Markerless bacterial artificial chromosome manipulation method by red proteins of phage λ mediated homologous recombination utilizing fluorescent proteins for both positive and counter selection.

Manipulating viral genomes is an essential technique in reverse genetics and recombinant vaccine development. A strategy for manipulating large viral genomes involves introducing their entire genome into bacterial artificial chromosomes and employing Escherichia coli genetic tools. For sequence manipulation on bacterial artificial chromosomes (bacterial artificial chromosomes recombineering), a well-established method that relies on the Escherichia coli strain GS1783, and the template plasmid, pEPKan-S, is often used. This method, known as markerless DNA manipulation, allows for the generation of a recombinant bacterial artificial chromosome that does not retain the selection markers used during recombination. Although this method is highly innovative, there remains room for improvement as the plasmid is currently only available for positive selection. Additionally, differentiating true recombinants from false negatives often proves time-consuming. Consequently, an improved method for bacterial artificial chromosomes recombineering, which utilizes fluorescent proteins, has been developed. This method's core comprises three plasmids containing the I-SceI recognition site, antibiotic resistance genes (ampicillin, kanamycin, and zeocin), and fluorescent genes (YPet, mOrange, and mScarlet). The success or failure of Red recombination can be confirmed via fluorescent signals. To validate this method, the Lassa virus genes were introduced into the bacterial artificial chromosomes, containing the entire genome of the vaccinia virus strain LC16m8. Consequently, the expression of fluorescent protein genes contributed to positive selection, such as blue-white screening and counter-selection during the first and second Red recombination.