Rescue of a tk-plasmid from transgenic mice reveals its episomal transmission

This communication demonstrates the usefulness of the plasmid rescue procedure for recovery of plasmids from transgenic mice. We have microinjected the plasmid pSK1 harbouring the Herpes simplex virus thymidine kinase gene into fertilized mouse oocytes and succeeded in recovering plasmids from newborns by transformation of E. coli either with HindIII cut cellular DNA or with uncut DNA. The majority of the rescued plasmids were indistinguishable from pSK1 by restriction analysis. The rescued plasmids proved to be functionally active in a transient expression assay in mouse Ltk- cells. The pSK1 DNA sequences were inherited by up to 90% of the second generation progeny mice, which is not in agreement with a Mendelian transmission of heterozygous markers integrated into a single site of the chromosome. These data support the assumption that germ line transmission of non-integrated episomal plasmids can occur.