Construction of recombinant vaccinia virus strains using single-stranded DNA insertion vectors

The ability of single-stranded (ss) DNA, isolated from recombinant M13 bacteriophage, to direct the insertion of foreign genetic elements into the vaccinia virus (VV) genome was examined. An identical chimeric transcriptional unit [VV promoter/chloramphenicol acetyl transferase (CAT) gene embedded in DNA sequences encoding vaccinia virus thymidine kinase (TK)] was inserted into either the previously characterized plasmid insertion vector, pGS20, or into M13mp18. It was found that the ss vector (M13mp18:TK/CAT) was four times more efficient than the plasmid vector (pGS20:CAT) in catalyzing homologous recombination of the cat gene by marker transfer into the VV genome. Furthermore, Southern blot analyses and CAT enzymatic activity assays confirmed that the structure of the M13-derived recombinant genomes were as expected and that the chimeric genes were fully active. Although the precise mechanism responsible for the ss DNA-catalyzed insertion event is not known, these results are discussed with respect to the advantages of using M13-based vectors with which to manipulate and insert genetic information into infectious VV recombinants.