We constructed two versions of an RCASBP-based retroviral shuttle vector, RSVP (RCASBP shuttle vector plasmid), containing either the zeocin or blasticidin resistance gene. In this vector, the drug resistance gene is expressed in avian cells from the long terminal repeat (LTR) promoter, whereas in bacteria the resistance gene is expressed from a bacterial promoter. The vector contains a bacterial origin of replication (ColE1) to allow circular viral DNA to replicate as a plasmid in bacteria. The vector also contains the lac operator sequence, which binds to the lac repressor protein, providing a simple and rapid way to purify the vector DNA. The RSVP plasmid contains the following sequence starting with the 5" end: LTR, gag, pol, env, drug resistance gene, lac operator, ColE1, LTR. After this plasmid was transfected into DF-1 cells, we were able to rescue the circularized unintegrated viral DNA from RSVP simply by transforming the Hirt DNA into Escherichia coli. Furthermore, we were able to rescue the integrated provirus. DNA from infected cells was digested with an appropriate restriction enzyme (ClaI) and the vector-containing segments were enriched using lac repressor protein and then self-ligated. These enriched fractions were used to transform E. coli. The transformation was successful and we did recover integration sites, but higher-efficiency rescue was obtained with electroporation. The vector is relatively stable upon passage in avian cells. Southern blot analyses of genomic DNAs derived from successive viral passages under nonselective conditions showed that the cassette (drug resistance gene-lac operator-ColE1) insert was present in the vector up to the third viral passage for both resistance genes, which suggests that the RSVP vectors are stable for approximately three viral passages. Together, these results showed that RSVP vectors are useful tools for cloning unintegrated or integrated viral DNAs.
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