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One of the most basic techniques in biomedical research is cDNA cloning for expression studies in mammalian cells.  Vaccinia  topoisomerase I-mediated cloning (TOPO cloning by Invitrogen) allows fast and efficient recombination of PCR-amplified DNAs. Among TOPO vectors, a pcDNA3.1 directional cloning vector is particularly convenient, since it can be used for expression analysis immediately after cloning. However, I found that the cloning efficiency was reduced when RT-PCR products were used as inserts (about one-quarter). Since TOPO vectors accept any PCR products, contaminating fragments in the insert DNA create negative clones. Therefore, I designed a new mammalian expression vector enabling positive blue white selection in  Vaccinia  topoisomerase I–mediated cloning. The method utilized a short nontoxic LacZα peptide as a linker for GFP fusion. When cDNAs were properly inserted into the vector, minimal expression of the fusion proteins in  E .  coli  (harboring  lacZΔM15 ) resulted in formation of blue colonies on X-gal plates. This method improved both cloning efficiency (75%) and directional cloning (99%) by distinguishing some of the negative clones having non-cording sequences, since these inserts often disturbed translation of  lacZ α. Recombinant plasmids were directly applied to expression studies using GFP as a reporter. Utilization of the P2A peptide allowed for separate expression of GFP. In addition, the preparation of  Vaccinia  topoisomerase I-linked vectors was streamlined, which consisted of successive enzymatic reactions with a single precipitation step, completing in 3 hr. The arrangement of unique restriction sites enabled further modification of vector components for specific applications. This system provides an alternative method for cDNA cloning and expression in mammalian cells.

plos one

An Alternative Method to Facilitate cDNA Cloning for Expression Studies in Mammalian Cells by Introducing Positive Blue White Selection in Vaccinia Topoisomerase I-Mediated Recombination