Flp-In™ Core System

We have observed in-house that in cells where the FRT site has integrated into a very transcriptionally active locus in the host cell genome (seen more commonly in Flp-In CHO and Flp-In 293 cells but can also happen in Flp-In 3T3 cells and any other Flp-In host cell line), there is some "read-through" transcription and translation of the lacZ-Zeocin ORF subsequent to the Flp-In reaction, even though the lacZ-Zeocin ORF does not have a bonafide promoter and ATG. In such cases, the hygromycin-resistant clones would also be lacZ-positive and Zeocin antibiotic-resistant. To make sure that the integration is FRT site-specific and not random, we recommend doing a parallel control transfection with no pOG44 present. This should yield no surviving clones upon hygromycin selection, indicating that all the hygromycin-resistant clones obtained in the presence of pOG44 are indeed Flp recombinase-dependent and hence have the gene of interest integrated at the FRT site. Also, a Southern blot analysis of these clones will help verify that they do indeed have proper FRT integration of the gene of interest despite the expression of lacZ (although this is typically not necessary). After the Flp-In reaction, as long as you see hygromycin-resistant clones, we recommend that you select them and assay them for expression of your gene of interest.

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The Flp-In 3T3 cell line is derived from NIH3T3 cells, which are mouse fibroblast cells. The CMV promoter is known to get silenced over time in murine cell lines and hence we would recommend using a Flp-In expression vector with a non-CMV promoter in these cells, such as the pEF5/FRT/V5-D-TOPO vector or the pEF5/FRT/V5-DEST vector.

Before giving up, we would suggest that you try using the pFRT/lacZeo2 vector to generate your host cell line. This vector contains a truncated version of the SV40 promoter driving the lacZ-Zeocin fusion. Use of this vector facilitates the isolation of clones that have integrated the vector near enhancer elements in the genome, thus resulting in higher levels of expression of the gene of interest.

The Jump-In system is PhiC31-integrase mediated and is a stable, targeted, and irreversible mammalian expression system. It consists of the Jump-In Fast system that involves a single integration step and the Jump-InTI (targeted integration) system that needs two integration steps, both of which are targeted and irreversible. In contrast, the Flp-In system is a stable, targeted mammalian expression system that is reversible. The first integration is random (integration of pFRT/lacZeo), and the second integration (integration of the Flp-In expression vector) is targeted but reversible.

In theory, one can get multiple integrations of the Flp-In expression construct—an FRT-specific integration event and a random, second-site integration. However, random integration is a relatively uncommon event. Limiting the amount of DNA in the transfection will reduce the chance of second-site integration. We have transfected 293 cells (lacking the FRT site) with the pcDNA5/FRT vector and have identified one potential second-site integrant after screening over 200 clones. DNA integrations can be detected by Southern blot. A single integrant will display a single band; double: two; triple: three, etc. We have maintained a number of Flp-In expression cell lines for over four months and have not observed any loss of the Flp-In expression construct, whether hygromycin selection was maintained or not.

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