vaccinia virus promoter without any further cloning steps. The gene of interest is amplified by PCR using oligonucleotide primers that provide an SfiI site at the 5' end and an RsrII site at the 3' end of the PCR product. Following digestion with these restriction enzymes, the PCR product is operationally linked to a synthetic early/late promoter within the viral genomic DNA via the unique SfiI/RsrII sites of the modified vaccinia virus genome. Using this approach, intermediate plasmid constructs and bacterial hosts are not required and time consuming screening steps can be omitted, because of 90% of the virus progeny carry the foreign DNA.